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Fourth-Order Bandpass Subwoofer Enclosure Design Calculator

 This calculator will design a bandpass subwoofer enclosure to achieve a specific F3(L) . If you don't understand what this means, click here for help.
Calculate Your Enclosure Here
 This calculator will design an enclosure using any value for F3(L) that you desire. If the "Gain" value comes up negative, you will have to increase the value you entered for F3(L), or choose a higher S factor.

Note : Too much gain can be hazardous to your speakers, try to keep it +3 dB or less unless you want to find out what your speakers look like with holes in them. If the calculated gain is too high, enter a lower number in the desired F3(L) box. If the gain returns a negative value, enter a higher F3.
Number of Drivers 
Isobaric Loading

Yes  No
  
Enter Qts
Enter Vas  (ft3 )
Enter Fs

Choose an S Factor

0.7  0.6   0.5
    

Enter Desired F3(L) 
Vf =(ft3 )
Vr(ft3 )
Qbp
Fb = Hertz
Passband Hertz
Gain = (dB SPL@1W/1M)

  If you want to experiment with different quantities of drivers, or the isobaric option, you don't need to enter all of the values again, just change the value you want to adjust and click the "Calculate Enclosure" button again. When you are finished, write down your enclosure specifications, (or copy & paste them), and go here to design the port(s) for your front chamber.
 

Instructions

  1. Make sure you have Java turned on in your browser.
  2. Enter required information.
  3. Click "Calculate" to get the answers.
  4. The calculators will give accurate results only for rectangular shaped objects.
      

Calculators

  1. Box Dimensions Calculator: Figures out third dimension of a box, given two known dimensions, required internal volume and material thickness.  For example, if you have the length and height of a part of your trunk, and need to figure out the width of the box: Enter the two dimensions you have measured, enter the internal volume, and the thickness of the wood.  If you are placing objects inside the box, such as braces, or a piece of wood to separate the box into two independent boxes for two woofers, use the "Solid Object Volume Calculator."
      
  2. Solid Object Volume Calculator: Figures out volume of an object to be placed inside a box. Then add result to "required internal volume" in the "Box dimensions Calculator".
      
  3. Conversion Factors: Metric to English units, Cubic inches to cu ft, etc.

 

Box Dimensions Calculator

External measurements: x x ? In.
Desired internal volume: Cu. Ft.
Thickness of box material: In.
External Box Dimensions: x x In.

 

Solid Object Volume Calculator

Object measurements: x x In.
Volume: Cubic Feet

 

Conversion Factors

- cu in / 1728 =   Cubic feet
- cu ft x 1728 = Cubic inches
- cu ft x 28320   = Cubic centimeters
- cm x 0.394 = Inches
- in x 2.540 = Centimeters
- ft x 30.48 = Centimeters
- ft x .3048 = Meters
- m x 3.281 = Feet

 

This page contains descriptions of the variations in port designs used in speaker enclosures.

 The most common type of port is a round tube. These are typically made of black plastic and look like a section of thin-walled pipe with a lip on one end.

Round Style Ported Speaker Enclosure

 The second most frequently used type of vent is a square port. The advantage of this design is that you are not limited by availability of only certain diameter tubes. You can also build the port out of the same material you are using to fabricate the enclosure.

Square Style Ported Speaker Enclosure

 The last type of vent is called a slot port. It is created using one wall of the enclosure as a wall of the vent. This can be very useful for bandpass enclosures which can be very hard to tune to a low enough frequency due to the small size of the front (vented) chamber. Friction created by the air flow in the enclosure traveling along the enclosure wall makes this type of port effectively longer than it physically is - which tunes the enclosure to a lower frequency so a slot port will be shorter for the equivalent vent area when compared to a simpler square vent.

Slot Style Ported Speaker Enclosure

Porting tips:

Keep the port(s) as far away from the enclosure sides and back as possible except for the slot type port.

Make sure the inside end of a tube or square port is at least the equivalent of one vent diameter away from the back of the box or any bracing materials.

Keep the vent free from anything that may affect air flow through it.

The port should also be placed as far away from the speaker cone as practical.

 

Sixth-Order Bandpass Subwoofer Enclosure Design Calculator

Explanation of Terms
Qts, Vas, and Fs, are the electro-mechanical parameters for your woofer. They are also commonly referred to as the "Thiele-Small" parameters. You can get these from the paperwork that came with your speakers, from the dealer where you purchased your speakers, or from the manufacturer of the speaker (they are sometimes hard to contact, however).
F3(L) - the frequency where the response is down by 3 dB (on the low end of the passband). This will be the first number in the "Passband" box when you calculate an enclosure.
F3(H) - the frequency where the response is down by 3 dB (on the high end of the passband). This will be the second number in the "Passband" box when you calculate an enclosure.
Vf - the internal volume of the front chamber of the enclosure.
Vr - the internal volume of the rear chamber of the enclosure.
Fb(F) - the frequency that the vent in the front chamber of the enclosure needs to be tuned to.
Fb(R) - the frequency that the vent in the rear chamber of the enclosure needs to be tuned to.
Passband - refers to the range of frequencies that will be allowed to pass through the speaker system (the frequencies you will be able to hear/feel). Any frequencies above or below the passband will be attenuated (reduced).
Gain - the amount of boost(or in some designs the amount of attenuation) in sound pressure level(SPL). This is usually referred to in speaker design with other terms, which serve to make it a valid reference point.

A typical specification for a bandpass enclosure would be :

Efficiency/Sensitivity : 94 dB @ 1W/1M.

 This means that this design is capable of creating a sound pressure level of 94 dB when a signal of 1 watt is applied to it, and is measured with the measuring instrument 1 meter (approximately 3.28 feet) away from the speaker system.

Bandpass Subwoofer Enclosure Image

Typical Sixth-Order Bandpass
Enclosure Design  
 This style of enclosure is patented by the Bose Corporation, information provided is for personal use only.
Calculate Your Enclosure Here
 One of the reasons bandpass enclosures are so popular is because of an inherent property they have that allows the enclosure designer the flexibility to trade bandwidth for efficiency and vice versa. This calculator will design a 6th order bandpass enclosure. This calculator is only for Qts values from 0.18 to 0.28.
 To use this calculator, enter your values for Qts, Vas, and Fs, and then click the "Calculate Enclosure" button. Your results will appear in the other boxes.

Note : Too much gain can be hazardous to your speakers, try to keep it +3 dB or less unless you want to find out what your speakers look like with holes in them.
Number of Drivers 
Isobaric Loading

Yes  No
  
Enter Qts Enter Vas  (ft3 )Enter Fs 
Vf = (ft3 )
Vr =  (ft3 )
Fb(F) = Hertz
Fb(R) =  Hertz
Passband =  Hertz
Gain = (dB SPL@1W/1M)

  If you want to experiment with different quantities of drivers, or the isobaric option, you don't need to enter all of the values again, just change the value you want to adjust and click the "Calculate Enclosure" button again. When you are finished, write down your enclosure specifications, (or copy & paste them), and go here to design the port(s) for your new subwoofer enclosure.
 

Vent Dimensions Calculator

Formulas
Minimum Usable Vent Diameter
 The first thing you should do when calculating a vent is to determine how big it has to be. "Port noise" can ruin the sound of an otherwise good enclosure design. If you do not have the Xmax figure for your driver(s), leave the default value (it is an average value suitable for most subwoofers).

 If you want to use this calculator, first choose what size driver(s) you are using, then enter how many are in your enclosure. Now, enter your Xmax figure (in millimeters), and the frequency that you need to tune your vent to. And, finally click the "Calculate Minimum" button to get your results. The "Minimum Diameter" box is for round ports, and the "Minimum Area" box is the minimum area required for a square vent. If you need to convert a measurement to/from English/Metric click here, then come back.

Driver Size (Inches)
6.5 1012 15 18
Is this a bandpass enclosure?
No  Yes
  
Enter Quantity of Drivers 
Enter Xmax mm
Enter Tuning Frequency Hertz
Minimum Diameter =  inches
Minimum Area = inches

Length of a side
for a square vent =
 inches
Calculate Your Vent Length

 To use this calculator, first you will need to choose whether you want a round or a square port. Then, enter your internal box volume (in cubic feet).

 Next, you enter either an inside diameter for the tube you are using if you want a circular port, or the inside dimensions of the square port you are going to build into your box (make sure you click the "Slot Port" button if you are designing a slot port - if you don't understand the difference between a square vent and a slot vent, click here). And finally, press the "Calculate Length" button at the bottom, and your answer will appear in the "Vent Length" box.

Select Your Port Type
Round  Square 
Is this a slot port ?
No  Yes 
Quantity of ports
Enter Enclosure Volume (ft3 )
Enter Desired Tuning Frequency  Hertz
Round Vent
Enter Desired Port Diameter
  Inches
Square Vent
Enter Height
  Inches
Enter Width Inches
Vent Length  Inches
  If you come up with a value too long for your enclosure, enter a smaller number(s) for the diameter, or the heighth/width of your vent(s), and click the "Calculate Length" button again to recalculate (you don't need to reload the page).
 

Sealed Speaker Enclosure Design Calculator

 This calculator is designed to tell you the Qtc of a sealed enclosure. It is useful for determining how a different woofer will function in an existing box (for example - if you ruined a speaker and a replacement is unavailable), or if you know your speaker parameters and just want to find out what the Qtc value of your speaker system is.

 If you don't understand anything on this page, click here to learn the basics of "sealed" speaker enclosure design.

Calculate Your Qtc Here

 To use this form, enter the Qts, Vas, and Fs parameters for your speaker, and then enter your enclosure volume (Vb) in the boxes below. Then click the "Calculate Qtc" button, and the results will appear in the empty text boxes.

Enter Qts
Enter Vas  (ft3 )
Enter Fs Hertz
Enter Vb (ft3 )

Qtc = 
Fc = Hertz
F3 = Hertz

 

warning.gif (487 bytes)Check with manufacturer or dealer for appropriate box volume and design.  Some subs can't be used in certain types of boxes, and have very small tolerances for box volume errors.  If a sub is installed in a box larger or smaller than what is supposed to, it will sound bad and could be destroyed.  Boxes can be built in any shape, but it is difficult to calculate volume for complex shapes.

Materials

A box has to be very rigid.  Most common building materials are 5/8" or thicker particle board or medium density fiberboard.

If building a box with Plexiglas, do not use anything less that 1/2 inch thick.

A common material used to mold complex shaped boxes is fiberglass, but it is a real pain to work with, and several layers need to be applied for a solid finish.

Gluing, Sealing

Use glue at all joints (cheapest and most used product is Liquid Nails).  Make sure there are no holes.  Any leaks will degrade the performance of your subs, not to mention the annoying noise air makes when being pushed out of a small hole.

Let glue cure for at least 24 hours before mounting the woofers.  The fumes of some products will eat up rubber and other materials subs are made of.

Holding Joints Together

Screw joints (use 2"  -  2-1/2" screws) every four inches or so. Pre-drill about 3/4" deep, so that screws do not split the wood at the edges, especially when working with particleboard.

A box for Each Sub?

Even though it is not necessary to have two separate chambers for two subs, it is best to take this approach for two reasons:  First, if one of the subs dies, then the volume of the box will be "twice" as big, as seen by the sub that is still working.  This could cause problems and even damage the other sub.   The second reason is bracing.  building a box with a divider in the middle will be much sturdier.

Making Ports

There are several way to build ports.  If a pre-made port is not available, the most common material is PVC tubing.  PVC tubing is very rigid, comes in different diameters, and is easily found at any hardware store.

Cut the tubing at the desired length.  Consider the volume the port takes up when calculating the box volume.  Cut a hole in the box.  Make sure the hole is as perfect as possible to minimize gaps between the box and the tube.   A couple wood braces can be added for screwing the port top the box.  Seal the gaps using plenty of Liquid Nails or similar product.

Bracing

Boxes that are more than a foot on width or length or height, should be braced (use a piece of wood maybe 3 or 4 inches wide across the box, so that box does not flex).  It is a good idea to put wood blocks on the corners for reinforcement. Always consider that blocks, braces, neon lights, etc. inside a box take up space and should be accounted for when calculating internal volume.

Damping/Filling

It is advisable to put damping material inside a box.   Pillow polyfill and fiberglass insulation are common, though polyfill is a lot easier on your skin.  This increases subwoofer efficiency by dissipating some energy that affects the sub, particularly the voice coil.  Polyfill also "fools" a sub into thinking it is in a bigger box.  Play around with different amounts of polyfill until you get the desired results.

Finishing the Box

Add wood filler to holes and sand the box to make a smooth surface.  If you are painting the box, It is a good idea to apply primer under the paint.

It is not necessary to sand the box if you are using carpet or padding under vinyl, since the thickness of the material will cover any small imperfections.  The best way to cut carpet or vinyl is with a good quality carpet knife.  Blades wear out pretty quickly, so buy a handful.  Cut a piece of carpet (or vinyl) big enough to cover the whole box.  Apply adhesive to both box and carpet (spray 3M adhesive 77 or 90 works great). Wait about a minute and place the fabric over the wood.  For a good fit, stretch the fabric when applying it.   The fabric should wrap around and end in a place of the box that will not be seen.  Do one side at a time, cutting excess carpet.  If possible, add staples to hold the fabric at the ends.

 

Fourth-Order Bandpass Subwoofer Enclosure Design Calculator

 This calculator will design a bandpass subwoofer enclosure to achieve a specific amount of gain (in dB). If you don't understand what this means, click here for help.
Calculate Your Enclosure Here
 This Javascript is designed to calculate a bandpass enclosure with any amount of gain that you desire. If the F3L is too high, try a higher S factor. If this still gives you a F3L that is too high, you will have to reduce the amount of gain you are asking for, or use a different speaker.

Note : Too much gain can be hazardous to your speakers, try to keep it +3 dB or less unless you want to find out what your speakers look like with holes in them.
Number of Drivers 
Isobaric Loading

Yes  No
  
Enter Qts
Enter Vas  (ft3 )
Enter Fs

Choose an S Factor

0.7  0.6   0.5
    

Enter Desired Gain  dB
Vf =(ft3 )
Vr(ft3 )
Qbp
Fb = Hertz
Passband Hertz

  If you want to experiment with different quantities of drivers, or the isobaric option, you don't have to enter all of the values again, just change the value you want to adjust and click the "Calculate Enclosure" button again. When you are finished, write down your enclosure specifications, (or copy & paste them), and go here to design the port(s) for your front chamber.
 

Sealed Speaker Enclosure Design Calculator

 This calculator will design a "sealed" speaker enclosure to obtain a specific Qtc value. If you don't understand what this means, click here for help.
 
Calculate Your Enclosure Here
 To use this form, enter the Qts, Vas, and Fs parameters for your speaker, and then enter your desired Qtc in the boxes below. Then click the "Calculate Enclosure" button, and the results will appear in the empty text boxes. If the calculated F3 (-3 dB point) is too high you will need to enter a lower value for desired Qtc. A Qtc value of 0.707 will give you the lowest possible F3 for the woofer or subwoofer you are using. This is a maximally flat alignment. The drawback is a larger enclosure. F3 increases for Qtc values higher than 0.707, below that it increases again.
 Note: Typical Qtc values for designs that are commonly available are from 0.9-1.2. This means that most designs you have heard (unless you are listening at high-end audio retailers) are likely to have a peak in frequency response in the 70-120 Hz range. Many people like this and will be disappointed by the sound of a speaker without this "boost" in the bass.
Number of Drivers : Isobaric
Yes  No
  
Enter Qts
Enter Vas  (ft3 )
Enter Fs Hertz
Enter Qtc
Vb =  (ft3 )
 F3 = Hertz

  If you want to experiment with different quantities of drivers, or the isobaric option, you  don't need to enter the values again, just change the value you want to adjust and click the "Calculate Enclosure" button again.
 

Sealed Rear Chamber Bandpass Sub Woofer Design Tips

Explanation of Terms
Qts, Vas, and Fs, are the electro-mechanical parameters for your woofer. They are also commonly referred to as the "Thiele-Small" parameters. You can get these from the paperwork that came with your speakers, from the dealer where you purchased your speakers, or from the manufacturer of the speaker (they are sometimes hard to contact, however).
S Factor - refers to the efficiency of the enclosure. 0.7 is the most efficient, however, it also has the narrowest passband. 0.5 is the least efficient, but it has the widest passband. Another drawback to the lower S Factor designs is that they have more passband ripple  (the sag in the frequency response plot in Figure 1). Higher S factor enclosure designs also exhibit better transient response.

 One of the reasons bandpass enclosures are so popular is this inherent property that allows the designer the flexibility to trade bandwidth for efficiency or vice versa.
Passband Ripple

Figure 1
F3(L) - the frequency where the response is down by 3 dB (on the low end of the passband   - see Figure 1). This will be the first number in the "Passband" box when you calculate an enclosure.
F3(H) - the frequency where the response is down by 3 dB (on the high end of the passband  - see Figure 1). This will be the second number in the "Passband" box when you calculate an enclosure.
Vf - the internal volume of the front (vented) chamber of the enclosure.
Vr - the internal volume of the rear (sealed) chamber of the enclosure.
Qbp - is the total resonance of the speaker system. Essentially the same as Qtc is in a sealed design.
Fb - the frequency that the vent in the front chamber of the enclosure needs to be tuned to.
Passband - refers to the range of frequencies that will be allowed to pass through the speaker system (the frequencies you will be able to hear/feel). Any frequencies above or below the passband will be attenuated (reduced).
Gain - the amount of boost(or in some designs the amount of attenuation) in sound pressure level(SPL). This is usually referred to in speaker design with other terms, which serve to make it a valid reference point.

A typical specification for a bandpass enclosure would be :

Efficiency/Sensitivity : 94 dB SPL @ 1W/1M.

 This means that this design is capable of creating a sound pressure level of 94 dB when a signal of 1 watt is applied to it, and is measured with the measuring instrument 1 meter (approximately 3.28 feet) in front of the speaker system.

Sealed Bandpass Enclosure Image

Typical Fourth-Order Bandpass
Enclosure Design  
General Design Tips
  •  Always add the volume displaced by the speaker's motor system (the magnet, frame, and the suspension), and all bracing materials to your enclosure's rear chamber volume when designing a speaker system.
  •  If you add damping material (fiberglass, Acousta-Stuf polyfill, Dacron, etc...), you can design the rear chamber of your enclosure 10% smaller than the suggested ideal size. I usually use 3/4-1 lb. per cubic foot (polyfill) of enclosure volume. I prefer not to use fiberglass because there is a chance that the fibers can get in the voice coil, and also because of the health hazards (skin & lung irritation) that are possible when working with this material.
  •  You should also take the volume consumed by your port(s) into account when designing the front chamber of your bandpass enclosure (unless you're designing an enclosure with the vent on the outside of the box).
  •  Some people feel that because a bandpass enclosure naturally attenuates the higher frequencies a lowpass filter is unneccessary, this is false. Bandpass enclosures create standing waves in the front chamber of the enclosure which cause peaks in the frequency response. In my opinion, sub woofers sound much better when they are not receiving any high frequencies. So, always use a lowpass filter (or a notch filter) on any bandpass sub woofer design.
  •  I do not put damping material in the front chamber of bandpass enclosures because it can shift around in the enclosure and interfere with vent operation unless it is very securely attached. I do however, cover all of the interior walls of the front chamber with carpet underlayment (padding - put the hard side toward the wood if the type you get has a side with a sealing layer on it). I believe this helps to suppress the standing waves created in the front chamber. I have also heard that felt works well when used in this application - I have never tried it though.
  •  Before you attach the last panel of your enclosure, apply silicone to all of the joints in the enclosure, and also seal the back of any wiring terminal cups (if a gasket was not provided with it). Let the silicone fully cure (read the label and follow all of the instructions) before putting on the last panel. I install the damping material at this time while waiting for the silicone to dry.
  •  Build your enclosure using the thickest, hardest, non-resonant material you can get your hands on. I use 3/4" (minimum) medium density fiberboard (MDF). Solid wood is not a good choice for enclosure construction - although hardwoods do make excellent bracing material. You can use some types of plywood - it has to be marine grade though (which can be expensive). The last choice is particle board - this material is flimsy and it is hard to make solid enclosure joints with, but if you cannot get MDF in your area you can use this (brace the enclosure very thoroughly if you do).
  • For some excellent advice on bracing your speaker enclosure, click here.
 
Minimum Vent Diameter

 The first two formulas shown here represent the minimum usable port diameter for subwoofer speaker enclosures - they are set up to avoid power compression and vent noise. The first one is the one I use for bandpass designs (from "The Loudspeaker Design Cookbook" by Vance Dickason - I divide the tuning frequency by two before I use it in this formula though), and the second is a more conservative formula from the works of Richard Small, which is useful for vented speaker designs.
 dv - is the required diameter of the port in inches.
 Fb - is the tuning frequency of your enclosure in Hertz.
Formula 1

Formula 2
 Vd - is the volume displaced by the driver (in cubic meters) traveling through it's full excursion (peak-to-peak). To figure out Vd for a speaker, find the Sd value for your driver in the table below, and multiply that number times the Xmax (in meters) of your speaker.
 
Driver Diameter Sd (M2)
18" 0.1300
15" 0.0890
12" 0.0530
10" 0.0330
8" 0.0220
6.5" 0.0165
6" 0.0125
5.25" 0.0089
Vent Length

 This formula is also from "The Loudspeaker Design Cookbook". It tells you how long you need to make the vent.
 Fb - is the tuning frequency of your enclosure in Hertz.
 Lv - is the length of your port in inches.
 R - is the inside radius of your vent tube.
 Vb - is the internal volume of your enclosure in cubic inches. To convert cubic feet to cubic inches, multiply by 1728.

 If you want to use multiple ports, divide your enclosure volume by the number of ports you want to use, then use the result of this calculation as your Vb in the formula below to find out how long each port should be (a tip from JL Audio).

Vent Length Formula
 If you want to calculate square vents, the formula below will give you the value of R to use in the formula above.

Conversion Formula
 In the formula above, a is the area of your square vent (height x width), and Pi (Pi) is approximately 3.141592.
 

Butterworth Fourth-Order Vented Speaker Enclosure Design Formulas & Calculator

Explanation of Terms
 Qts, Vas, and Fs, are some of the electro-mechanical parameters for your woofer. They are also commonly referred to as the "Thiele-Small" parameters. You can get these from the paperwork that came with your speakers, from the dealer where you purchased your speakers, or from the manufacturer of the speaker (they are sometimes hard to contact, however).
Vb - the required internal volume of the enclosure.
F3 - the frequency that the response is down by 3 dB at.
Fb - the frequency that the vent needs to be tuned to.
Enclosure Design Formulas
Vented Enclosure Image
Typical Vented Enclosure Design  
Vb = Qts 2.87 x 15 x Vas
Fb = Qts -0.9 x 0.42 x Fs
F3 = Qts -1.4 x 0.26 x Fs

Calculate Your Enclosure Here
 To use this form, enter the Qts, Vas, and Fs parameters for your speaker in the boxes below. Then click the "Calculate Enclosure" button, and the results will appear in the empty text boxes.
Number of Drivers : Isobaric
Yes  No
  
Enter Qts
Enter Vas  (ft3 )
Enter Fs Hertz
Vb = (ft3 )
F3 Hertz
Fb  Hertz
  If you want to experiment with different quantities of drivers, or the isobaric option, you  don't need to enter all of the values again, just change the value you want to adjust and click the "Calculate Enclosure" button again. When you are finished, write down your enclosure specifications, (or copy & paste them), and go here to design your port.
 

Measurement Unit Conversions


 To use this unit conversion calculator, just type in the number that you have in the appropriate unit box and hit the "Convert It" button. If you want to convert more numbers, just enter a new measurement unit in the box it belongs in, and press the button again.

Linear Units
Millimeter Centimeter Meter Inch Foot Yard Mile

 

Square Units
Millimeter 2 Centimeter 2 Meter 2 Inch 2 Foot 2

 

Cubic Units
Milliliter 3 Liter 3 Meter 3 Inch 3 Foot 3 Yard 3

 

Mass Units
Milligram Gram Kilogram Ounce Pound
 
Selecting a Subwoofer Alignment

 There are three basic subwoofer enclosure alignments, sealed, vented, and bandpass. There are however, a number of variations of each of these styles. Most subwoofers are designed to function optimally in a specific type of box. Contrary to popular opinion, you don't just select a subwoofer and then decide you want to put it in a sixth-order bandpass enclosure because you heard this type of subwoofer box means the loudest or lowest bass.

 An inexpensive subwoofer loaded in the proper enclosure will sound much better than an expensive one in the wrong box. If you choose the wrong type of box for your speaker, the speaker may sound bad or suffer mechanical damage. There are some general rules to help you decide which alignment is the right one for your subwoofers. You need to compare the Thiele-Small parameters of your driver to the guidelines below.

 Sealed box (and sealed bandpass) woofers should have the following characteristics:

  • Qts => 0.40
  • Fs <= 35 Hz
  • Xmax => 4 mm

 Vented or ported box (and ported bandpass) woofers should have the following characteristics:

  • Qts <= 0.40
  • Fs <= 45 Hz
  • Vas should be relatively low (less than 4 cubic feet unless enclosure size is not a factor).

 An additional calculation to help you determine the proper enclosure is the efficiency bandwidth product. This is figured by dividing the Fs value by the Qes value. If the result is close to one hundred, a vented enclosure is the right choice. Closer to fifty means this driver will function best in a sealed alignment.

 The best results (widest passband) for bandpass subwoofers are obtained when the Fs value divided by the Qts parameter is ninety or higher.

 
 This page is designed to help you determine the proper dimensions for a speaker enclosure. There are some ratios which are typically used to determine an enclosure shape that will help to minimize standing waves in the familiar rectangular speaker box. Never build a cube shaped box, which is the absolute worst enemy of a smooth frequency response curve.
Subwoofer Enclosure with correct proportions
The "Golden Ratio"
 To use this calculator, enter the volume of your enclosure (in cubic feet) in the first box. Then click the "Dimensions" button to see the proper internal dimensions for your new speaker box design.
Box volume =  (Ft3) Depth = inches
Height =  "
Width = "
Total =  (Ft3)
 Due to rounding, the total number shown above will come out smaller than the value you entered. This is normal. This calculator is only intended to give you some "ballpark" figures to use in the calculator below.
  

 

The Actual Box
 To use this calculator, add a little bit to the dimensions from the calculator above. Then click the "Box Volume" button to see the volume obtained from these dimensions. You won't need to re-enter all the values to re-calculate, you can change only one or as many as you desire. Then click the button again to calculate another time.

Inch MeasurementDecimal Equivalent
1/160.0625
1/80.125
1/40.250


Depth = inches
Height =  "
Width = "
Box volume =  (Ft3)

 Due to rounding, the total number shown above will come out smaller than the value you entered. This is normal. This calculator is only intended to design a speaker box with the correct proportions in an easy to build shape.

  
 

Calculating Vent Air Speed

 This calculator is intended to help you determine the speed of the air in a port. This will help you decide if a particular port is too small for your application, which will cause noise at high volume levels. This is not intended for use on bandpass designs. The answer is in percentage of the speed of sound. If you don't need to know the air speed in the vent, click here to automatically calculate a port that is the proper size for your speaker enclosure.
 

Speaker Reference Efficiency
 

A general rule regarding port air speed is to try to keep it 10 percent or less of the speed of sound.
 

Explanation of Terms
W is the acoustic power output of the speaker in Watts. You can calculate this by multiplying the reference efficiency times the power handling of the speaker.
Fb is the tuning frequency of the enclosure.
R is the radius of the port.
Mach is the speed of sound in air (344.8 m/sec or 1131.2 ft/sec).
 
Calculate
 To use this form, enter the values for W, Fb, and port radius. Then click the "Calculate Values" button, and the answer will appear in the empty text box.
Enter W
Enter Fb Hz
Enter Radius  inches
 percentage of Mach
 
 

Sealed Speaker Enclosure Design

Explanation of Terms
Qts, Vas, and Fs, are the electro-mechanical parameters for your woofer. They are also commonly referred to as the "Thiele-Small" parameters. You can get these from the paperwork that came with your speakers, from the dealer where you purchased your speakers, or from the manufacturer of the speaker (they are sometimes hard to contact, however).

Qtc - is the total resonance of the speaker system. An enclosure with a Qtc value of 0.707 will give you the flattest frequency response and the lowest possible F3 for a given driver, use this value if you want the your music to sound as close to the original recording as possible.

 In the frequency response plot below, the relative efficiency of the speaker is 90 dB. So,   the frequency of F3 is determined by first finding the proper SPL (sound pressure level) to look at (90 dB - 3dB = 87 dB) on the left scale, and then following this line over to the right until you run into the frequency response plot (the colored lines) of the enclosure you are interested in. Then you draw a line from this point down to the bottom scale to find the frequency of F3.

Qtc comparison

 Most people like the sound of boxes designed with Qtc values ranging from 0.9-1.1 which produces a slightly emphasized bass range. Values above 1.2 are undesirable because they tend to sound unnatural (the peak around the 100 Hertz range in the plot above is what most people refer to as "boomy" sound), and they also can be physically hard on the speaker.

Important Qtc relationships :

 F3 increases for values above or below 0.707.
 Enclosure volume (Vb) requirements decrease with higher Qtc values.
 Mechanical power handling increases for Qtc values up to 1.1 .

 Mechanical power handling is not the same as the manufacturers' "power rating" (which only tells how much power the voice-coil can handle without burning up, and has nothing to do with how much input power the speaker can take before it bottoms out). The speaker's suspension and the enclosure design determine the mechanical power handling of the system.

Fc - the resonant frequency of the driver when mounted to an enclosure.
Vb - the required internal volume of the enclosure.
F3 - the frequency that the response is down (from the reference level) by 3 dB at. In the frequency response plot below, the reference level is 90 dB (left scale). So, the frequency (bottom scale) of F3 is at the intersection of 87 dB and 40 Hertz. Therefore, this enclosure design has an F3 of 40 Hertz.

2nd-Order Frequency Response Plot

 
Alpha - The Greek letter alpha. In speaker design, this term is used to reflect the ratio of speaker compliance (Vas) to enclosure volume (Vb).
Second-Order Speaker Enclosure Design Formulas
Sealed Enclosure Image
Typical Sealed Enclosure Design  
 The following formulas will allow you to design a sealed speaker enclosure for any Qtc value you desire.
Alpha= (Qtc / Qts)2 - 1
Vb = Vas / Alpha
Fc = (Qtc x Fs/ Qts
F3 Formula

 If you don't want to/can't calculate an enclosure using the formulas shown above, click here to use the Javascript calculator, which does all of the math for you.
 

An introduction To Amplifiers!

Amplifiers take the signal from the head unit and makes it large enough to be able to drive your speakers. It is preferable to use separate amps for high and low frequencies but it is not necessary. The problem with using one amp for all frequencies is that you cannot adjust the levels among different frequency ranges as easily as you can adjust outputs of separate amps. Many people start their system with an amplifier for the low frequncies (bass) and use their head units built-in power to drive the higher frequency speakers. This is adequate but the built-in power in a head unit is usually not strong enough for high volume listening and not clean enough for the discerning ear. There are many options when choosing an amplifier.

Power?
There are different ways in which power is measured by amplifier manufacturers to make people think that their amps have more power than others. Laws of physics tell us that Power can be obtained by multiplying Current and Voltage. For example, if your amplifier gets 12 volts, and it draws 20 amps, then power would be 240 watts, right? Not exactly. In the real world, amplifiers waste 50% or more of the power in the form of heat. That leaves you with only 120 watts.

Power Diagram
Click to View

Things get more complicated than that. There are different ways to measure power. Power can be measured for top to bottom of the signal (Peak, or Max, etc). Another way to measure power is From the zero-level to the top half (usually called music power). The most accurate way to measure power is RMS (root mean square) watts. The RMS value is obtained by squaring the value of the signal, taking the average, then the square root. This is the equivalent of the actual power delivered. Most reputable manufacturers use the RMS rating.

To get RMS power from peak or max power just divide by three. Music power is just half of peak power. For example, an amplifier is rated at 100w (peak) per channel. The so called Music power would be only 50w per channel. The RMS power would be 33w per channel. Big difference, isn't it? Be careful when checking specifications of amps before buying, to see what you are really getting. Always ask for the RMS power of an amplifier.

Confused enough? There is more. Some companies rate their amplifiers using unrealistic conditions, for example calculating power at 15 volts, under 2 ohms, at 10% distortion, etc. Make sure you see the actual test voltages and loads.

How to tell if I am getting a good amp?
Shop for reputable brands. Look at the size, weight of the amp. The more power the amp puts out, the more wasted heat, and the bigger area it will need to dissipate that heat (bigger heatsinks). This alone can't be enough to determine if the amp is good or not. Watch out for companies that use bigger heat sink than needed, giving the idea of a more powerful amp.

Look at the fuses that are either plugged into the amp, or specified by the instruction book. If you see a 400w amplifier with a 5-amp fuse, you should be suspicious. Remember what was said above, multiply size of the fuse by around 6 (12v at 50% efficiency), and that will give you a rough idea of what you are dealing with in terms of maximum possible RMS power.

How much power do I need?
For mids and highs, anywhere from 30 to 50 watts (RMS) per channel would be a minimum. For subs you would need at least 80 - 150 watts (or more) per subwoofer. There should always be more total power going to the subwoofers than  the rest of the speakers, since human ears are more sensitive to higher frequencies than lower. For example, if you have 4 x 50 watts going to all your mids and tweeters (total=200 W), then you should have at least 200 W or more going to your subs.

A lot of people wonder if too much amplifier power can burn up the speakers. What damages speakers most of the time is distortion, not power. If the speakers have the proper crossovers and are not distorting, then it is really hard to blow them. A bigger amp just gives you the opportunity to go to higher volumes without distortion.   Get the biggest amplifiers you can afford and your car's electrical system can handle.  More power means louder sound, but most importantly, cleaner sound.

What Else to Look For in an Amplifier

It is a good idea to get an amp with a built-in crossovers, so that you don't have to spend extra money later on crossovers. If you are going to be using multiple speakers, make sure the amp is 2-ohm stable (or less). A bridgeable amplifier could come in handy in the future if you are planning to upgrade. Overheat, short-circuit, overload protections are good features that any good amplifier should have.  Look for a low THD (total harmonic distortion) rating.

Amplifier Classes
There are different amplifier designs: Class A, A-B, B and D

Class A amplifiers are the most sonically accurate. On the other hand, they have some drawbacks that make them a rare breed. Class A amplifiers use only one output transistor that is turned "on" all the time, giving out tremendous amounts of heat. Class A amplifiers are very inefficient (less than 25%). More heat means more heatsink area, so even though most class A amps have built-in cooling fans, they are big.  Class A amplifiers are usually and expensive choice.

Class B amplifiers are the most common by far.  They use two output transistors. One for the positive and one for the negative part of the cycle. Both signals are then "combined". The problem with this design is that at the point when one transistor stops amplifying and the other one kicks in (zero volt line), there is always a small distortion on the signal, called "crossover distortion". Good amplifier designs make this crossover distortion very minimal. Since each transistor is "on" only half of the time, then the amplifier does not get as hot as a class A, yielding to a smaller size and better efficiency (typically 50%).

Class A-B amplifiers are a combination of the two types described above. At lower volumes, the amplifier works in class A. At higher volumes, the amplifier switches to class B operation.

An increasingly popular kind is the class D amplifier (known as digital amplifier). These amplifiers are not really digital (there is no such thing), but operate similarly in the same manner as a digital-to-analog converter. The signal that comes in is sampled a high rates, and then reconstructed at higher power. This type of amplifiers produce almost no heat and are very small in size, but really expensive.  Although there are full-range class D amplifiers available, most high-end manufacturers are designing amps for low frequency applications.  These amps are capable of over 1000 Watts.  Efficiency is much higher in class D amplifiers (~80%).

 

How To Install An Amplifier!

Remote Turn-on Wire
The remote turn on wire goes to the head unit.  When the radio is on, it puts out 12 volts that turn the amplifier on.  If you are using a factory radio that does not have a remote turn on (or power antenna wire) you can tap into, hook it up to the ignition, so that the amplifier does not remain on when you turn the car off.

If you are using multiple devices (amplifiers, crossovers, equalizers, fans, etc), you might have to add a relay, since typical turn-on wires in a radio can't handle more that 300mA.
  

Power Wiring
Even though amplifiers are easy to install, a lot of things could go wrong. The most important thing to consider is where to get the power from: Straight from the battery.  ALWAYS put a fuse as close to the positive battery terminal as possible. If the wire going to the back of the car shorts out, then the fuse will blow. If you don't install a fuse or breaker and the wire shorts out, then the wire will carry so much current that the insulation will melt and could catch your car on fire. The size of the fuse should be the same rating as the fuses used by the amp(s) or less. The ground (-) should be hooked-up to a metal part of the car. It is not necessary to run a ground wire all the way to the battery.

It is not essential to spend a lot of money in getting 99.999999% copper 0-gage wire and gold connectors unless you are installing a competition system. Here's a table to help decide what gauge wire to use, based on total current draw and length of wire:

Power Cable Calculator
Total Amperage
Draw of System
Up to 4 ft. 4 to 7 ft. 7 to 10 ft. 10 to 13 ft. 13 to 16 ft. 16 to 22 ft. 22 to 28 ft.
0 - 20 14 12 12 10 10 8 8
20 - 35 12 10 8 8 6 6 4
35 - 50 10 8 8 6 4 4 4
50 - 65 8 8 6 4 4 4 2
65 - 85 6 6 4 4 2 2 0
85 - 105 6 6 4 2 2 2 0
105 - 125 4 4 4 2 0 0 0
125 - 150 2 2 2 0 0 0 00
The above chart shows wire gauges to be used if no less than a .5 volt drop is accepted.
Cable size calculation takes into account terminal connection resistance.
  

RCA Wiring
When running power wires to the amp, keep them as far away from the RCA wires (see alternator noise section for more info), ideally on the other side of the car. It is OK to run the turn-on wire from the radio along with RCA's, since it carries very little current.
  

Mounting
Amplifiers produce a lot of heat and need to receive plenty of fresh air.  If the amplifier is to be mounted under a seat, upside down, in a rack or enclosed, a fan or two might need to be used to increase air flow.

To avoid noise problems, it is good practice to mount the amplifier itself to a piece of wood or other non-conducting material.  That way the only ground it gets is from the ground wire and not the mounting screws.

 

Amplifier Terms!

RMS Power: The power output of an amplifier should be roughly matched to what the amp will be used for and what speakers it will be driving. Oddly enough, the most common problem with matching speakers and amps is using an amp that is too weak to power the speaker. When an underpowered amp is used to power a speaker, the listener tends to turn the volume up higher in order to get more output of the amplifier. Eventually the amplifier runs into its limit and begins to distort. This distortion can cause the output from the amplifier to become DC for short periods of time and DC signals of even low power can destroy a speaker. Underpowering a speaker in this way can be more dangerous than overpowering it! Also more power is usually necessary when powering subwoofers because of their large size and excursion. Do not plan on using an amp of less than 75watts per channel to drive a subwoofer. The converse holds true for higher frequencies (midrange and treble) only 25-50watts per channel are necessary to drive speakers in those frequency ranges, however more power will not hurt, it just probably will not be used. Another factor in power output is stability in low resistance loads. Sometimes you can wire mutiple subwoofers to a single channel on an amplifier but the amp will have to work harder to drive this kind of load. Many moderately priced amps can drive loads as low as 2 ohms or less, with 4 ohms being the typical load of a single speaker.

Power Supply Regulation: The power supply in an amplifier converts the 12volt DC that is available in your car's electrical system to something the amp can use to produce more power. Several designs are employed by manufacturers today. Two classifications are regulated and unregulated. A regulated supply produces the same power regardless of whether your car's electrical system voltage sags (which a capacitor will help prevent). An amp using a stiffly regulated power supply will be able to supply full power even when the input voltage dips below 12volts. However, it will not gain any power if the input voltage goes above 12volts. An unregulated supply's power output depends directly on the input voltage. This causes changes in the maximum output power with changes in the car's electrical system. I recommend getting an amp with a regulated power supply so power output will be constant regardless of input voltage changes. This changes if you have a stiffening capacitor or another regulation device (Accumatch) to smooth out your car's electrical system. In this case, buy an amp with an unregulated supply. Some cheap amps use unregulated supplies to save money but provide none of the benefits of a typical unregulated supply. One way to determine whether an amp has a regulated supply or not is to view the power output specs for 12volt and 14.4volt inputs. If they are the same then the amp probably has a regulated supply otherwise it has an unregulated one.

Tri-Mode: Some amps can play in what is called "tri-mode." In this mode, 2 channels are used to drive a pair of high frequency speakers and one subwoofer. The subwoofer receives power from both channels. This is a very efficient way to use an amp for more than one purpose. A special crossover is required to separate the two ranges of frequencies and it should have a way of adjusting the output level between the high frequency speakers and the subwoofer. This can be a nice way to save money on your system although it wastes a little bit of amplifier power because of the crossover and it can be more difficult to adjust the relative level between the high and low frequency outputs.

Other Specs: THD (Total Harmonic Distortion) is a spec that often shows up with the power output spec. An example would be "45wattsx2 @ 0.01% THD" This spec says that at an output level of 45watts into each channel the THD will be no more than 0.01%. Sometimes manufacturers will quote the power spec at a THD of 1%. Be wary of this, 1% THD is poor and either implies that the amp is not very high quality or that the manufacturer is artificially inflating the power output spec by running the amp into a higher distortion region where it does produce more power but more distortion as well. Either way it is a sign of a poor amp or marketing that decieves. Anything less than 0.1% is negligible.

Built-in Crossovers: These allow you to use the amp to only amplify certain frequencies and dedicate the amp to a subwoofer or some other specialized speaker. By using an amplifier's built-in crossover you eliminate the need for a separate one which can save you considerable money. There are sophisticated amps on the market today that combine multiple channels and built-in crossovers so that you can use them in place of multiple amps and a separate crossover. They are expensive but often cheaper than buying separate components.

Pre-amp Outputs: Some amps have pre-amp outputs which allow you to "daisy-chain" multiple amps together without splitting the pre-amp output from your head unit. Also, if the amp has a built-in crossover, you can use it to drive another amp. For example if you have an amp you are going to use to drive a subwoofer with a built-in crossover at 90Hz, you can use its built-in crossover to set the amp to only amplify signals below 90Hz for the subwoofer and then have a pre-amp output that only has frequencies above 90Hz which you can connect to an amp that does not have a built-in crossover. That amp can then be used to power the high frequency drivers.

Input Sensitivities: I have received a number of questions about input sensitivities and their importance especially as to why 4 volt outputs on a head unit are better. Here's what an amp does: it takes its input and makes it larger so it can drive speakers. How much larger it can make the input signal is set by the input sensitivity and the maximum power output of the amp. You can turn the input sensitivity all the way up but that does not make the amp put out more power than its max, it just gets to that max level with a smaller input voltage. To see why 4 volt head units are better lets say we have 2 head units, model A puts out a 1 volt signal and model B puts out a 4 volt signal max. We're connecting these head units to a 25 watt amp. The amp puts out 10 volts.

Power = Voltage^2/Resistance = 10^2/4 = 25watts.

To get maximum output from head A, the gain needs to be 10 (10volts out per 1volt in, 10/1 = 10). Now let's say there's 0.1 volt of noise in the signal. With our gain set at 10 with our input sensitivity control we have amplified the noise to 1 volt. Consider what happens with head B. The gain needs to be only 2.5 to get full output. We still get 10 volts of output but the noise is only 0.25 volts. This noise level is 4 times lower than with head A. By using a higher voltage head unit you can set the gain on your amp lower and thus amplify less noise. Also lets say you left the input sensitivity set for a gain of 10 and you used 4 volt head unit at its max. If this did not make the input stage distort it would try to make the amp put out 40 volts (10*4) which would be 400watts! Obviously the amp can't do that and just hits its 25watt limit. To set your input sensitivity, turn you amp's input sensitivity almost all the way down. Now start with your head unit at its lowest volume and turn it up until you hear distortion and then back off some. Some head units will let you go to full volume without distorting the pre-amp level outputs. Now with your head unit putting out its max clean voltage, turn the input sensitivity up until you get to the loudest your system will play without distortion or the loudest you ever care to listen, whichever is lower. Now your amp is set to amplify the least amount necessary to produce full volume making it amplify noise the least.

 

Amplifier Power Specifications!

This page is intended to explain amplifier power specifications in more detail. I have a BS in Electrical Engineering so I do not know how much of this the average Joe is going to understand. I am also human so there may be mistakes below.  

Amplifier power ratings are important in determining whether an amp will satisfy your system's needs or not. It is necessary for the amp manufacturer to give out a power specification which clear and complete. Otherwise you are just guessing.  An example of a good power amp spec for a 4 channel amp is:

"50watts X 4 RMS all channels driven continuously into 4 ohms with less than 0.1%THD from 20Hz to 20kHz"

Every part of that spec is important and without any part of it the power rating is virutally meaningless. Many times amp manufacturers do not give this much information but you have to judge for yourself whether they are hiding anything. Head unit power ratings are notorious for being very misleading. Now I'll go into what  each part of the spec means and why each is important.

"50watts X 4 RMS all channels driven continuously into 4 ohms with less than 0.1%THD from 20Hz to 20kHz"

The "50watts" part is the one we notice first and everything else qualifies how that "50watts" was measured. Having enough power is what most people look for in an amp. However, other things come into play. If the you are going to run a load less than 4 ohms, then the current capability of the amp is definitely important and most specs do not give a current capability. A power rating into 2 ohms can help though. If the power doubles into 2 ohms then you know that the amp is built strongly enough that it can deliver enough current to drive a 2 ohm load. You may think that this is not important if you are not going to drive 2 ohm loads but it is important. Speakers (woofers, midranges, tweeters, etc) are not purely resistive. They have capacitive and inductive properties as well. Depending on the music and your setup, the impedance may dip well below 4 ohms for a nominally 4 ohm speaker.

Whether you amp can supply current fast enough to reproduce the music faithfully depends partially on the amp's slew rate (how fast its output can change), its damping factor (how easily it can control the speaker) and its current capability. For these reasons 2 ohm power is important even when driving 4 ohm speakers. Slew rates of 100V/microsec and damping factors above 100 (referenced with a 4 ohm load) are good but that information is usually not given out by the amp manufacturer. I hope it is clear now that the number of watts an amp can produce is only one factor in determining whether an amp is capable of the performance you desire.

On a final note on this part of the spec, most head units use IC (integrate circuits or chips) for the built-in amp's output stage. Those chips rarely can provide adequate current which is why even most novices know not drive subwoofers from a head unit. Real amps often have ICs in them as well but the output stages are almost always discrete, meaning they are built from transistors, resistors, capacitors and not integrated together inside tiny ICs.  Advances in IC technology always making them better though.

"50watts X 4 RMS all channels driven continuously into 4 ohms with less than 0.1%THD from 20Hz to 20kHz"

The "X 4" implies that the amp has 4 output channels. The "RMS" stands for "root mean square" and is a method of measuring an AC waveform. More importantly here it implies that the power rating is not just a peak rating but continuous. "all channels driven" means that the power measurement was made with all channels of the amp driven to their maximum level at the same time. This means that the power supply is strong enough to allow all 4 output channels to produce 50watts at the same time.

This is a common place where head unit specs "cheat." They leave off the "all channels driven" and measure only 1 channel at a time which often gives a higher number. I've seen head units claming "30x4" which is meaningless but most people take it to mean that the head unit produces 30watts each into 4 channels. That's 120 watts from a head unit. No amp is 100% efficient so let us say it draws 150 watts to do this (80% efficiency which is still high). With a 12V power input, the head unit amp's power supply would be drawing 12.5 amps. I guarantee you that it is not easy to design a power supply that fits into a head unit leaving enough room for everything else (including the amp stages themselves) for any reasonable price that can deliver that kind of power. That is one reason why I say not preferable to use the head unit's power.

"50watts X 4 RMS all channel driven continuously into 4 ohms with less than 0.1%THD from 20Hz to 20kHz"

"continuously" implies that the measurement was made using a continuous (probably sine wave) test signal and not just a quick burst. An amp capable of producing higher power for short amounts of time will have a higher power rating if they measure power with short bursts instead of a continuous input.

The argument can be made that continuous power is not as important because music by nature is dynamic and therefore the peak power is what we really should concentrate on. My response to this is that there is no standardized burst input which all amp manufacturers would use to measure "peak" power. In the end to make their power ratings look higher they would use extremely short pulses which would not represent the amp's performance with music. Because no standard currently exists for peak power we must rely on continuous power ratings for consistancy and to be able to compare amps with each other.

"50watts X 4 RMS all channel driven continuously into 4 ohms with less than 0.1%THD from 20Hz to 20kHz"

"into 4ohms" means that the power measurement was done using a dummy 4 ohm resistor as the load. This is not the same as a 4 ohm speaker but provides a standard which everyone uses to measure power. Sometimes (but not very often) amp manufacturers will measure power specs into 2 or 3 ohm loads and not say "into 4ohms" only to make the power rating look bigger than it actually is but this is rare. 4 ohms is what car audio amp manufacturers almost always give their power ratings for.

"50watts X 4 RMS all channel driven continuously into 4 ohms with less than 0.1%THD from 20Hz to 20kHz"

"with less than 0.1% THD" tells something about the distortion the amp is producing at this power level. Most amps have an intrinsic distortion that occurs at a near constant level for most of its power range and then when the amp starts to get overdriven the distortion rises quickly. THD stands for "total harmonic distortion" which is one way of measuring distortion that is standardized.

Often a power spec without the THD number was made with the amp driven until the THD reached 1% or more. This gives a higher power rating but you probably would not want to use the amp at that level because it would be distorted. This is a common ploy used when you see a 400watt amp for $50 at a flea market or discount store. This is often another way that head unit amp specs are inflated.  

"50watts X 4 RMS all channel driven continuously into 4 ohms with less than 0.1%THD from 20Hz to 20kHz"

"from 20Hz to 20kHz" tells us the frequency range into which this amp can produce its rated power. Some amps have power curves that fall off at low and high frequencies. Having this part of the spec present gives you reassurance that the amp can produce its power anywhere in the normal audio range. A power spec that says "into 1kHz" or leaves it off could be inflated. Many amps just put the frequency response as a separate datum on the spec sheet and not with the power rating. It should be with the power spec as well. A "+/- 1dB" or something similar should accompany the frequency response so you know how flat the frequency curve is.

That is it for amplifer power specs and be careful with incomplete specs. Even the best manufacturers put out incomplete specs and then it is up to you to figure out whether the amp is well designed or not but it should not be too difficult. You get what you pay for but look at the construction and "feel" of the amp as well to help make your decision. Also, keep in mind that these explanations are valid for home amplification equipment as well, although the FTC has more stringent requirements for power claims of home audio equipment.

 
Amplifiers
Your amplifier takes the signal from your head unit and makes it large enough to be able to drive your speakers. It is preferable to use separate amps for high and low frequencies but it is not necessary. The problem with using one amp for all frequencies is that you cannot adjust the levels among different frequency ranges as easily as you can adjust outputs of separate amps. Many people start their system with an amplifier for the low frequncies (bass) and use their head units built-in power to drive the higher frequency speakers. This is adequate but the built-in power in a head unit is usually not strong enough for high volume listening and not clean enough for the discerning ear. There are many options when choosing an amplifier.

I've also created a page for understanding power amplifier specifications here.

RMS Power:
The power output of an amplifier should be roughly matched to what the amp will be used for and what speakers it will be driving. Oddly enough, the most common problem with matching speakers and amps is using an amp that is too weak to power the speaker. When an underpowered amp is used to power a speaker, the listener tends to turn the volume up higher in order to get more output of the amplifier. Eventually the amplifier runs into its limit and begins to distort. This distortion can cause the output from the amplifier to become DC for short periods of time and DC signals of even low power can destroy a speaker. Underpowering a speaker in this way can be more dangerous than overpowering it! Also more power is usually necessary when powering subwoofers because of their large size and excursion. Do not plan on using an amp of less than 75watts per channel to drive a subwoofer. The converse holds true for higher frequencies (midrange and treble) only 25-50watts per channel are necessary to drive speakers in those frequency ranges, however more power will not hurt, it just probably will not be used. Another factor in power output is stability in low resistance loads. Sometimes you can wire mutiple subwoofers to a single channel on an amplifier but the amp will have to work harder to drive this kind of load. Many moderately priced amps can drive loads as low as 2 ohms or less, with 4 ohms being the typical load of a single speaker.

Power Supply Regulation:
The power supply in an amplifier converts the 12volt DC that is available in your car's electrical system to something the amp can use to produce more power. Several designs are employed by manufacturers today. Two classifications are regulated and unregulated. A regulated supply produces the same power regardless of whether your car's electrical system voltage sags (which a capacitor will help prevent). An amp using a stiffly regulated power supply will be able to supply full power even when the input voltage dips below 12volts. However, it will not gain any power if the input voltage goes above 12volts. An unregulated supply's power output depends directly on the input voltage. This causes changes in the maximum output power with changes in the car's electrical system. I recommend getting an amp with a regulated power supply so power output will be constant regardless of input voltage changes. This changes if you have a stiffening capacitor or another regulation device (Accumatch) to smooth out your car's electrical system. In this case, buy an amp with an unregulated supply. Some cheap amps use unregulated supplies to save money but provide none of the benefits of a typical unregulated supply. One way to determine whether an amp has a regulated supply or not is to view the power output specs for 12volt and 14.4volt inputs. If they are the same then the amp probably has a regulated supply otherwise it has an unregulated one.

Tri-Mode:
Some amps can play in what is called "tri-mode." In this mode, 2 channels are used to drive a pair of high frequency speakers and one subwoofer. The subwoofer receives power from both channels. This is a very efficient way to use an amp for more than one purpose. A special crossover is required to separate the two ranges of frequencies and it should have a way of adjusting the output level between the high frequency speakers and the subwoofer. This can be a nice way to save money on your system although it wastes a little bit of amplifier power because of the crossover and it can be more difficult to adjust the relative level between the high and low frequency outputs.

Other Specs:
THD (Total Harmonic Distortion) is a spec that often shows up with the power output spec. An example would be "45wattsx2 @ 0.01% THD" This spec says that at an output level of 45watts into each channel the THD will be no more than 0.01%. Sometimes manufacturers will quote the power spec at a THD of 1%. Be wary of this, 1% THD is poor and either implies that the amp is not very high quality or that the manufacturer is artificially inflating the power output spec by running the amp into a higher distortion region where it does produce more power but more distortion as well. Either way it is a sign of a poor amp or marketing that decieves. Anything less than 0.1% is negligible.

Built-in Crossovers:
These allow you to use the amp to only amplify certain frequencies and dedicate the amp to a subwoofer or some other specialized speaker. By using an amplifier's built-in crossover you eliminate the need for a separate one which can save you considerable money. There are sophisticated amps on the market today that combine multiple channels and built-in crossovers so that you can use them in place of multiple amps and a separate crossover. They are expensive but often cheaper than buying separate components.

Pre-amp Outputs:
Some amps have pre-amp outputs which allow you to "daisy-chain" multiple amps together without splitting the pre-amp output from your head unit. Also, if the amp has a built-in crossover, you can use it to drive another amp. For example if you have an amp you are going to use to drive a subwoofer with a built-in crossover at 90Hz, you can use its built-in crossover to set the amp to only amplify signals below 90Hz for the subwoofer and then have a pre-amp output that only has frequencies above 90Hz which you can connect to an amp that does not have a built-in crossover. That amp can then be used to power the high frequency drivers.

Input Sensitivities:
I have received a number of questions about input sensitivities and their importance especially as to why 4 volt outputs on a head unit are better. Here's what an amp does: it takes its input and makes it larger so it can drive speakers. How much larger it can make the input signal is set by the input sensitivity and the maximum power output of the amp. You can turn the input sensitivity all the way up but that does not make the amp put out more power than its max, it just gets to that max level with a smaller input voltage. To see why 4 volt head units are better lets say we have 2 head units, model A puts out a 1 volt signal and model B puts out a 4 volt signal max. We're connecting these head units to a 25 watt amp. The amp puts out 10 volts.

Power = Voltage^2/Resistance = 10^2/4 = 25watts.
To get maximum output from head A, the gain needs to be 10 (10volts out per 1volt in, 10/1 = 10). Now let's say there's 0.1 volt of noise in the signal. With our gain set at 10 with our input sensitivity control we have amplified the noise to 1 volt. Consider what happens with head B. The gain needs to be only 2.5 to get full output. We still get 10 volts of output but the noise is only 0.25 volts. This noise level is 4 times lower than with head A. By using a higher voltage head unit you can set the gain on your amp lower and thus amplify less noise. Also lets say you left the input sensitivity set for a gain of 10 and you used 4 volt head unit at its max. If this did not make the input stage distort it would try to make the amp put out 40 volts (10*4) which would be 400watts! Obviously the amp can't do that and just hits its 25watt limit. To set your input sensitivity, turn you amp's input sensitivity almost all the way down. Now start with your head unit at its lowest volume and turn it up until you hear distortion and then back off some. Some head units will let you go to full volume without distorting the pre-amp level outputs. Now with your head unit putting out its max clean voltage, turn the input sensitivity up until you get to the loudest your system will play without distortion or the loudest you ever care to listen, whichever is lower. Now your amp is set to amplify the least amount necessary to produce full volume making it amplify noise the least.  
Amplifier Power Ratings
Amplifier power ratings are important in determining whether an amp will satisfy your system's needs or not. It is necessary for the amp manufacturer to give out a power specification which clear and complete. Otherwise you are just guessing.  An example of a good power amp spec for a 4 channel amp is:

"50watts X 4 RMS all channels driven continuously into 4 ohms
with less than 0.1%THD from 20Hz to 20kHz
"

Every part of that spec is important and without any part of it the power rating is virutally meaningless. Many times amp manufacturers do not give this much information but you have to judge for yourself whether they are hiding anything. Head unit power ratings are notorious for being very misleading. Now I'll go into what each part of the spec means and why each is important.

The "50watts" part is the one we notice first and everything else qualifies how that "50watts" was measured. Having enough power is what most people look for in an amp. However, other things come into play. If the you are going to run a load less than 4 ohms, then the current capability of the amp is definitely important and most specs do not give a current capability. A power rating into 2 ohms can help though. If the power doubles into 2 ohms then you know that the amp is built strongly enough that it can deliver enough current to drive a 2 ohm load. You may think that this is not important if you are not going to drive 2 ohm loads but it is important. Speakers (woofers, midranges, tweeters, etc) are not purely resistive. They have capacitive and inductive properties as well. Depending on the music and your setup, the impedance may dip well below 4 ohms for a nominally 4 ohm speaker.

Whether you amp can supply current fast enough to reproduce the music faithfully depends partially on the amp's slew rate (how fast its output can change), its damping factor (how easily it can control the speaker) and its current capability. For these reasons 2 ohm power is important even when driving 4 ohm speakers. Slew rates of 100V/microsec and damping factors above 100 (referenced with a 4 ohm load) are good but that information is usually not given out by the amp manufacturer. I hope it is clear now that the number of watts an amp can produce is only one factor in determining whether an amp is capable of the performance you desire.

On a final note on this part of the spec, most head units use IC (integrate circuits or chips) for the built-in amp's output stage. Those chips rarely can provide adequate current which is why even most novices know not drive subwoofers from a head unit. Real amps often have ICs in them as well but the output stages are almost always discrete, meaning they are built from transistors, resistors, capacitors and not integrated together inside tiny ICs.  Advances in IC technology always making them better though.

The "X 4" implies that the amp has 4 output channels. The "RMS" stands for "root mean square" and is a method of measuring an AC waveform. More importantly here it implies that the power rating is not just a peak rating but continuous. "all channels driven" means that the power measurement was made with all channels of the amp driven to their maximum level at the same time. This means that the power supply is strong enough to allow all 4 output channels to produce 50watts at the same time.

This is a common place where head unit specs "cheat." They leave off the "all channels driven" and measure only 1 channel at a time which often gives a higher number. I've seen head units claming "30x4" which is meaningless but most people take it to mean that the head unit produces 30watts each into 4 channels. That's 120 watts from a head unit. No amp is 100% efficient so let us say it draws 150 watts to do this (80% efficiency which is still high). With a 12V power input, the head unit amp's power supply would be drawing 12.5 amps. I guarantee you that it is not easy to design a power supply that fits into a head unit leaving enough room for everything else (including the amp stages themselves) for any reasonable price that can deliver that kind of power. That is one reason why I say not preferable to use the head unit's power.

"continuously" implies that the measurement was made using a continuous (probably sine wave) test signal and not just a quick burst. An amp capable of producing higher power for short amounts of time will have a higher power rating if they measure power with short bursts instead of a continuous input.

The argument can be made that continuous power is not as important because music by nature is dynamic and therefore the peak power is what we really should concentrate on. My response to this is that there is no standardized burst input which all amp manufacturers would use to measure "peak" power. In the end to make their power ratings look higher they would use extremely short pulses which would not represent the amp's performance with music. Because no standard currently exists for peak power we must rely on continuous power ratings for consistancy and to be able to compare amps with each other.

"into 4ohms" means that the power measurement was done using a dummy 4 ohm resistor as the load. This is not the same as a 4 ohm speaker but provides a standard which everyone uses to measure power. Sometimes (but not very often) amp manufacturers will measure power specs into 2 or 3 ohm loads and not say "into 4ohms" only to make the power rating look bigger than it actually is but this is rare. 4 ohms is what car audio amp manufacturers almost always give their power ratings for.

"with less than 0.1% THD" tells something about the distortion the amp is producing at this power level. Most amps have an intrinsic distortion that occurs at a near constant level for most of its power range and then when the amp starts to get overdriven the distortion rises quickly. THD stands for "total harmonic distortion" which is one way of measuring distortion that is standardized.

Often a power spec without the THD number was made with the amp driven until the THD reached 1% or more. This gives a higher power rating but you probably would not want to use the amp at that level because it would be distorted. This is a common ploy used when you see a 400watt amp for $50 at a flea market or discount store. This is often another way that head unit amp specs are inflated.

"from 20Hz to 20kHz" tells us the frequency range into which this amp can produce its rated power. Some amps have power curves that fall off at low and high frequencies. Having this part of the spec present gives you reassurance that the amp can produce its power anywhere in the normal audio range. A power spec that says "into 1kHz" or leaves it off could be inflated. Many amps just put the frequency response as a separate datum on the spec sheet and not with the power rating. It should be with the power spec as well. A "+/- 1dB" or something similar should accompany the frequency response so you know how flat the frequency curve is.

That is it for amplifer power specs and be careful with incomplete specs. Even the best manufacturers put out incomplete specs and then it is up to you to figure out whether the amp is well designed or not but it should not be too difficult. You get what you pay for but look at the construction and "feel" of the amp as well to help make your decision. Also, keep in mind that these explanations are valid for home amplification equipment as well, although the FTC has more stringent requirements for power claims of home audio equipment.
 
Understanding Ohm's Law
Understanding what ohms are and how they relate to car audio is most helpful when trying to determine optimal wiring configurations for connecting multiple speakers to an amp. There are some basic electrical engineering formulas I will use on this page. Fortunately they are fairly simple.

Ohm's law: Current = Voltage / Resistance or I = V / R
You can rearrange this formula in a couple ways: V = I * R and R = V / I

Power equation: Power = Voltage^2 / Resistance or P = V^2 / R
Rearranging this formula gives us: R = V^2 / P and V = SQRT(P * R)

Using Ohm's law we can derive some more power equations: P = I^2 * R
Rearranging gives us: I = SQRT(P / R)

Abbreviations used on this page are:
I : current, measured in amps
V: voltage, measured in volts
R: resistance, measured in ohms
P: power, measured in watts

With that out of the way we can get down to business. As stated earlier, ohms are a measure of electrical resistance. It helps in this discussion if we consider amplifiers to be perfect voltage sources.

Consider this 25 watt amp:

P = V^2 / R

Most amps are rated into 4 ohm loads so we now have:

25 watts = V^2 / 4 ohms

Solving for voltage we get: V = SQRT(25 * 4) = 10 volts

So our 25 watt amp can be considered here to have the ability to produce up to 10 volts output.

Now let us consider what happens when we connect two speakers in parallel to the amp. At this point I am going to introduce some more formulas. We are considering speakers to just be simple 4 ohm resistors for this discussion. There are formulas which dictate what happens when you combine resistors in various ways. Before that we need to explain what is meant by parallel and series ConneXions.

Each channel of an amplifier has a positive (+) and negative (-) connection. The amp develops a voltage between these two terminals and this voltage is what drives the speakers. The equivalent resistance of what you connect to the amp is referred to as the load.

Normally when connecting a single speaker to a single amp channel you merely connect the "+" terminal of the speaker to the "+" terminal of the amp. Then do the same thing for the "-" terminals. Things get more complicated when you are connecting multiple speakers to a single amp channel. In a parallel configuration you connect both "+" terminals of the speakers to the "+" terminal of the amp. Then do the same thing for the "-" terminals.

A series connection is a little more complicated. First, you connect the "+" terminal of the amp to the "+" terminal of one of the speakers (let us call it speaker A). The next thing you do is connect the "-" terminal of speaker A to the "+" terminal of the other speaker (speaker B). Lastly, you connect the "-" terminal of speaker B to the "-" terminal of the amp. You can see in this connection that power from the amp goes through both speakers one after the other, hence the name "series."

Finally, here are the formulas that tell you what resistance load you end up with when wiring multiple speakers:

For two speakers in parallel:

1 / Rt = 1 / Ra  +  1 / Rb

where Rt is the total equivalent resistance or load and Ra and Rb are the resistances of the two speakers. You can see that you can use speakers of different resistances but there other implications of doing that which are usually undesirable because the power will not be spread evenly between the speakers.

Working through the math if you put two 4 ohm speakers in parallel you get:

1 / Rt = 1 / 4  +  1 / 4  =  1 / 2,  Rt = 2 ohms

The equivalent resistance is exactly half of what we started with. We will look at the implications of this a little later. Let us do another example first with three 4 ohm speakers in parallel:

1 / Rt = 1 / 4  + 1 / 4  +  1 / 4  =  3 / 4, Rt = 4 /3 = 1.33 ohms

You can see that as you put more and more speakers in parallel the equivalent resistance will drop further.

Next lets look at the equivalent resistance for speakers in series:

Rt = Ra + Rb

Now that is easy! You just add the resistances for each speaker so putting two 4 ohm speakers in series will you give a single load of 8 ohms.

With that background out of the way we can look at what effect these different wiring combinations have on the amplifier. Going back our 25 watt (10 volt) amplifier with a single 4 ohm speaker we have:

Current = Voltage / Resistance = 10 / 4 = 2.5 amps

So when this amp is producing maximum power (25 watts) into a 4 ohm load, the load will draw 2.5 amps from the amp.

Now let us look at what happens when we connect two 4 ohm speakers in parallel (which gives us a 2 ohm equivalent load) to this amp:

Power = Voltage^2 / Resistance = 10^2 / 2 = 50 watts

This is seems great! Our 25 watt amp is now producing 50 watts but there are some complications. Let us see the current now:

Current = Voltage / Resistance = 10 / 2 = 5 amps

Even though our voltage is still the same (10 volts) our current has now doubled from 2.5 amps to 5 amps.  If the amp has the capability to produce this much current and dissipate the heat that this will generate then everything will be fine.  One way to determine if your amp is capable of this is to look for power ratings that are given into 2 ohms in addition to the normal 4 ohm rating.  Further if the power doubles into the 2 ohm rating then the amp has ample current capacity.  Another clue to tell whether the amp will work with 2 ohm loads is look for the phrase "2 ohm stable."  Being 2 ohm stable only means that the amp will function with 2 ohm loads; it does not necessarily mean that the amp will produce more power into 2 ohms.  If you attempt to use a 2 ohm load with an amp that cannot handle it a well designed amp will shut itself off or blow a fuse and a poor one could be permanently damaged.

Next let us look at a series connection with two 4 ohm speakers. This will give us an 8 ohm load and we will use our 25 watt (10 volt) amplifier again:

P = V^2 / R = 10^2 / 8 = 12.5 watts

Our amp is now producing only half its power rating! And the current is:

I = V / R = 10 / 8 = 1.25 amps

The current is 1/2 its original value as well. Series ConneXions are not used as often as parallel ConneXions because they reduce power. However, they are easier for your amp to drive since they draw less current.

Another option that is often available is to bridge an amplifier. This process takes 2 amp channels and combines them to act as a single more powerful amp channel. How to do this to an amp and wire everything varies so please do not ask me how to bridge your amp. I can explain the effects of it though.

What typically happens when you bridge an amp is that the voltage it can produce doubles. Our 25 watt (10 volt) amp can now produce 20 volts. Let us look at how that affects power:

P = V^2 / R = 20^2 / 4 = 100 watts

This is 4 times the original power of the amp but let us look at the current situation:

I = V / R = 20 / 4 = 5 amps

So now we see even with a regular 4 ohm load the current is already double what the normal value (2.5 amps) was. For other reasons the power usually does not usually quadruple when you bridge an amp but will typically at least double. Connecting a 2 ohm load to a bridged amp raises the current requirement even more. However, if your amp can handle it then you will be squeezing a lot of power out of the amp.

In fact, many people use that kind of setup in competition where the classes are judged by the power rating of the amps in the system. A 25 watt amp can produce many times more power when bridged and driving low resistance loads so the competitor gets more power than what appears on the surface. Zapco and Phoenix Gold make amps which are able to drive such low resistance loads (sometimes as low as 0.5 ohm!)

Some final notes:
  • On this page I have considered amplifiers to be perfect voltage sources. They are not though and they have some internal resistance which lowers power output slightly.
  • I have also considered speakers to be perfect 4 ohm resistors. In actuality the resistance of the speaker depends on the frequency the speaker is playing. For example, a speaker may have a 3 ohm resistance at 80 Hz and a 9 ohm resistance at 300 Hz. If you were to make a plot of resistance versus frequency you would get what is called the impedance curve of the speaker. Also, speakers act in some ways as inductors and capacitors so a true model of a speaker must include those components as well. How does this affect the sound you ask? Well if you have an amp that has very weak current capability it may work fine into perfect 4 ohm loads but when you connect our real speaker which has an impedance dip at 80 Hz the amp may have difficulty and smear sounds that have 80 Hz components. These are minor but audible effects. This is why it is good to get a 2 ohm stable amp even if you never plan on running 2 ohm loads.
  • Placing speakers in parallel and bridging amp channels are effective methods for increasing the power in your system assuming your amp can handle the increased demand.
  • Use a series configuration when you need to raise the effective resistance of the load. This occurs more often when you are using dual voice coil speakers.
  • Dual voice speakers have two speaker ConneXions on them. This typically increases power handling capability and gives you more wiring options. For example, if you have two dual 4 ohm voice coil speakers you can get a single 4 ohm load which is suitable for connecting to a bridged amp. You would do this by connecting the voice coils on each speaker to each other in series. This would give you two 8 ohm speakers. Next you put those two 8 ohm speakers in parallel and this will give you a single 4 ohm equivalent load.
  • Be aware that using lower resistance loads and bridging produces a greater load on the amp. Well designed amps that cannot handle the demand will either shut themselves off or blow a fuse. A poorer designed amp can permanently damage itself. Also, even if an amp works in these configurations it will probably generate more heat so ventilation is even more important.
  • As mentioned earlier ohms are a measure of electrical resistance. You should be able to understand why resistance changes affect the amp power as it does. If you raise the resistance the amp is not able to drive as much current through the load and thus you get less power. If you lower the resistance of the load math says that more current will be drawn from the amp. Assuming the amp can handle this you get more power.
  • To understand why series and parallel configurations have the effect on resistance that they do consider this. When you connect speakers in series current must flow through both speakers and so it hits the resistance of both speakers. When you have speakers in parallel, the current has multiple paths since it can go through either speaker so the equivalent resistance is always lower than that of either speaker alone.
 

Ohm's Law

Voltage (volts) is the force that moves electrons, forcing a current.  Voltage can be compared to a tank of water elevated at a certain height (potential).   If the tank is placed low (low voltage), water will not flow very quickly (low current).  If the tank is raised to a higher location (higher voltage), the water will flow rapidly (high current).

Current (Amperes) is, in simple terms a measurement of how many electrons flow through a device.  In the water tank analogy, current would be water flow rate.

Resistance (Ohms) slows down current flow.  The higher the resistance of a circuit, the lower the current will be.  Resistance would be equivalent to pipe size.   If you have the water tank at a high level, but the pipe is very small in diameter (high resistance), not much water will flow.  If you use a big pipe (low resistance), then the water flow rate will be larger.

Ohm's Law (481 bytes)Knowing the relationships between voltage, current and resistance brings us to ohm's law:  "Current is proportional to Voltage divided by Resistance".  This equation can be manipulated to obtain any value knowing the other two.  For example, by measuring the voltage and current of a circuit, resistance can be calculated by dividing voltage by current.  When a circuit is open (disconnected), the resistance is infinite (zero current).  The formula also shows what happens when a circuit is shorted (resistance = 0): The battery will put out as much current as it can instantaneously (not a good sight).

DC vs. AC Circuits

On a DC circuit, current flows in one direction only.   Voltage can remain at a level or change, but it always has the same polarity.   A car's battery produces DC voltage.

AC circuits are a bit more complicated to understand.  The voltage supply reverses its polarity switching from positive to negative.  The current produced goes in one direction while the voltage is positive and then flows in the opposite direction when voltage is reversed.  AC circuits have a frequency associated with them.  The frequency (Hertz or Hz) is how many times per second (cycles) the current (and voltage) switch from positive to negative and back.  The higher the frequency, the faster the circuit will switch polarity.  AC voltage in the car is produced by the alternator, which is converted to DC voltage to charge the battery (even though some of the AC energy from the alternator remains in the electrical system, this is what causes alternator whine when the car is running).  The audio signal that comes from the head unit, gets amplified and drives the speakers is also an AC signal.

A Typical DC Circuit

Electrical Circuit (889 bytes)Electrons flow in a circuit from the negative side of the battery to the positive side of the battery (that is why physicists will argue with the direction of the current in the circuit).  Engineers represent current in the opposite direction of electron flow, as in the diagram.  It does not matter what convention you follow for current direction.  The important thing to keep in mind is how much current flows through the circuit, and that you stick to only one of the models when analyzing a circuit.
For a circuit to have current, there has to be a path (i. e. wire) and a battery.  A circuit also has a resistance, which slows down flow of electrons.
If the path is broken, current can not flow.  The battery supplies the voltage.  The top portion of the circuit in a car is represented by the positive battery cable going to the fuse box and to all the accessories (radio, wipers, lights, etc).  Each accessory has a resistance.  As more accessories are added, the resistance drops, and more current flows through the circuit.  To save money, car manufacturers use the car metal for the bottom part of the circuit, instead of running a ground wire to every device.

Common Engineering Notations

To represent very high or low values, zeros or decimal points are represented by letters.  These are the most common used in car audio:

Symbol:  Value: Used mainly For: Example:
  µ (micro)   millionth   Capacitors, which are measured in Farads   0.000001F = 1µF
  m (mili)   thousandth     Capacitors (F), inductors (Henries), voltage (V), current (A)     0.001Volts = 1mV
  k (kilo)   thousand   Resistance (Ohms), frequency (Hertz), power (Watts)   1000W = 1kW
  M (mega)     million   Frequency (Hz), resistance(Ohms)   1,000,000 Hz = 1MHz
 
Voltage Calculations

 This portion of this site is intended to help you calculate voltage levels. You can calculate any unknown value if you know the other two in the equation.

Explanation of Terms

E - stands for voltage, the unit is the Volt.
I - stands for current, the unit is the Amp.
P - stands for power, the unit is the Watt.
R - stands for resistance/impedance, the unit is the Ohm.

The Formulas

E = I x R
E = P / I
E =

Voltage

  Click the unknown measurement type (Current/Power/Resistance), then enter the two values that you do know, and press the "Calculate Power" button to get your answer. If you have more than one value to calculate, just change the value(s) you want to adjust and click the button again (if you change to a new measurement type - make sure you click the reset button).

Which number is unknown ?

Current  Power  Resistance 

Enter Current Amps
Enter Power Watts
Enter Resistance  Ohms

Voltage =  Volts

 

About Relays?

Operation
Relays do exactly the same thing a regular switch.   Instead of being moved by a finger, they are moved by an electromagnet that attracts a metal switch.  When the "switch" is inactive (no power through electromagnet) the common and normally closed contacts are tied together.

Inactive Relay Diagram
  

When the relay is activated (12 volts across electromagnet), the switch is pulled, disconnecting normally closed contact, and connecting the normally open contact with common terminal.  As soon a power is removed from the coil, the contacts go back to their original position.

Activated Relay Diagram

Do you need a relay?

Most head units have an output to turn on amplifiers, etc.  These outputs are designed to turn on a small number of devices, so they provide very little current.

On high-end systems, when many devices (amplifiers, crossovers, equalizers, processors, fans, etc) have to be connected to this turn-on wire the current output might not be enough.  If the circuit is overloaded, it can blow a fuse or even damage the head unit.

There is an easy way around this problem:  Add a relay.

How many devices are too many?  Depends on how much current each device draws and how much current the head unit provides.  Check the specifications section of the manuals to see.  Typically, pieces of equipment such as amplifiers, crossovers and equalizers draw very little current, since their turn-on switches are either solid state or small relays.  If you are hooking other devices that draw more current such as neon lights, fans, actuators, motors, etc, then you definitely need to add a relay.  An easy way to tell how much current devices are drawing is to check with a current meter.
  

Connections

Turn-on RelayThe diagram to the left shows the connections required to get the turn-on output.  The relay can be located either behind the radio, trunk, or elsewhere in the car.  Usually, it is easier behind the radio because wires going to the relay are shorter.

Terminal 87 goes to constant power (+12v).  It can be obtained from the same wire where the radio's memory backup is connected.

Terminal 86 goes to ground (negative wire going to the head unit or to a metal part that is connected to the chassis of the vehicle).

Terminal 85 is connected to the remote turn-on wire output at the head unit. Lastly, terminal 30 is run to all the components that need to be turned on.

 

About Diodes?

There are different types of diodes for different applications.   For vehicles, we are mostly interested in rectifier diodes.  Rectifier diodes, in simple terms, are "valves" that allow current to flow in one direction only.

Uses
Different circuits can be isolated to avoid undesired interactions.   Diodes are also used to protect circuits.  In car applications, diodes are used to isolate switches and sensors in security systems.  Diodes can also eliminate current transients in inductive components such as relays.

Purchasing the Correct Diode

Diode SymbolThe top image on the left is the symbol commonly used in circuit diagrams to represent diodes.
Most rectifier diodes are small black tubes with a white line at one end and look like the bottom image on the left.  The white line is commonly called the "negative" side (or cathode).  The other end of a diode is called the anode.

Diodes can be obtained at any electronic parts store such as Radio Shack.  Low current diodes (1 amp or less) are very cheap.  Several diodes can be purchased for a dollar.  Higher current diodes do cost more.

Current Ratings
For circuit isolation purposes (i.e. isolating switches or alarm sensors), 1 amp diodes would suffice. Other applications, such as isolating parking lights circuits in cars require diodes that can handle more current, say 3 amps.

By hooking up diodes in parallel, larger current capabilities can be obtained. For example, by connecting two 1 amp diodes in parallel, a total of 2 amps can be passed through the diodes.

Modes of Operation

Forward Bias Mode If you want a diode to conduct current, hook it up in the "forward bias" mode.

In the forward bias mode, the diode will behave as a short circuit (i.e. being replaced by a wire).

Reverse Bias Mode If the purpose of a diode in a circuit is to block current, then try the "reversed bias" operation.

In the reverse bias mode, it can be represented by an open circuit (i.e., cutting the wire in the circuit).

Voltage Considerations
Diodes do have shortcomings. A typical diode has a voltage drop of 0.7 volts. This is not critical for most car applications, but should be taken into consideration for other applications.

Exceeding voltage limits of diodes is not a concern in car applications because we deal with only 12 volts. For higher voltage circuits, the diode's maximum voltage ratings on forward and reverse bias should be observed.

 

ohms?

This page is intended to help people understand "ohms" in more detail. It also addresses parallel / series wiring configurations along with bridging amplifiers.  Dual voice speakers are mentioned in the notes as well.  I have a BS in Electrical Engineering so I do not know how much of this the average Joe is going to understand. I am also human so there may be mistakes below.

Understanding what ohms are and how they relate to car audio is most helpful when trying to determine optimal wiring configurations for connecting multiple speakers to an amp.  There are some basic electrical engineering formulas I will use on this page.  Fortunately they are fairly simple.  Here they are:

Ohm's law: Current = Voltage / Resistance   or    I = V / R
You can rearrange this formula in a couple ways:   V = I * R    and    R = V / I

Power equation:  Power = Voltage^2 / Resistance    or    P = V^2 / R
Rearranging this formula gives us:   R = V^2 / P    and    V = SQRT(P * R)
Using Ohm's law we can derive some more power equations:  P = I^2 * R
Rearranging gives us:    I = SQRT(P / R)

Abbreviations used on this page are:
I : current, measured in amps
V: voltage, measured in volts
R: resistance, measured in ohms
P: power, measured in watts

With that out of the way we can get down to business.  As stated earlier, ohms are a measure of electrical resistance.  It helps in this discussion if we consider amplifiers to be perfect voltage sources.  Consider this 25 watt amp:

P = V^2 / R

Most amps are rated into 4 ohm loads so we now have:

25 watts = V^2 / 4 ohms

Solving for voltage we get:  V = SQRT(25 * 4) = 10 volts

So our 25 watt amp can be considered here to have the ability to produce up to 10 volts output.

Now let us consider what happens when we connect two speakers in parallel to the amp.  At this point I am going to introduce some more formulas.  We are considering speakers to just be simple 4 ohm resistors for this discussion.  There are formulas which dictate what happens when you combine resistors in various ways.  Before that we need to explain what is meant by parallel and series connections.

Each channel of an amplifier has a positive (+) and negative (-) connection.  The amp develops a voltage between these two terminals and this voltage is what drives the speakers.  The equivalent resistance of what you connect to the amp is referred to as the load.

Normally when connecting a single speaker to a single amp channel you merely connect the "+" terminal of the speaker to the "+" terminal of the amp.  Then do the same thing for the "-" terminals.  Things get more complicated when you are connecting multiple speakers to a single amp channel.  In a parallel configuration you connect both "+" terminals of the speakers to the "+" terminal of the amp.  Then do the same thing for the "-" terminals.

A series connection is a little more complicated.  First, you connect the "+" terminal of the amp to the "+" terminal of one of the speakers (let us call it speaker A).  The next thing you do is connect the "-" terminal of speaker A to the "+" terminal of the other speaker (speaker B).  Lastly, you connect the "-" terminal of speaker B to the "-" terminal of the amp.  You can see in this connection that power from the amp goes through both speakers one after the other, hence the name "series."

Finally, here are the formulas that tell you what resistance load you end up with when wiring multiple speakers:

For two speakers in parallel:

1 / Rt = 1 / Ra  +  1 / Rb

where Rt is the total equivalent resistance or load and Ra and Rb are the resistances of the two speakers.  You can see that you can use speakers of different resistances but there other implications of doing that which are usually undesirable because the power will not be spread evenly between the speakers.

Working through the math if you put two 4 ohm speakers in parallel you get:

1 / Rt = 1 / 4  +  1 / 4  =  1 / 2,  Rt = 2 ohms

The equivalent resistance is exactly half of what we started with.  We will look at the implications of this a little later.  Let us do another example first with three 4 ohm speakers in parallel:

1 / Rt = 1 / 4  + 1 / 4  +  1 / 4  =  3 / 4, Rt = 4 /3 = 1.33 ohms

You can see that as you put more and more speakers in parallel the equivalent resistance will drop further.

Next lets look at the equivalent resistance for speakers in series:

Rt = Ra + Rb

Now that is easy!  You just add the resistances for each speaker so putting two 4 ohm speakers in series will you give a single load of 8 ohms.

With that background out of the way we can look at what effect these different wiring combinations have on the amplifier.  Going back our 25 watt (10 volt) amplifier with a single 4 ohm speaker we have:

Current = Voltage / Resistance = 10 / 4 = 2.5 amps

So when this amp is producing maximum power (25 watts) into a 4 ohm load, the load will draw 2.5 amps from the amp.

Now let us look at what happens when we connect two 4 ohm speakers in parallel (which gives us a 2 ohm equivalent load) to this amp:

Power = Voltage^2 / Resistance = 10^2 / 2 = 50 watts

This is seems great!  Our 25 watt amp is now producing 50 watts but there are some complications.  Let us see the current now:

Current = Voltage / Resistance = 10 / 2 = 5 amps

Even though our voltage is still the same (10 volts) our current has now doubled from 2.5 amps to 5 amps.  If the amp has the capability to produce this much current and dissipate the heat that this will generate then everything will be fine.  One way to determine if your amp is capable of this is to look for power ratings that are given into 2 ohms in addition to the normal 4 ohm rating.  Further if the power doubles into the 2 ohm rating then the amp has ample current capacity.  Another clue to tell whether the amp will work with 2 ohm loads is look for the phrase "2 ohm stable."  Being 2 ohm stable only means that the amp will function with 2 ohm loads; it does not necessarily mean that the amp will produce more power into 2 ohms.  If you attempt to use a 2 ohm load with an amp that cannot handle it a well designed amp will shut itself off or blow a fuse and a poor one could be permanently damaged.

Next let us look at a series connection with two 4 ohm speakers.  This will give us an 8 ohm load and we will use our 25 watt (10 volt) amplifier again:

P = V^2 / R = 10^2 / 8 = 12.5 watts

Our amp is now producing only half its power rating!  And the current is:

I = V / R = 10 / 8 = 1.25 amps

The current is 1/2 its original value as well.  Series connections are not used as often as parallel connections because they reduce power.  However, they are easier for your amp to drive since they draw less current.


Another option that is often available is to bridge an amplifier.  This process takes 2 amp channels and combines them to act as a single more powerful amp channel.  How to do this to an amp and wire everything varies so please do not ask me how to bridge your amp.  I can explain the effects of it though.

What typically happens when you bridge an amp is that the voltage it can produce doubles.  Our 25 watt (10 volt) amp can now produce 20 volts.  Let us look at how that affects power:

P = V^2 / R = 20^2 / 4 = 100 watts

This is 4 times the original power of the amp but let us look at the current situation:

I = V / R = 20 / 4 = 5 amps

So now we see even with a regular 4 ohm load the current is already double what the normal value (2.5 amps) was.  For other reasons the power usually does not usually quadruple when you bridge an amp but will typically at least double.  Connecting a 2 ohm load to a bridged amp raises the current requirement even more.  However, if your amp can handle it then you will be squeezing a lot of power out of the amp.

In fact, many people use that kind of setup in competition where the classes are judged by the power rating of the amps in the system.  A 25 watt amp can produce many times more power when bridged and driving low resistance loads so the competitor gets more power than what appears on the surface.  Zapco and Phoenix Gold make amps which are able to drive such low resistance loads (sometimes as low as 0.5 ohm!)

Some final notes:

  • On this page I have considered amplifiers to be perfect voltage sources.  They are not though and they have some internal resistance which lowers power output slightly.

  • I have also considered speakers to be perfect 4 ohm resistors.  In actuality the resistance of the speaker depends on the frequency the speaker is playing.  For example, a speaker may have a 3 ohm resistance at 80 Hz and a 9 ohm resistance at 300 Hz.  If you were to make a plot of resistance versus frequency you would get what is called the impedance curve of the speaker.  Also, speakers act in some ways as inductors and capacitors so a true model of a speaker must include those components as well.  How does this affect the sound you ask?  Well if you have an amp that has very weak current capability it may work fine into perfect 4 ohm loads but when you connect our real speaker which has an impedance dip at 80 Hz the amp may have difficulty and smear sounds that have 80 Hz components.  These are minor but audible effects.  This is why it is good to get a 2 ohm stable amp even if you never plan on running 2 ohm loads.

  • Placing speakers in parallel and bridging amp channels are effective methods for increasing the power in your system assuming your amp can handle the increased demand.

  • Use a series configuration when you need to raise the effective resistance of the load.  This occurs more often when you are using dual voice coil speakers.

  • Dual voice speakers have two speaker connections on them.  This typically increases power handling capability and gives you more wiring options.  For example, if you have two dual 4 ohm voice coil speakers you can get a single 4 ohm load which is suitable for connecting to a bridged amp.  You would do this by connecting the voice coils on each speaker to each other in series.  This would give you two 8 ohm speakers.  Next you put those two 8 ohm speakers in parallel and this will give you a single 4 ohm equivalent load.

  • Be aware that using lower resistance loads and bridging produces a greater load on the amp.  Well designed amps that cannot handle the demand will either shut themselves off or blow a fuse.  A poorer designed amp can permanently damage itself.  Also, even if an amp works in these configurations it will probably generate more heat so ventilation is even more important.

  • As mentioned earlier ohms are a measure of electrical resistance.  You should be able to understand why  resistance changes affect the amp power as it does.  If you raise the resistance the amp is not able to drive as much current through the load and thus you get less power.  If you lower the resistance of the load math says that more current will be drawn from the amp.  Assuming the amp can handle this you get more power.

  • To understand why series and parallel configurations have the effect on resistance that they do consider this.  When you connect speakers in series current must flow through both speakers and so it hits the resistance of both speakers.  When you have speakers in parallel, the current has multiple paths since it can go through either speaker so the equivalent resistance is always lower than that of either speaker alone.

 
Electrical Current Calculations

 This portion of this site is intended to help you calculate current levels in a circuit. You can calculate any unknown value if you know the other two in the equation.

Explanation of Terms

E - stands for voltage, the unit is the Volt.
I - stands for current, the unit is the Amp.
P - stands for power, the unit is the Watt.
R - stands for resistance/impedance, the unit is the Ohm.

The Formulas

I = E  / R
I = P  / E
I =

Current

  Click the unknown measurement type (Voltage/Power/Resistance), then enter the two values that you do know, and press the "Calculate Power" button to get your answer. If you have more than one value to calculate, just change the value(s) you want to adjust and click the button again (if you change to a new measurement type - make sure you click the "Reset" button first).

Which number is unknown ?

Voltage  Power  Resistance 

Enter Voltage Volts
Enter Power Watts
Enter Resistance   Ohms

Current =  Amps

 
Impedance/Resistance Calculations

 This portion of this site is intended to help you calculate impedance/resistance levels. You can calculate any unknown value if you know the other two in the equation.

Explanation of Terms

E - stands for voltage, the unit is the Volt.
I - stands for current, the unit is the Amp.
P - stands for power, the unit is the Watt.
R - stands for resistance/impedance, the unit is the Ohm.

The Formulas

R = E / I
R = P / I2
R = E2 / P

Impedance/Resistance

  Click the unknown measurement type (Voltage/Current/Power), then enter the two values that you do know, and press the "Calculate Power" button to get your answer. If you have more than one value to calculate, just change the value(s) you want to adjust and click the button again (if you change to a new measurement type - be sure to click the reset button or the values shown will be incorrect).

Which number is unknown ?

Voltage  Current  Power 

Enter Voltage Volts
Enter Current Amps
Enter Power Watts

Resistance/Impedance =  Ohms

 
Ohm's Law Calculations
This portion of this site is intended to help you with electrical calculations required when working on electronics or mobile audio installations.

 

Power (Watts) Current (Amps)
Power Calculations Current Calculations
Voltage Calculations Resistance Calculations
Voltage (Volts) Resistance (Ohms)
 
Power Calculations

 This portion of this site is intended to help you calculate power levels. You can calculate any unknown value if you know the other two in the equation.

Explanation of Terms

E - stands for voltage, the unit is the Volt.
I - stands for current, the unit is the Amp.
P - stands for power, the unit is the Watt.
R - stands for resistance/impedance, the unit is the Ohm.

The Formulas

P = E x I
P = I2 x R
P = E2 / R

Power

  Click the unknown measurement type (Voltage/Current/Resistance), then enter the two values that you do know, and press the "Calculate Power" button to get your answer. If you have more than one value to calculate, just change the value(s) you want to adjust and click the button again (if you change to a new measurement type - make sure you reset the form or the values will be incorrect).

Which number is unknown ?

Voltage  Current  Resistance 

Enter Voltage Volts
Enter Current Amps
Enter Resistance  Ohms

Power =  Watts

 

Planning Your System

This part of the game can be fun or disappointing depending on what you can do. First I'm going to give you an example system that I believe includes everything you need to have a pretty good system. After that I will show you how to make compromises and leave out parts that may not be as important to you to keep your system within your budget. If you want to go beyond my basic system you probably already know more than what this site can tell you. Also, you do not have to get everything at once. I put my system together over a few years. With a little planning you can upgrade your system in steps and that way its like getting a new system every time you change something instead of getting everything at once!

Basic System: This is my opinion only but I think that a good system should start off with a good head unit that either has a CD player and/or is connected to a CD changer. A good system sounds best when playing CDs, tapes just do not cut it. Next I think component sets are made with fewer compromises than coaxial speakers so I suggest getting a good midrange/tweeter set for the front. Head units generally do not put out enough clean power so you will want an amp to drive the component set. In the rear where you only need some "fill" for ambiance you can get away with cheaper coaxials and set their level lower than the fronts to keep the sound stage in front. A modest (50x4) 4 channel amp is a good choice here for powering the component set up front and the rear speakers. You could use a good 2 channel amp and run the front and back in parallel on the amp but it would harder to adjust the level between them. Midranges sound best when they do not play bass so you will want a 2 way electronic crossover and use the high pass output to drive your 4 channel amp. I did not forget the bass! Most people are happy with a single 10" woofer or a pair of 12"s. Use an appropriate enclosure and a big amp (at least 75x2, preferably even more). Throw in installation and wiring accessories (like fuses and distribution blocks). Here's an approximate price break down of what this costs in my area. Your prices may be significantly different.

    head unit: $300-$500

    4 channel amp for highs: $250-$400

    2 channel amp for lows: $300-$700

    sub(s) (1-10" to 2-12"): $150-$450

    enclosure for sub(s): $0 (free air) - $250 (custom)

    component set: $200-$500

    coaxials for rear fill: $100-$400

    crossover/equalizer: $100-$500

    wiring and accessories: $50-$250

    installation: $0 (do it yourself) - $100 (basic)

This comes out to $1450-$3950! I realize that this is a lot of money and that most people do not spend nearly this much money on their car stereo. However, the things listed above are what I feel is necessary to have a system with only a few compromises. If you are less concerned about highs, get coaxials in front instead of the component set and power them off of the head unit and use some bass blockers on them. This will save you about $400. Getting a bargain head unit can save you some money as well. If you are really not into bass much you can forgo all the bass related equipment and run your component set full range. This will still give you clean sound but not much bass. However, you will save $550-$2300. I would start with what I have listed above and take out parts you do not care about as much. Only you know what kind of system you can be happy with.

Please do not email me asking for recommendations about specific brands. There is a lot of equipment out there that I have not used so I will not comment on them. I am happy with the components that I have but that is as far as I can go with recommendations. When buying equipment try to spend time listening to it before you buy, especially with speakers. Also try to use equipment that is similar to yours when listening in a store. As for amps, it costs money to build a good amp so if you see some awesome price on an amp you have never heard of, it is probably a piece of junk. Stick with good names with amps.

Finally, if you are on a budget (aren't we all?) it works better to upgrade in steps.  The most important thing is to have a car audio system that sounds good to you not someone else.  If you are happy with just changing the factory speakers and stopping there then just do that.  There is a level when that new amp or speaker is not going to make a difference so it is not necessary to always upgrade.  There are people who think my system is terrible but it works well enough for me and anything else I do to it would be a minor gain and not worth my trouble.  Do not let a salesperson talk you into something you do not need!  Good luck!

 

Depending on your budget and personal tastes, there are many different system configurations to suit your needs.  Here's some examples:

System 1: Basic

A lot of times less is better.  Less components means lower system cost and the ability to spend a few extra bucks on the components that really count.

When it comes to sound quality, less speakers is definitely better.   The more speakers you have, the more harmful interactions and cancellation of sound waves will occur.
Start with a good set of components (two tweeters and mids) that can go down to 60 Hz with no problem in a properly designed enclosure (i.e. custom kick panel pods).  Rear speakers are not essential in most cases.

Get a good head unit with a clean signal. If most of your music is in CD format, it is better to spend your money in a good single CD player than a cheaper set of tape player/CD changer combo.

Unhappy with the front speakers sound level?  Not to worry, get a good quality amplifier.  100 watts per channel should be plenty.  Don't be too concerned about the power rating on your speakers, unless you drive speakers with ridiculously high power levels.  As long as you have good speakers and protected by a crossover with a steep slope (i.e. 24 dB/Octave) on the lower end, you should be fine.

The system should sound pretty darn good by now. If not, fix any system design/installation flaws.

Now for the last part: Bass.  You need two things: subwoofer(s) and an amplifier.  For audiophile quality sound two 10" subs will satisfy most people.  The trick is a properly designed enclosure and lots of power.  200 watts or more per sub should add plenty of punch for the bottom end of your system.

Subwoofers need more power than speakers because they are bigger and have to move more air.  If you have limited finances, go with a mid-end (i.e. Sony, Pioneer, Kenwood) amplifier for the front speakers and a better high-current amplifier for the subs.

If everything is installed properly and tweaked carefully, you will be very happy with the results.
  

System 2: Competition Level

Competition level systems require deeper pockets and more components.  System presentation also becomes a big issue.

The guidelines of the previous system apply, but you need more gear.

Head units and amplifiers need to have impeccable sound quality and no noise (background noise or alternator noise) whatsoever.  Most people prefer to take advantage of head units capable of high line level voltages (3 to 8 volts) for minimum background noise.

Equalizers become a necessity to fine tune system's response.   Most competitors prefer to use a mono 30 or 31 band equalizer per channel.   Many people have multiple sets of equalizers to quickly change the system's response from sound quality to high SPL or RTA judging.

If the audio system is to be played when the car is not running for extended periods of time, extra batteries should be added to the car.  You might need to add a high power alternator if the car's electrical system can't handle the extra loads.

All the components should be very neatly installed.  Every detail of the installation must be meticulously executed.  Wiring and connectors should be neat and clean.  The car should also be treated against rattles and road noise.
  

System 3: SPL

SPL systems are very different.  The idea here is to be as loud as possible, especially in the lower frequencies.  For a system to be loud, it needs three things:  Lots of power, a lot of speakers and a closed place where all that sound can be concentrated.  This costs, as you can imagine, plenty of money.

The first upgrade here is the vehicle's electrical system.   Alternators, capacitors and batteries become essential.

Most people into SPL competition don't consider staging and imaging as important.  Multiple speakers are placed up front, wired in series/parallel combinations to maximize amplifier power output.

A very important aspect of an SPL vehicle are the subwoofers.   Subs need to have a big cone area and high excursion (Xmax) to be able to move as much air as possible.  The amplifiers moving the subs must be able to handle high current demands and to have low impedance capability.

SPL vehicles should also be treated to be as stiff as possible to minimize loses.  Flexing body parts take a toll on output.  Serious SPL competitors replace glass with thick Plexiglas, and reinforce the whole inside of the vehicle with steel, concrete, etc.  To minimize air leaks many competitors use bolts to hold doors in place, keeping door seals tightly closed.

 

How Do Speakers Work?

Moving Speaker Speakers are air pistons that move back (on the negative cycle of the signal) and forth (on the positive cycle), creating different degrees of air pressure at different frequencies. The amplifier (either separate or built-in your radio), produces electrical impulses that alternate from positive and negative voltages (AC).  This current reaches the voice coil inside the speaker, creating an electro-magnet that will either be repelled, or attracted by the fixed magnet at the bottom of the speaker.  The voice coil is attached to the cone, moving it back and forth, creating sound.  The surround (rubbery circle that joins top of the cone and metal basket) and the spider (usually yellow corrugated circle joining bottom of cone to magnet) make the cone return to its original position.

Speaker Sensitivity, measured in dB, is how loud a speaker plays (usually 1 Watt, 1 meter).  A higher Sensitivity rating means that the speaker will play louder using the same power as a speaker with a lower rating.

The back and front parts of the speaker should be isolated from each other.  When the front of the cone is pushing air, the bottom is pulling air, creating a canceling effect.  Ideally every speaker should be in an enclosure.   If you are mounting a speaker in a big hole, make sure you build a panel to isolate the front and back of the speaker (baffle).
  

Imaging, Staging and Directivity

Imaging - is being able to pick certain sounds from different places.  The singer would normally be located towards the middle of the car, guitars, trumpets, and other instruments towards the sides of the car.  If you scatter speakers all around the car your imaging would be very poor, since you would be producing the same sound at different places.  If you have a system with good imaging, the sound should seem to come from different instruments and voices, not speakers.

Staging - is the ability of a system to "fool you" into thinking that everything (including bass) is in front of you.  The sound should be similar to a stage in a concert, where the singer would be in the front center, and the rest of the instruments and background vocalists would be located to the left and right (but always on the front).

Good staging and imaging are not so easy to implement. It takes a lot experimenting with speaker location and direction.

Directivity - of sound is related to frequency.   At higher frequencies it is easier to pinpoint where the sound is coming from than lower frequencies.  This can be used to our advantage in car stereo.  Tweeters are the most important part of getting good staging.  They should be aimed towards the middle of the car.  A way to "bring" the bass to the front of the car is to fool our ears by overlapping frequencies played by midbases and subs, so that your midbases actually "pull" the bass to the front, since lower bass in not too directional.  You should crossover your midbases as low as you can (without getting distortion).  Then cut your subs at a bit higher frequency (preferably 60 HZ or less).  This will mix the bass coming from the front and rear, making the bass seem to come from the front.  Adding a center channel also improves staging, if it is set up correctly.
  

Types of Speakers

Coaxials - Coaxial speakers (or three-ways) are two (or more) speakers built-in the same frame.  They are cheaper than separate woofer and tweeters and also easier to install.  There is no need to worry about crossovers, since they are already built-in (you might still need to add a crossover to block bass if you are using high-power amplifiers).  A disadvantage of coaxials is the lack of flexibility.  For example, if the coaxial is all the way in the kick panel, or door panel aiming at your feet, you will not have good staging or imaging.  Some manufacturers try to compensate for this by making adjustable tweeters.  You should usually consider coaxial speakers for the back of the car, and separates for the front, unless you only have one speaker hole and don't plan to cut any more holes in the car.

Separates - Separates consist of a tweeter and woofer, and [most of the time] come with an external crossover.  The woofer is usually mounted in the factory hole in the door or kick panel.  The tweeters can be mounted in different places. The most common place to install tweeters is towards the top front corner of the door panel, aiming (if possible) between both front seat head rests.   Another popular location for tweeters is in the dash, either surface mounted, or in factory dash holes.  Yet another location where tweeters are commonly mounted is in the blank plastic piece on the top front side of the doors (where the mirror is on the outside).  You would have to experiment with angle and location to achieve the best possible imaging and staging.

Horns - Horns are very good at directing sound and have high efficiencies.  Horns are usually mounted under the dash.  By doing this, difference in distance from left and right speakers are greatly reduced over conventional mounting locations.  Since horns play mids and highs, tweeters are not needed.  Horns cost more than conventional speakers and require customization.  In many installations a good equalizer is required to compensate for their high sensitivity.
Horns are not for everyone though.  Many audiophiles complain of unnatural sound.   It is very hard to properly setup a set of horns.

Midbases - Midbases are usually 5, 6 or 8 inch speakers that are designed to go lower in frequency and are part of a three way system with a mid and tweeter. The problem is that 3-way arrangements require more complicated crossovers.  Midbases are most commonly mounted in the doors.

Subwoofers - Subwoofers add lower frequencies to the system.  They have to be enclosed in a box, with the exception of free air subwoofers, which use the trunk as an enclosure.  There are many different types of boxes and implementations discussed in the "subwoofers" section.
  

Mounting Locations

Front Speakers - The best place to mount speakers in the front, in custom kick panels.  By doing this, the path between the speakers and ears is minimized giving the best possible sound without having to add time delay circuitry.  If this is not possible, try to point the speakers towards the center of the car, and try to minimize the distance between the right and left speakers to your ears.  Custom kick panels are usually built from fiberglass or molded plastic, and are available from some manufacturers such as Ai Research.

Rear Speakers - Rear speakers should give a sense of space to the music, but not overpower the front speakers.  You should be able to barely hear the rear speakers.  If you are using rear speakers to add more bass/midbass to the system, at least use a crossover to cut off higher frequencies.  A lot of hi-end systems don't even have any rear speakers.  Tweeters are not necessary for the rear, a set of coaxials will work good for rear fill.

Center Channels - Center channels consist of a midrange speaker (3 or 4 inch) mounted in the middle of the dash (usually) on the top.   Center channels play a mono (Left + Right) signal between 350 - 500 and 3500 Hertz (voice range). The purpose of the center channel is to raise the sound stage, by creating the sensation of the singers "being" in the front of the car, and not in the door panels.  Center channels are hard to implement: First, a bandpass crossover is needed. Left and right channels have to be summed up.  There are various commercially available center-channel processors (many with built-in amplification).  The volume level of the center channel should be lower than the other speakers, since it is only supposed to make subtle changes to the total sound image.
  

Sizes and Shapes

There are many speaker sizes ranging from 1-inch tweeters to 18-inch (or bigger) subwoofers.  A smaller speaker will reproduce higher frequencies better than a bigger one.  The wavelength of a 20,000 Hz signal is very small, while the length of a lower (bass) note moving in the air could be as big as 40 feet.  That explains why a 4-inch speaker can't really put out bass (the lower the frequency, the more air mass that has to be moved by the speaker).  Tweeters are designed to play frequencies from 3500, 4500 or even 6000 Hz, all the way up to 20,000 Hertz.   Midranges (3, 4 or 5 inchers) play music from around 300, 500 Hz, to where the tweeters start in the upper level.  Midbases (5, 6, 8 inches) play from around 50 Hz to 500 (and even 1000) Hz.  Subs handle frequencies below 120-60 Hertz.

Do round speakers sound better than oval-shaped speakers (i.e. 6x9's)?  The answer is yes for most practical purposes. A round cone is more rigid than an oval-shaped one, so at higher levels, an oval-shaped speaker will distort more.   The reason why there are oval-shaped speakers is because of rear deck space considerations by manufacturers.  An advantage of a 6x9 speaker over a 6-inch speaker is that it has a bigger area, so it will move higher air volume, producing more bass.
  

Power Considerations

Most people think that if they use a 50 watt per channel amplifier on their factory speakers, the speakers will be damaged.  This may be true if the speakers do not have crossovers blocking off frequencies speakers were not designed to play.  What destroys speakers is distortion.  If you turn the volume all the way up on the radio, there will be distortion.  If you start hearing distortion, turn the volume down.  A high power amplifier allows the volume in the system to be higher, while the volume control on the radio is down in the range where no distortion is present.   It is better to have more power than what you need to get cleaner sound.

So how much power do you really need? As much as you can afford.   At a minimum, 30 to 50 Watts (each) would be OK for your front and rear speakers, while a little bit more (100-150 Watts) should be applied to each sub.  If you are powering up your tweeters independently, they require less power (20 - 40 Watts). Example: A four-channel set-up with separates in the front and coaxials in the rear with two subs will need about 40 Watts on each channel (Total=160W), and 100W going into each sub (Total=200W).  Notice that total power going to subs is more than total power going to the rest of the speakers.  This is because our ears are less sensitive to bass.

 

Subwoofer Boxes

A box ranges in complexity from the "plain vanilla box" (sealed) to bandpass and even more exotic enclosures.  Each enclosure has advantages and disadvantages and should be designed accordingly to the individual speaker parameters (the "one size fits all" rule DOES NOT apply to subwoofers and boxes).

Subwoofers need more amplifier power than everything else in the system.  This is because human ears are less sensitive at lower frequencies, so a higher bass level is needed for everything to sound even.  A low-pass crossover is required to block off high frequencies.

What type of subwoofer is better?  A bigger subwoofer gives more bass, but needs a bigger box.  Since most people like to have a trunk, 10 and 12-inch woofers are most common.  When buying a subwoofer always keep in mind that bigger size is not necessarily better.  A good quality 8-inch sub will outperform a cheap 12-incher.  Big subs (12", 15") have slower responses, yielding to boomier bass.  Small subs (8", 10") have a tight and more controlled sound.
  

Types of boxes

Free Air - subwoofers are either mounted under the rear deck or behind the rear seat of a car. This configuration will not work very well for hatchbacks. Holes have to be cut where the woofers are to be mounted. Since the woofers use the whole trunk as a box, the trunk has to be as sealed as possible from the cabin. Trunk can be isolated usually by putting particle board under the deck and behind the seat.
The drawback of free air subwoofers is that bass will not be very accurate (especially at lower frequencies), and more amplifier power will be required than with a regular box, but then again, you still have a full trunk.
Free Air Config.

 
Sealed - is the most common box and easiest to build. These boxes will give the flattest frequency response, and best overall sound quality (especially at lower frequencies). The box internal volume should be as close as possible to the recommended by the manufacturer. If a box is smaller than what it is supposed to be, the sound will be tighter, but more amplifier power will be required. If the box is too big, then the sound will be muddy.
Sealed Box

 
Ported - boxes are usually bigger in size than sealed and have a "tube" (port) that lets some air out of the box. The idea of a ported box is that the speaker port pushes (or pulls) air at the same time as the woofer, reinforcing bass. The box itself acts as an amplifier, yielding to more bass than a sealed enclosure (3 to 4 dB). Ported boxes do not have a linear frequency response.  If the box is not built according to specifications, it will not sound good. The box design acts as a filter, cutting off lower frequencies.
Ported Box

 
Isobaric - configuration is a good way to get bass in a smaller box. This is done by building a box about half the volume of a sealed box, and placing two woofers facing each other. Note that everything must be sealed, including space between woofers. A spacer between both woofers must be used in most cases to avoid subs hitting each other. When wiring, make sure that woofers are out of phase: Wire one of them backwards (negative to positive, and positive to negative), so that both pull or push at the same time. An isobaric configuration will NOT put out much more power than a box using a single woofer.  Its main purpose is to reduce box size.  Another drawback is that since one of the subs is exposed, it is more prone to damage.
Isobaric Loading

 
Band Pass -  enclosures consist of a woofer between a sealed and ported box.  Bandpass boxes  will yield more bass than sealed and ported boxes (especially at lower frequencies), but over a narrower frequency range. Since the box acts as a filter, mechanically blocking lower and upper frequencies, a crossover is not needed in most cases.  These enclosures are usually big, and very unforgiving when precise volumes and port sizes are not followed.  Bandpass boxes also tend to mask distortion.  If you can't hear distortion and turn your stereo down in time, you could damage your subs.
Bandpass Box

 
Aperiodic - Very small boxes that "breathe" through a moving membrane.  Both the membrane and cone can not be in the same exterior space.  Either the membrane part has to be isolated by cutting a hole in the car so that it is outside, or the subwoofer has to be isolated from the rest of the trunk in a similar fashion to free air woofers.  The "box" has to be as small as possible (ideally the membrane should be right up against the sub), since it is used only for coupling the sub and membrane.  Aperiodic membrane configurations are very hard to design and tune, but give good frequency response and respond faster to transients, giving accurate and tight bass as opposed to boomy sound.  They are not ruled by Thiele-Small parameters like other designs, so any woofer would work with the membrane.
  
Aperiodic Membrane

Amplified Bass Boxes

A good choice for small cars and (ideal) for hatchbacks and pickup trucks.  They usually take up very little room, putting out to fairly good bass. The most known manufacturer is Bazooka® for it's Bass Tubes®.  Their design is a ported box. The woofer has to be close to a wall or, better yet, to a corner.   To fine-tune, the bass tube is moved either closer, or farther form the wall or corner.

It is convenient to get an amplified tube, since amplifier, crossover and subwoofer are all integrated in a small package. If you buy the components separately, you will end up spending more money. Another good feature of tubes is the fact that they can be easily and quickly installed and removed.

If you decide to get one, keep in mind that even though they all look the same, cheaper brands will not sound good. A decent tube will run in the $300's (amplified), and in the $100's for a non-amplified.
   

Custom Bass Boxes

Many manufacturers such as JL and MTX are making custom boxes (with subs included) to fit in center consoles, under seats, or in other small spaces.  Although these boxes do cost a lot of money, most give superb performance and integrate easily in a car without taking up too much room.

 
What is a Crossover?

Crossovers are an essential part of an audio system, often ignored.   A crossover splits frequencies so that each speaker receives a certain range of frequencies.  This is done for two reasons:

  1. Avoid speaker damage: Speakers are designed to play only a certain range of frequencies. If other frequencies are played, then the speaker will produce distortion, which eventually will destroy it.
  2. Overall balance: If a system with subwoofers, and full range speakers doesn't have a crossover, then the subs will be playing, for example, from 20 to 1000 Hz, while the full-range speakers will be playing from 60 Hz, all the way up to 20,000 Hz. As it can easily be seen, there is an "overlap" of frequencies between 50 and 1000 Hz. In this overlap region, the levels are higher than levels below 50 and above 1000Hz, yielding to a non-balanced system.

There are 3 types of crossovers:  High-pass, low-pass and band-pass.  As it can be deducted from the names, a high-pass crossover will block low frequencies, a low-pass will block high frequencies, and band-pass will block low and high frequencies below and above crossover points.

From the description above, crossover operation sounds very simple, but it is a bit more complicated.  Crossovers do not block undesired frequencies completely (unless you are using digital crossovers).  Crossovers cut frequencies progressively.  A crossover "slope" describes how effective a crossover is in blocking frequencies.  The minimum slope is 6dB/octave.  For example, a high pass crossover at 1000 Hz, will let anything above 1000Hz pass.  The farther lower frequencies are from 1000Hz the lower levels will be.  At 500 Hz (1 octave), the level at the speaker would be 6dB less.  A steeper slope (i.e. 24dB/octave) will block undesired frequencies more effectively, but will cost more than a lower slope crossover.

If a speaker will be played near it's frequency range limit, then you need a high slope.  For example, a midbass rated at 50Hz on the lower range could be crossed over at 55 or 60HZ with a 24dB/Oct crossover.  If you want to use a lower slope crossover, then the frequency would need to be higher (i.e. 100 Hz at 6dB/Oct).

So what are good crossover frequencies? It largely depends on the car, speakers, and speaker location. Typical crossover frequencies are 100Hz (bass), 350Hz (midbass), 3500 - 5000Hz (highs). For more details of what frequencies to choose, see the speakers section.
  

Active Crossovers

Active crossovers (and equalizers) need external power to operate and work at low signal voltage levels (RCAs). Signal from the head unit's RCA's is split it into low-frequencies (bass), mid-frequencies (mids), and high frequencies (tweeters), to go to different amplifiers.

What are the advantages of active crossovers? Signal is not affected as much as with passive crossovers, since everything is done at low voltages. There is much more flexibility, since all that is needed to adjust crossover frequencies is to turn a knob, while on passive crossovers, the components have to be replaced. The problem is that more amplifier channels are needed to go to all the speakers.
   

Passive Crossovers

Passive crossovers work after the amplifiers, receiving high signal levels. Since all the frequency splitting is done after the amplifiers, more speakers can run off an amplifier channel, obtaining maximum power by playing with the resistances "seen" by the amplifiers.

Passive Crossovers are capacitors and inductors either in parallel or series, or combinations that are added to cut off highs and/or lows. A capacitor stores voltage, acting as an open circuit (blocks off signal) at lower frequencies, and acts as a short at higher frequencies (lets signal pass). An inductor, on the other hand, stores current, acting in exactly the opposite way of a capacitor. Inductors act as shorts at lower frequencies, and open circuits at high frequencies.

If a capacitor is hooked up in series with a speaker, it will be a high-pass crossover (signals at lower frequencies will be blocked, and higher frequency components of the signal will be allowed to pass). An inductor in series with a speaker will be a low-pass filter. Subwoofers need inductors in series (low-pass), while midranges will need both a high-pass (cut bass off) and a low-pass (cut higher frequencies that tweeters will be taking care of). Tweeters need also high-pass filters, to block lower frequencies.

An octave is double the frequency: i.e: 20Hz --> 40Hz --> 80Hz --> 160Hz, etc. The more components are added, the more effectively the filter will be, so if both an inductor and capacitor are used, the cutoff slope will be 12 dB-per-octave, then 18 dB-per-octave, then 24 and so on.  There are many types of passive crossovers that can be implemented, starting with the capacitor or inductor in series, and getting complex as more capacitors and inductors are added.

Capacitors and inductors also dissipate power, wasting energy that speakers could be using. Low order passive crossovers are not very expensive: Capacitors run $1 - $5, inductors run $10 - $20. Higher order (i.e. 24dB/Oct) crossovers can get really expensive, especially at low frequencies/high power applications.  Passive crossovers have another drawback, that is ignored most of the time for practical purposes: They introduce phase shifts, which put voltage and current out of phase with respect to each other, affecting delivered power to the speakers and affecting overall speaker "timing".  A 6dB/Oct crossover has a phase shift of 90 degrees, 12dB/Oct = 180, 18dB/Oct = 270, 24dB/Oct = 0.  Try to stay with even-order crossovers.  If you have a 180 degree shift (2nd order crossover = 12dB/Oct, hook up the speakers out of phase (+ to - and - to +).  On 4th order crossovers (24dB/Oct), there is no phase shift.

 

Equalizers are used to fine tune a system, not to fix design flaws.  If you have to use a lot of equalization, there is a problem with the system that should be solved first by relocating speakers, changing crossover frequencies, amplifier gains, etc.  Equalizers are valuable instruments to flatten a system's frequency response (making the levels the same at all frequencies).  In competition, a measurement is taken on how flat the response of a system is.  More points are given to a competitor with a flatter response.  In a real system, a flat frequency response is a starting point, but does not ultimately mean perfect sound since human ears are not sensitive at the same level to all frequencies.

Many people believe that an equalizer is to boost power by raising signal levels.  95% of the time an equalizer should be used to cut levels rather than boost them.  In a well designed system the settings on an equalizer should not be too far from the zero dB line. 

Frequency

Frequency Chart (4.7kb)
Click to View
Frequency is how many times per second a signal (AC) switches from positive to negative and back, measured in Hertz (see page on "Electrical Concepts" for a more extensive explanation).  The frequency range in which we are interested for audio is from 20 Hertz to 20,000 (20k) Hz.  The lower the frequency, the slower the signal oscillates.

The frequency spectrum is read using a logarithmic scale, and is divided in octaves (doubling of the frequency).   Octaves are for example, 20, 40, 80, 160, 315, 630 Hz and so on.  Equalizers are divided in octaves, 1/2 octaves or 1/3 octaves.  A 1 octave equalizer can only control 7-9 bands (frequencies), while a 1/2 octave equalizer can control 15 bands. A 1/3 octave equalizer would give you the most control over the system, by being able to adjust 30 -31 bands.

Q

Equalizer Diagram (8.6k bytes)Q is a measurement of how much the equalizer band affects a range of frequencies.  A high Q means that the EQ can control a lower "envelope" of frequencies, while a low Q is a larger envelope.  Looking at the image on the right, Q is the thickness of the affected frequencies.  A smaller Q means a wider range of frequencies boosted or cut, while a larger Q is a narrower shape.  Typical Q values are 1, 2 and 3.

Parametric vs. Graphic Equalizers

A graphic equalizer has usually fixed frequency and Q value.   The layout of a graphic equalizer is the typical sliding controls arranged by frequency.  The advantage of a graphic equalizer is that in the way it is laid out, it is easy to see what frequency is being boosted or cut and any person without much experience can adjust it.  Since a graphic EQ has fixed frequencies and Q, it has limitations on what it can control.

A parametric equalizer consists of knobs that are turned to desired levels, have adjustable frequencies and (usually) Q.  The advantage of parametric equalizers is a much greater control, since frequencies and Q values can be adjusted.  On the other hand, a parametric equalizer is much harder to adjust than a graphic EQ, requiring an experienced person and measuring equipment.

Mono and Stereo Equalizers

The main difference between mono and stereo EQs is that a mono EQ has only one input and one output, and a stereo has two inputs and two outputs.  They both have their advantages and disadvantages:   A stereo EQ controls your whole system (both left and right channels) and it is easy to adjust: Just turn the knob or slider and both left and right channels are taken care of.  If you want to adjust left and right channels independently, you can't!

A mono EQ controls only one channel, so you need two of them for the whole system.  Since you have now two EQs it takes a lot more time to setup the system.  Many people use mono EQs for the greater control they give over the system.  Since left and right speakers are not exactly at the same distance to our ears, two mono EQs can help compensate for time delays and problems caused by speaker placement.  Buying two mono equalizers is more expensive than buying one stereo EQ.

Low Level and High Level Output

A high level output equalizer takes either high level (speaker) or low level (RCA) inputs and has a built in amplifier.  The output goes directly to the speakers and can not be hooked up to another amplifier.  These equalizers are cheap and cause more damage than good to the sound system.  They do "boost" signals, but all this does is add distortion to the overall sound.

A low level input EQ takes RCA signals from the radio and has RCA outputs that get hooked up to amplifiers. Since these equalizers work at low signal levels, they introduce very little distortion, if any to the system.  They do cost more and require more wiring than a high level EQ.

A third kind of equalizer gets hooked up directly to the head unit via a special cable and is controlled by the head unit.  These EQs use low level signals and are usually of good quality.  The drawback is that if you want to upgrade the head unit or change brands, the EQ will not be compatible with other brands or even with different models from the same manufacturer.

 

The Head Unit is the most important part of the audio system.   You will always be staring at the head unit and touching it to control your sound system.  It is a good idea to look for something that is aesthetically pleasing, integrates to your car, has a logical button layout and has features that best suit your needs.

Head Units Features

CD Changer Controls - Whether you get a CD, cassette or MD head unit get a model that has CD changer controls, they only cost a bit more, but give you the opportunity of simply plugging in a changer in the future.

Power - Probably the most overrated feature in head units.  The power on head units is seldom given in RMS watts (see amplifiers section for definition).  Typically a head unit has an output of about 5-7 watts RMS per channel, while a high-powered head unit goes up to 13-15 watts RMS per channel (even if they claim 35 or 40 watts).

RCA Outputs - If you are planning to run the speakers from the head unit's built-in amplifier, you don't have to worry about RCAs, but if you plan to add amplifiers in the future, get one with a set of RCAs (left and right), three sets preferably (left and right for front, rear and subwoofer).  An important feature to look for is high-voltage RCA outputs.  Typically RCA signals are less than 1 volt.  High-voltage RCA signals are 2, 3 or even 4 volts.  This allows for better noise immunity and gives you a higher headroom for amplifier gain settings.  Most high-end manufacturers are selling units with high-voltage RCA outputs which are frequently used in competition.

Security - There are many security options for head units nowadays.  None of them is 100% effective in deterring theft.  Detachable faces are the most common option.  The front part of the radio comes off, rendering the rest of the unit useless.  The problem is that after a while people forget to take the face off, or simply tuck it under the seat or in the glove box.

Another option is codes, key CDs (i.e. Blaupunkt, Eclipse).  If power is cut off, the unit asks for a code or a predetermined CD used as a key.  If the incorrect code is entered, it locks the radio up, requiring service from the manufacturer.  This has proven to be an inconvenience when the owner loses their code or forget which CD they used to program the radio.  Some radios, such as Blaupunkt are using a smart card, that when removed, renders the unit useless, but again, people forget to take it out or lose it causing aggravation to the consumer.

Yet another security protection pioneered by Kenwood is a flat panel that covers the radio when the ignition key is turned off.  While this will fool some people into thinking there is no radio in, it won't fool most thieves.
  

RF modulated CD Changers

FM modulated changers can be hooked up to any radio that has an FM tuner, whether factory or aftermarket.  They use the radio's antenna to introduce the signal.  They are usually simple to hook up and consist on the changer itself which is mounted in the trunk or under seats, the control box and the display/remote control.  The drawback is that the sound of the CD changer will not be "CD quality", it will be as good as the FM tuner is.  The signal coming from the CD player has a wide frequency range but the FM tuner limits the signal, cutting the lower and upper ends of the spectrum.

 

Before you purchase any component, plan your system very carefully.   You need to consider if you are going to buy the whole system all at once or piece by piece, how much you want to spend and what quality and quantity of sound you want.  Are you doing a flashy or stealth installation?  Are you keeping your factory panels or are willing to cut your car to achieve better sound?   Are you doing the installation yourself, or leave it to a professional?

Sources

The most important part of the system.  Get a good head unit from a name brand.  If you skimp here, your whole system will suffer.  For people that are on a budget:  Get good quality head unit without all the bells and whistles.   A flip down face with a colorful display looks great, but it won't necessarily sound better that a regular plain head unit.  If you are planning to get amplifiers in the future, get a head unit with RCA outputs.
  

Speakers

The second most important part of the system.  If you are on a budget, just get a nice set of speakers up front and don't even worry about the rear speakers, amplifiers, etc until you have some more money later on.

Speaker installation is definitely the most important aspect that determines how your whole system sounds.  No equalizer or processor can compensate for poorly installed speakers.

Factory locations are usually not acceptable for audiophile quality sound. Speakers should ideally be pointing straight at you.   Speakers on each side should be as close to each other as possible with no obstructions. Speakers should be mounted on a good baffle (preferably an enclosure).   Difference between left and right speaker distances to your ears should be as small as possible.

The front speakers should also play as low as possible in frequency (ideally 60Hz or less), being able to handle full power.  This is where crossovers with high slopes come in to protect the speakers.
  

Amplifiers

Amplifiers do not only make a system sound louder, they make it sound BETTER.  The more power you get, the cleaner the signal going into the speakers.   A common misconception is that if a 100 watt amplifier is used on 50 watt speakers, the speakers will burn.  This is not true, as long as there is no distortion and the speakers are properly protected with crossovers.  More power is always better.

For systems with a lot of power, you might also have to upgrade the car's electrical system, by getting a high output alternator, capacitors, etc.
  

Subwoofers

Subwoofers cover low frequencies in the audio spectrum.  Subwoofers need to be installed in a box designed specifically for them.  Put a subwoofer in the wrong type or size box and it will not perform as it should and could be destroyed.

Subwoofers need a lot of power to play at acceptable levels without distortion.
  

Matching subs (and speakers) to amplifiers

This is a very important aspect of system planning that is often overlooked.  Amplifiers are designed to provide maximum power at a certain impedance.  An amplifier at this maximum level will be under more stress and produce more heat, so mounting location also becomes important.  Professional installers wire subs (and speakers) in parallel and/or series combinations to obtain a load that will make the amplifier perform at full power.
  

Processors

Many people believe that they need to have an equalizer, center channels, rear speakers, etc for better sound and compromise by buying cheaper components.  A properly designed system will sound great without the need for all this other components.

If you have the money and are an audiophile or into competition, then this "extra" components can become important.
  

Upgrading

Always keep in mind future upgrades when buying audio gear.  For example, let's say you are low on funds and want to add two subwoofers and an amplifier.   Since powerful amplifiers are expensive, you can get a 2-channel amplifier to drive the subwoofers at acceptable levels.  Later on, when you have more money, you can buy an identical amplifier and power each sub with an amplifier in bridged mode for more bass.   If you planned carefully, the impedance's of your subwoofers will match the amplifiers for maximum output in the bridged configuration.
  

Cheap Components

Buying better quality components will definitely increase system performance.  Although name brands are more expensive, they are more reliable (read: will last longer).

For people on tight budgets, it is better to save for a better component and take longer building a better system one component at a time.
  

Installation

Even though you will save money and learn something new by doing the installation yourself, sometimes it is better to pay a professional to do things that might be a bit over your head.  An experienced installer has many years of experience that will definitely make a difference in your system's performance and reliability.  If something goes wrong, you can always go back and have them fix the problem.  Many manufacturers offer and extended warranty period if the equipment is installed by an authorized professional.

 
Planning Your System
This part of the game can be fun or disappointing depending on what you can do. First I'm going to give you an example system that I believe includes everything you need to have a pretty good system. After that I will show you how to make compromises and leave out parts that may not be as important to you to keep your system within your budget. If you want to go beyond my basic system you probably already know more than what this site can tell you. Also, you do not have to get everything at once. I put my system together over a few years. With a little planning you can upgrade your system in steps and that way its like getting a new system every time you change something instead of getting everything at once!

Basic System: This is my opinion only but I think that a good system should start off with a good head unit that either has a CD player and/or is connected to a CD changer. A good system sounds best when playing CDs, tapes just do not cut it. Next I think component sets are made with fewer compromises than coaxial speakers so I suggest getting a good midrange/tweeter set for the front. Head units generally do not put out enough clean power so you will want an amp to drive the component set. In the rear where you only need some "fill" for ambiance you can get away with cheaper coaxials and set their level lower than the fronts to keep the sound stage in front. A modest (50x4) 4 channel amp is a good choice here for powering the component set up front and the rear speakers. You could use a good 2 channel amp and run the front and back in parallel on the amp but it would harder to adjust the level between them. Midranges sound best when they do not play bass so you will want a 2 way electronic crossover and use the high pass output to drive your 4 channel amp. I did not forget the bass! Most people are happy with a single 10" woofer or a pair of 12"s. Use an appropriate enclosure and a big amp (at least 75x2, preferably even more). Throw in installation and wiring accessories (like fuses and distribution blocks). Here's an approximate price break down of what this costs in my area. Your prices may be significantly different.
  • head unit: $300-$500
  • 4 channel amp for highs: $250-$400
  • 2 channel amp for lows: $300-$700
  • sub(s) (1-10" to 2-12"): $150-$450
  • enclosure for sub(s): $0 (free air) - $250 (custom)
  • component set: $200-$500
  • coaxials for rear fill: $100-$400
  • crossover/equalizer: $100-$500
  • wiring and accessories: $50-$250
  • installation: $0 (do it yourself) - $100 (basic)
This comes out to $1450-$3950! I realize that this is a lot of money and that most people do not spend nearly this much money on their car stereo. However, the things listed above are what I feel is necessary to have a system with only a few compromises. If you are less concerned about highs, get coaxials in front instead of the component set and power them off of the head unit and use some bass blockers on them. This will save you about $400. Getting a bargain head unit can save you some money as well. If you are really not into bass much you can forgo all the bass related equipment and run your component set full range. This will still give you clean sound but not much bass. However, you will save $550-$2300. I would start with what I have listed above and take out parts you do not care about as much. Only you know what kind of system you can be happy with.

When buying equipment try to spend time listening to it before you buy, especially with speakers. Also try to use equipment that is similar to yours when listening in a store. As for amps, it costs money to build a good amp so if you see some awesome price on an amp you have never heard of, it is probably a piece of junk. Stick with good names with amps.

Finally, if you are on a budget (aren't we all?) it works better to upgrade in steps.  The most important thing is to have a car audio system that sounds good to you not someone else.  If you are happy with just changing the factory speakers and stopping there then just do that.  There is a level when that new amp or speaker is not going to make a difference so it is not necessary to always upgrade.  There are people who think my system is terrible but it works well enough for me and anything else I do to it would be a minor gain and not worth my trouble.  Do not let a salesperson talk you into something you do not need!  Good luck!
 
Speaker Power Ratings and Amplifier Power Ratings
This page is intended to help people understand the relationships between speaker power ratings and amplifier power ratings.  A question that comes up in designing a system is "how much power do I need for my subs?" and "how much power do I need to run my other speakers?" I have a BS in Electrical Engineering so I do not know how much of this the average Joe is going to understand. I am also human so there may be mistakes below.

When most people consider how they are going to match their speakers and amps together they usually only consider matching the power levels.  There are many more factors that come into play.  A big factor is the sensitivity rating of the speaker.  The sensitivity (efficiency) rating of a speaker gives you a rough idea of how loud the speaker will play given a certain amount of power.  Let's consider a speaker with this sensitivity rating:

87 dB / 1 watt / 1 meter

What this spec means is that the speaker will produce sound at 87 dB 1 meter away from the speaker when it is given input power of 1 watt.  Typically the input sound's frequency is 1 kHz.  Depending on the type of enclosure and other factors the speaker may not produce 87 dB but it's still a useful spec for comparison with other speakers.

It takes a doubling of input power to produce TDB more sound (assuming the speaker is not reaching its limits).  Therefore we can make a table for how loud the speaker will play given a certain amount of power like this:

Power in watts Volume in dB
1 87
2 90
4 93
8 96
16 99
32 102
64 105
128 108
256 111
512 114

You can see how it starts to take a lot of power to make a speaker play very loud.  Fortunately even 32 watts of power gets us decent volume.

When you ask yourself how much power you need for your system you need to ask yourself how loud you want your system to play and plan accordingly.  Going with higher power amps or more sensitive (efficient) speakers will make your system play louder.

There are some subjective items to consider as well.  Designing a quality speaker is a process fraught with many compromises.  For example, a speaker whose cone is stiffer tends to produce less distortion at high output levels but the added weight of a stiffer cone can smear quick transient response.  Speaker cones have been made out of something as simple as stiff paper (typical of poor factory speakers) to exotic materials like Kevlar (some fairly high end aftermarket speakers).

Sensitivity is another factor when designing a speaker.  Typically factory speakers and aftermarket speakers meant to be driven from a head unit are very sensitive because they must be able to play loudly with only small amounts of input power.  The compromises that are made to create highly sensitive speakers can have a negative impact on the quality of sound the speaker produces.  Some of the higher end speakers have low sensitivities because it was easier to design a high quality speaker that had low sensitivity than one that sounded good and had high sensitivity.  Also, it is presumed that a high end speaker will be driven by a proper aftermarket external amplifier with more power than a head unit.

As for matching power ratings between speakers and amplifiers, it is not necessary.  Most speakers can accept clean input power in huge amounts before destroying themselves.  Any quality amp that can produce enough power for your loudness expectations should work fine.  The only advantage a 200 watt amp holds over a 100 watt (of the same design) is the ability to play 3 dB louder.

Finally, one other item to consider when choosing an amplifier is whether it is 2 ohm stable.  One may not think this matters if you are going to be using normal 4 ohm speakers but it can still be a factor.  When a speaker is rated at 4 ohms, that is just a nominal rating.  The actual impedance will change with frequency and is also affected by the type of enclosure the speaker is in.  There can be frequencies where the impedance dips well below the nominal 4 ohm value.  Having an amplifier that is stable to 2 ohms assures that your amp will be able to provide the current necessary for the speaker to reproduce sound accurately at those frequencies.

So in the end the basic answer to the question of how much power you need for your speakers is based on how loud you want your system to play and how sensitive the speakers are that you are going to use.  Also consider that subs can be less efficient than other speakers so you will probably want to give your subs more power than the higher frequency speakers in your system.  The lower the frequency the more power that is required by the speaker to reproduce it.
 

Two-Way Crossover Network Design

 The first and most important concept for you to understand before you design any passive crossover is that all the formulas are derived with the assumption that you are terminating into a constant impedance load. This means that the values these formulas give are only correct for the impedance value you input into the formula.

 This is great if you are using a speaker with ferro-fluid (a magnetic fluid in the voice-coil gap which modifies the impedance to a relatively flat value),  however, in general a speaker's impedance varies greatly over the operating range of the driver. So before you calculate a passive crossover network, you must first correct the inherent impedance problems common in dynamic loudspeakers.

 The first problem is the impedance peak present at the resonant frequency of the speaker. The exact frequency and shape of this peak is variable depending on the type of enclosure loading you are using with the driver. The graphic below will illustrate this point clearly.

 As you can see, using the manufacturer's rated impedance is not a good idea. However, you can correct this impedance peak so the formulas are useful again. I would also like you to notice the impedance curving upwards in the higher frequency range. This is caused by the inductance of the voice-coil. This too can be corrected.

 The first problem we will address is the peak at resonance. This can be fixed using a contour network. This filter type is covered here.

To correct the rising impedance in the higher frequencies, you need to utilize a different type of impedance shaping filter.

Explanation of Terms Used in the Formulas
C1, C2, C3, etc... are the capacitor values required for your crossover network.
L1, L2, L3, etc... are the inductor values required for your crossover network.
Rh is the impedance of the speaker at the frequency you want to apply your high pass filter.
RL is the impedance of the speaker at the frequency you want to apply your low pass filter.
f is the frequency which you are calculating the crossover for.
A bandpass filter can be used to limit how high or low the frequency range being sent to a speaker is. This can be useful when limited excursion drivers are used for the bass range in a two-way speaker system.
 
Links to the Calculators/Diagrams/Formulas
1st Order Butterworth
2nd Order Butterworth 2nd Order Linkwitz-Riley
3rd Order Butterworth
4th Order Butterworth 4th Order Linkwitz-Riley
6th Order Linkwitz-Riley
 

Driver Attenuation Circuit

 This calculator is intended to help you design an "L-pad" type circuit. It can be useful if you need to attenuate (reduce) the sound level of one speaker so that it will match the output of another driver.
 

Circuit Diagram
Shelving Network

 This circuit is commonly used on tweeters because they are generally more efficient (higher sensitivity) than woofers.

 It can also be applied to the "rear fill" speakers in a mobile audio application. It is very undesirable to have the soundstage in the rear of the vehicle (which will happen if your rear speakers are too loud).

Note : You can accomplish this using only 1 resistor in series with the driver, however, the impedance load presented to your amplifier will be increased. When you use a L-Pad, the impedance load stays the same as it was before.
 

Explanation of Terms
R1, and R2 are resistors.
 
Calculate
 To use this form, enter the impedance of the driver you want to attenuate, and then enter how many dB you want to reduce the output by (3 dB is the equivalent of cutting the power to the speaker in half). Then just click the "Calculate Values" button, and your answer will appear in the empty text boxes.
Enter Impedance Ohms
Enter Desired Attenuation  dB SPL
R1 =  Ohms
R2 =  Ohms
 
 

Crossover Network Design Formulas & Calculator

 This calculator will design a two-way third-order Butterworth crossover network for you. If you don't understand what any of the terms mean, click here for help.
 
3rd Order Butterworth
High Pass Filter Schematic
 High Pass Filter
Low Pass Filter Schematic
Low Pass Filter
Bandpass Filter Schematic
Bandpass Filter
C1 = .1061 / ( Rh x f )
L1 = (.1194 x Rh ) / f
C2 = .3183 / ( Rh x f )
L2 = (.2387 x RL ) / f
C3 = .2122 / ( RL x f )
L3 = (.0796 x RL ) / f
Calculate Your Crossover Here

 To use this form, enter the impedance values (at the crossover frequency) for your speakers, and then enter the frequency you want your filter to crossover at. Now, click the "Calculate Values" button, and your results will appear in the empty text boxes.

Enter Tweeter Impedance Ohms
Enter Woofer Impedance Ohms
Enter Desired Crossover Frequency  Hertz
HighpassLowpass
C1 =   Microfarads
L1 =   Millihenries
C2 =   Microfarads
L2 =   Millihenries
C3 =   Microfarads
L3 =   Millihenries
 

Series Notch Filters

 A series notch filter can be used to control the impedance peak present at the resonant frequency (Fs) of a speaker. For an explanation of loudspeaker impedance and passive crossovers, click here. If your driver has Ferrofluid in the voice-coil gap, this circuit will not be beneficial because the peak at resonance is controlled mechanically.

A high-pass crossover with a corner frequency (the frequency utilized in the crossover network design) less than two octaves above the resonance of the driver will not function properly. All crossover design formulas are accurate only when the loudspeaker impedance is at the level specified in the formula. If the impedance is higher/lower than the value used in the formula, the filter roll-off is affected.

 The black line in the image below shows the impedance curve of a speaker in a closed box. The red line shows the impedance of the same speaker/enclosure combination with a properly designed series notch filter.

Series Notch Filter

 The upward curve at higher frequencies is caused by the voice-coil inductance and is corrected using a Zobel or parallel notch filter.

When you design a crossover from a formula and apply it in a situation similar to the one above, the results are less than spectacular.

 For this simulation, I modeled a driver whose resonant frequency (Fs) is 53 Hz. The component values were determined using 8 ohms for impedance and 200 Hz for the crossover frequency. This frequency was chosen so it would be close enough to the resonance of the driver that the crossover would be affected by it.

 The following image shows the desired response for a second-order Linkwitz-Riley crossover network in red. The black curve is the response you would obtain if you used the components specified by the crossover design formulas without impedance correction.

Resonance Interaction


 The third graphic shows how much closer to the desired response you will get by correcting the impedance of the driver.

Excellent Crossover


The crossover filter goes in the circuit before the notch filter (closer to the source).

The Formulas

Capacitor FormulaInductor Formula
Resistor Formula
C is the value for the capacitor
L is the value for the inductor.
R is the value for the resistor.
Series Notch Filter Diagram


Calculate Your Series Notch Filter Here
 To use this calculator, enter the values of Fs, Qes, Qms, and Re for your speaker. Then click the "Calculate" button and the values will show up in the boxes below.
Fs : Hz
Qes :
Qms :
Re : Ohms
C :  uF
L :  mH
R :  Ohms
 
This type of filter is generally used on midranges/tweeters. It can however, be used on woofers, but the component values required are large and would be very expensive.
 
Crossover Network Design Formulas & Calculator
 This calculator will design a two-way second-order Linkwitz-Riley crossover network for you. If you don't understand what any of the terms mean, click here for help.
 
 Please note the reversed polarity wiring on the high pass filter. Second-order filters will usually give you a better overall frequency response when they are wired this way because this type of filter shifts the phase of the input signal 180°.
2nd Order Linkwitz-Riley
Second-Order High Pass Filter Schematic
Second-Order High Pass Filter
Second-Order Low Pass Filter Schematic
Second-Order Low Pass Filter
Bandpass Filter Schematic
Second-Order Linkwitz-Riley Bandpass Filter
C1 = .0796 / ( Rh x f )
L1 = (.3183 x Rh ) / f
C2 = .0796 / ( RL x f )
L2 = (.3183 x RL ) / f
Calculate Your Crossover Here

 To use this form, enter the impedance values (at the crossover frequency) for your speakers, and then enter the frequency you want your filter to crossover at. Now, click the "Calculate Values" button, and your results will appear in the empty text boxes.

Enter Tweeter Impedance Ohms
Enter Woofer Impedance Ohms
Enter Desired Crossover Frequency  Hertz
HighpassLowpass
C1 =  Microfarads
L1 =  Millihenries
C2 =   Microfarads
L2 =   Millihenries
 

Crossover Network Design Formulas & Calculator

 This calculator will design a two-way sixth-order Linkwitz-Riley crossover network for you. If you don't understand what any of the terms mean, click here for help.
 
6th Order Linkwitz-Riley
Sixth-Order High Pass Filter Schematic
Sixth-Order High Pass Filter
Sixth-Order Low Pass Crossover Filter Schematic
Sixth-Order Low Pass Filter
Bandpass Filter Schematic
Sixth-Order Linkwitz-Riley Bandpass Crossover Filter
  
C1 = .08841 / ( Rh x f )
  
L1 = (.08594 x Rh ) / f
C2 = .10810 / ( Rh x f )
L2 = (.14200 x Rh ) / f
C3 = .21880 / ( Rh x f )
L3 = (.63660 x Rh ) / f
C4 = .29470 / ( RL x f )
L4 = (.28650 x RL ) / f
C5 = .17830 / ( RL x f )
L5 = (.23440 x RL ) / f
C6 = .03979 / ( RL x f )
L6 = (.11570 x RL ) / f
Calculate Your Crossover Here

 To use this form, enter the impedance values (at the crossover frequency) for your speakers, and then enter the frequency you want your filter to crossover at. Now, click the "Calculate Values" button, and your results will appear in the empty text boxes.

Enter Tweeter Impedance Ohms
Enter Woofer Impedance Ohms
Enter Desired Crossover Frequency  Hertz
HighpassLowpass
C1 =   Microfarads
L1 =  Millihenries
C2 =   Microfarads
L2 =  Millihenries
C3 =   Microfarads
L3 =  Millihenries
C4 =   Microfarads
L4 =  Millihenries
C5 =   Microfarads
L5 =  Millihenries
C6 =   Microfarads
L6 =  Millihenries
 

Zobel Loudspeaker Impedance Correction Circuit

 This page will help you understand what a Zobel filter does, and how it can be useful in a loudspeaker crossover network design. If you need more help, click here.
 A "Zobel" circuit can correct the rising impedance load presented to the amplifier. This allows a speaker to reproduce high frequencies better because more power is available for the driver due to the fact that the impedance is relatively constant. It also helps to keep the crossover point from shifting due to impedance variations.
 The cause of this impedance rise is the inductance of the voice-coil of the speaker. A voice-coil is a coil of wire wrapped around the former (which is connected to the back of the cone.) It looks like an inductor wound with small gauge wire. The reason inductors are used in crossover design is because the impedance of an inductor increases as the frequency of the signal applied to it goes up. This reduces the power applied to the speaker at higher frequencies. This is how an inductor "rolls-off" the sound.
 The line with an impedance of approximately 37.5 ohms around 2000 Hz is this driver resonant frequency (Fs). The plot with the greatest impedance centered at approximately 1350 Hz is the impedance of the same speaker with a Zobel filter. Notice on this driver the Zobel filter also lowered the impedance at resonance. Since a flat impedance curve is very desirable, I altered the component values so this one filter is could do the work of two (the lowest line). This reduced the component count of the impedance correction portion of this crossover network from five to two!
 The other (unnecessary in this case) filter is a resonance equalization circuit. This speaker is a good example of how useful a crossover design program can be. The crossover design equations are all useful to get in the "ballpark", but if you are going to design advanced crossover networks, you need a good program to see what is really happening in your crossover. Personally I use CALSOD™ from Audiosoft, L.E.A.P.™ is generally regarded as the best loudspeaker design program available, but I can't afford it yet. Maybe someday :).
Explanation of Terms
Le is the inductance of your driver's voice-coil (in henries).
Re is the D.C. resistance of the voice-coil (ohms). The design equations for this type of filter are:
Capacitor = Le/Rc2
Resistor (Rc) = 1.25 X Re
Zobel Impedance Correction circuit
Zobel Filter Diagram
The Formulas and the Calculator
 To use this calculator, enter the D.C. resistance and the inductance of your driver, click the "Calculate Values" button and the required component values will appear in the remaining text boxes.
Enter D.C. Resistance Ohms
Enter Voice-Coil Inductance  Millihenries
Capacitor Microfarads
Resistor = Ohms

 

Crossover Network Design Formulas & Calculator

 This calculator will design a two-way fourth-order Linkwitz-Riley crossover network for you. If you don't understand what any of the terms mean, click here for help.
 
4th Order Linkwitz-Riley
Linkwitz-Riley High Pass Filter Schematic
Fourth-Order High Pass Filter
Fourth-Order Linkwitz-Riley Low Pass Crossover Filter Schematic
Fourth-Order Low Pass Filter
Bandpass Filter Schematic
Linkwitz-Riley Bandpass Filter
C1 = .0844 / ( Rh x f )
L1 = (.1000 x Rh ) / f
C2 = .1688 / ( Rh x f )
L2 = (.4501 x Rh ) / f
C3 = .2533 / ( RL x f )
L3 = (.3000 x RL ) / f
C4 = .0563 / ( RL x f )
L4 = (.1500 x RL ) / f
Calculate Your Crossover Here

 To use this form, enter the impedance values (at the crossover frequency) for your speakers, and then enter the frequency you want your filter to crossover at. Now, click the "Calculate Values" button, and your results will appear in the empty text boxes.

Enter Tweeter Impedance Ohms
Enter Woofer Impedance Ohms
Enter Desired Crossover Frequency  Hertz
HighpassLowpass         
C1 =   Microfarads
L1 =  Millihenries
C2 =   Microfarads
L2 =  Millihenries
C3 =   Microfarads
L3 =   Millihenries
C4 =   Microfarads
L4 =   Millihenries
 

Parallel Notch Filter Circuit

Explanation of Terms
C is the capacitor value required for your filter.
L is the required inductor value for your filter.
R is the desired resistor value for this filter.
F is the frequency (in Hertz) halfway between F1 and F2.
F1 is the frequency point below F that is -3dB SPL relative to F.
F2 is the frequency point above F that is -3dB SPL relative to F.
 For the example driver below, the center point of the response peak (F) would be at about 3583 (Hertz). F1 and F2 would be located at approximately 1015 and 6150 respectively.
 
Good Candidate for a Notch Filter
Frequency Response Example
 At first glance, the response of the speaker above does not look like a very good candidate for a driver in a quality loudspeaker system. A broad peak in the frequency response of a speaker like the example above can be easily corrected through the use of a notch filter, however.
Note : This filter can be used on any type of speaker, it is not only for midranges - this was just an easy example to model.
Logarithmic Paper
 If you need some logarithmic paper to aid in your SPL measurements, I have made two graphics to help. The first one can be used for low frequency plotting, and the second is a full-range scale. Just click the link for the one you need, and press the "print button" on your browser after the image is done loading. I will add some notes about the methods used to properly measure frequency response as soon as I can.
Calculate Your Notch Filter
 To use this form, enter F, F1, and F2. Then, click the "Calculate Values" button, and your results will appear in the empty text boxes.
 
Enter F Hertz
Enter F1 Hertz
Enter F2 Hertz
C Microfarads
L = Millihenries
R = Ohms

 
High Pass Crossover Filter Schematic
Highpass Filter
To determine the crossover frequency a certain amount of capacitance will give you, use the formula:

0.159/(C x Rh) = F

Explanation of Terms
C - is the capacitance value (in Farads) - to convert to Farads divide the value shown on the side of the cap in uF by one million.
Rh - is the impedance of the load (speaker) you will be using.
F - is the crossover frequency you will get.

 

by frequency
Crossover Frequency (Hertz) Capacitance uF (microFarad)
8 Ohms 4 Ohms 2 Ohms
80 248.44 496.88 993.75
100 198.75 397.50 795.00
120 165.63 331.25 662.50
150 132.50 265.00 530.00
200 99.38 198.75 397.50
280 70.98 141.96 283.93
400 49.69 99.38 198.75
600 33.13 66.25 132.50
800 24.84 49.69 99.38
1000 19.88 39.75 79.50
1200 16.56 33.13 66.25
2000 9.94 19.88 39.75
4000 4.97 9.94 19.88
5000 3.98 7.95 15.90
6000 3.31 6.63 13.25
8000 2.48 4.97 9.94
10000 1.99 3.98 7.95
12000 1.66 3.31 6.63

 

by capacitor
Capacitance uF (microFarad) Crossover Frequency (Hertz) approximate
8 Ohms 4 Ohms 2 Ohms
2.2 9034 18068 36136
3.3 6023 12045 24091
4.7 4229 8457 16915
6.8 2923 5846 11691
10.0 1988 3975 7950
22 903 1807 3614
50 398 795 1590
100 199 398 795
200 99 199 398

 

Calculate for any Value
 To use this calculator, enter the amount of capacitance you have available in the first box. Then enter the impedance of the speaker you will be using with this filter in the box below that. Finally, click the "Calculate It" button to see the frequency this filter will be -3 dB.
Capacitor =  uF
Rh = Ohms
Answer =  Hertz
  

 

Capacitors in Series/Parallel
 To use this calculator, enter the amount of capacitance in the first two boxes, they can be different values - this is how you obtain the odd values in the charts above. Then pick a series or a parallel connection. Now, click the "Capacitance" button to see the resulting value.
Capacitor 1 =  uF
Capacitor 2 =  uF
Connection
ParallelSeries
Answer =  uF
  
 
Low Pass Speaker Crossover Filter Schematic
Lowpass Filter
To determine the crossover frequency a certain amount of inductance will give you, use the formula:

RL/(6.283 x L) = F

Explanation of Terms
RL - is the impedance of the load (speaker) you will be using.
L - is the inductance value (in Henries) - to convert to Henries divide the value shown on the inductor in mH by one thousand.
F - is the crossover frequency you will get.

 

by frequency
Crossover Frequency (Hertz) inductance mH (milliHenry)
8 Ohms 4 Ohms 2 Ohms
80 15.92 7.96 3.98
100 12.73 6.37 3.18
120 10.61 5.31 2.65
150 8.49 4.24 2.12
200 6.37 3.18 1.59
280 4.55 2.24 1.14
400 3.18 1.59 0.8
600 2.12 1.06 0.53
800 1.59 0.8 0.4
1000 1.27 0.64 0.32
1200 1.06 0.53 0.27
2000 0.64 0.32 0.16
4000 0.32 0.16 0.08
5000 0.25 0.13 0.06
6000 0.21 0.11 0.05
8000 0.16 0.08 0.04
10000 0.13 0.06 0.03
12000 0.11 0.05 0.03

 

by inductor
inductance mH (milliHenry) Crossover Frequency (Hertz) approximate
8 Ohms 4 Ohms 2 Ohms
0.1 12733 6366 3183
0.2 6366 3183 1592
0.27 4716 2358 1179
0.3 4244 2122 1061
0.33 3858 1929 965
0.4 3183 1592 796
0.5 2547 1273 637
0.6 2122 1061 531
0.7 1819 909 455
0.8 1592 796 398
0.9 1415 707 354
1.0 1273 637 318
1.2 1061 531 265
1.8 707 354 177
2.0 637 318 159
2.2 579 289 145
2.7 472 236 118
3.0 424 212 106
3.3 386 193 96
4.0 318 159 80
4.7 271 135 68
5.0 255 127 64
6.0 212 106 53
7.0 182 91 45
8.0 159 80 40
9.0 141 71 35
10.0 127 64 32
12.0 106 53 27
15.0 85 42 21

 

Calculate for any Value

 To use this calculator, enter the amount of inductance you have available in the first box. Then enter the impedance of the speaker you will be using with this filter in the box below that. Finally, click the "Calculate It" button to see the frequency this filter will be -3 dB.


Inductor =  mH
RL = Ohms
Answer =  Hertz
  

 

Inductors in Series/Parallel
 To use this calculator, enter the amount of inductance in the first two boxes, they can be different values - this is how you can obtain the odd values in the charts above. Then pick a series or a parallel connection. Now, click the "Inductance" button to see the resulting value.
Inductor 1 =  mH
Inductor 2 =  mH
Connection
ParallelSeries
Answer =  mH
  
 

Crossover Network Design Formulas & Calculator

 This calculator will design a two-way second-order Butterworth crossover network for you. If you don't understand what any of the terms mean, click here for help.
 
 Please note the reversed polarity wiring on the high pass filter. Second-order filters will usually give you a better overall frequency response when they are wired this way because this type of filter shifts the phase of the input signal 180°.
2nd Order Butterworth
High Pass Butterworth Filter Schematic
High Pass Filter
Second-Order Low Pass Filter Schematic
Low Pass Filter
Passive Bandpass Crossover Filter Schematic
Bandpass Filter
C1 = .1125 / ( Rh x f )
L1 = (.2251 x Rh ) / f
C2 = .1125 / ( RL x f )
L2 = (.2251 x RL ) / f
Calculate Your Crossover Here

 To use this form, enter the impedance values (at the crossover frequency) for your speakers, and then enter the frequency you want your filter to crossover at. Now, click the "Calculate Values" button, and your results will appear in the empty text boxes.

Enter Tweeter Impedance Ohms
Enter Woofer Impedance Ohms
Enter Desired Crossover Frequency  Hertz
HighpassLowpass
C1 =  Microfarads
L1 =  Millihenries
C2 =   Microfarads
L2 =   Millihenries
 

Crossover Network Design Formulas & Calculator

 This calculator will design a two-way first-order Butterworth crossover network for you. If you don't understand what any of the terms mean, click here for help.
 
1st Order Butterworth
High Pass Butterworth Crossover
Highpass Filter
First-Order Low Pass Filter
Lowpass Filter
Bandpass Butterworth Crossover
Bandpass Filter
C1 = .159 / ( Rh x f )
L1 = RL / ( 6.28 x f )
Calculate Your Crossover Here

 To use this form, enter the impedance values (at the crossover frequency) for your speakers, and then enter the frequency you want your filter to crossover at. Now, click the "Calculate Values" button, and your results will appear in the two empty text boxes.

Enter Tweeter Impedance Ohms
Enter Woofer Impedance Ohms
Desired Crossover Frequency  Hertz
C1 Microfarads
L1 Millihenries

 

Instructions

  1. Make sure you have Java turned on in your browser.
  2. Enter high and low pass speaker impedances.
  3. Enter desired crossover frequency.
  4. On the second-order crossover calculator you must select type of crossover.
  5. Click on the "calculate" button to get the answers.
  • Impedance is the nominal resistance of the speaker (typically 4 Ohms).
  • Enter frequency in Hertz (not kHz).
  • Capacitor value(s) are given in millionths of a Farad (µF).
  • Inductor value(s) are given in thousands of a Henry (mH).
  • For the Zobel circuit, enter inductance in Henries (not mH).

Calculators *

  • First Order Crossover (6db/octave).
  • Second Order Crossover (12db/octave).
  • Third Order Crossover (18db/octave).
  • Fourth Order Crossover (24db/octave).
  • Zobel Circuit (Impedance Stabilization).
  • L-pad Circuit (Speaker Attenuation).

First Order (6db/octave) Two-Way Crossover

High Pass Impedance: Ohms
Low Pass Impedance: Ohms
Frequency Hz



6dB Crossover Diagram     C1= µF




  L1= mH

  • Phase shift on a first-order crossover is 90 degrees.

 


Second Order (12db/octave) Two-Way Crossover

Linkwitz-Riley Butterworth Bessel

High Pass Impedance: Ohms
Low Pass Impedance: Ohms
Frequency Hz



12dB Crossover Diagram     C1 = µF

  L1 = mH


  C2 = µF

  L2 = mH

  • Linkwitz-Riley crossovers match attenuation slopes so that system response is flat at crossover point.
  • Butterworth crossovers yield to a peak at the crossover frequency.
  • Bessel crossovers have a frequency response between Linkwitz-Riley and Butterworth crossovers.
  • The phase shift on a second-order crossover is 180 degrees (reversed polarity).

 


Third Order (18db/octave) Two-Way Crossover

High Pass Impedance: Ohms
Low Pass Impedance: Ohms
Frequency Hz



18dB Crossover Diagram     C1 = µF
  C2 = µF
  L1 = mH

  L2 = mH
  L3 = mH
  C3 = µF

  • Phase shift on a third-order crossover is 270 degrees (-90 degrees).

 


Fourth order (24dB/octave) Two-Way Crossover

High Pass Impedance: Ohms
Low Pass Impedance: Ohms
Frequency Hz



24dB Crossover Diagram     C1 = µF
  C2 = µF
  L1 = mH
  L2 = mH

  C3 = µF
  C4 = µF
  L3 = mH
  L4 = mH

  • The phase shift on a fourth-order crossover is 360 degrees = 0 degrees (no phase shift).

 


Zobel Circuit (Impedance Stabilization)

DC resistance (Re): Ohms
Inductive Equivalent (Le): Henries



Zobel Circuit Diagram     C1= µF


  R1= Ohms

  • Even though speakers are rated at a certain "resistance" (i.e. 4 Ohms), the actual impedance varies with frequency (speakers have inductance). To compensate for the non-linearity of speakers (on mainly subwoofers), Zobel circuits are used.
  • Re is the DC resistance of the woofer (can be measured with an ohmmeter)
  • Le (or Lces) is the electrical inductive equivalent of the driver.

 


L-pad (Speaker Attenuation)

Driver Impedance = Ohms
Desired Attenuation = dB



l-pad.gif (1013 bytes)     R1 = Ohms


  R2 = Ohms

  • An L-pad circuit will attenuate a speaker.
  • L-pads keep the load "seen" by the amplifier constant, affecting only the power delivered to the speaker.  The power delivered by the amplifier remains constant.
  • Since L-pads are made from resistors, it does not induce any phase shifts, or affect frequency response.
 

The easiest (and cheapest) crossovers to build are 6 dB/Oct, made of either an inductor or capacitor inline.  While this might work as a simple fix, or at a crossover frequency that is not close to the speaker frequency response limits, it is not the best solution.  For higher slope crossovers, complexity and cost add up quickly.

Parts of a Crossover Network

Parts of a Xver

  1. Filter: This is the real crossover.  It blocks undesired frequencies by increasing impedance seen by the amplifier.  Made up of capacitors and inductors.  There are three types:  High pass, low pass and bandpass (high pass and low pass filters used together).
      
  2. L-pad:  Attenuates the output of a speaker, while presenting a constant load to the amplifier.  L-pads are made using two resistors that dissipate power that would go to the speaker.  It is used to match "volume" levels of different speakers.
      
  3. Impedance Stabilization:  Commonly known as a Zobel circuit.  It uses a capacitor and resistor to compensate for the inductive effects of the speaker coil, making the speaker play in a more linear fashion.  This also makes the amplifier see a more stable load (speaker impedance varies with frequency).   Zobel networks are used on speakers that play lower frequencies, not tweeters.
      

How to Pick Crossover Components

  1. Capacitors: If you can afford them, try to get Mylar or polypropylene capacitors, especially when they are used in series (high-pass filters).  For low pass filters, or high capacitance values non-polarized electrolytic capacitors have to be used.
      
  2. Inductors: Most critical in low-pass filters, when they are placed in series.  For audiophile quality sound, CFAC (Copper-foil-air-core) inductors are probably the best choice, but at a high cost.   Most cost/quality effective option is air core inductors for high-end crossovers.   At larger inductance values, the series DC resistance in air core inductors becomes a problem.   This is when iron core inductors would have to be used.
      
  3. Resistors: High-power resistors are bulky.   Always pick a higher wattage than you would need on average conditions.  Get non-inductive resistors for best performance.
      
  4. L-pads: There are commercially available variable L-pads, or a set can be fabricated using two high-power resistors.  If you buy an L-pad, it is very important that you get the right value.  I.e:  For 4-ohm speakers, get a 4-ohm L-pad.  A dual 8-ohm L-pad could be wired in parallel to operate with a single 4-ohm speaker, at twice power handling.
      
  5. Circuit Boards:  Chose double sided copper boards.  The thicker the copper coating and insulating material, the better.   You can either etch the board with chemicals, or with a Dremel® tool.
      

Crossover Design Tips

  • Capacitor voltage/power ratings: Typically, 50-volt capacitors can handle up to 70 RMS Watts, 100v can handle 200w and 250v up to 300w.
      
  • Inductors gauge/power ratings: In inductors, the gauge of the wire used determines power handling.  Common values are:  20 gauge = 180 watts, 18ga = 250 to 300w, 16ga = 500w and 14ga = 800w.
      
  • Series/parallel inductors add up in the same way as resistors, capacitors add up in the opposite way (capacitance increases in parallel, decreases in series).  For formulas, look at the "speaker/sub wiring" page.
      
  • In crossovers with large non-polarized electrolytic caps, sound quality can be improved by bypassing them with a small value (0.01 to 0.47 µF) film or foil polypropylene capacitor in parallel (tip courtesy of Parts Express).
      

Crossover Initial Design

Plan very carefully which frequency you are using for the crossover.  Once you buy the components, you can't change it.  Consider car's response, speaker's response, slope, acoustical effects, etc.

Whether it is plugging numbers into formulas, or having a computer figure out the values, you will come up with a set of inductor and capacitor values.  Most likely, the values you have are not commercially available.  You will have to play around with frequencies and commercially available values to compromise on a good design without much deviation from theoretical data.

Once you have your values figured out, you need to calculate power.  Always over-engineer crossovers and pick your inductor's wire gauge and capacitor's max. voltage accordingly.  Any weak component will cause problems in the overall crossover design.

If you have the resources, try to run a simulation of the crossover's interaction with the speakers parameters, see what comes up and how shifts in voltage and current will affect the response.
  

Building The PCB

Take your time figuring out the best component layout possible.   A good layout takes up a least amount of space while avoiding traces to cross each other.  Two sided-copper boards are easier to work with.  If there is no way to avoid traces crossing, both sides can be used to keep from soldering jumper wires.

Once you have a tentative layout and have analyzed that the layout matches the schematics, draw each component's outline with a pencil.  Mark where component leads and input/output wires need to be drilled in the board.  Drill all component holes and test fit all the components without soldering.  Etch or grind away parts of the board to create traces.  If there is a possibility of a component shorting out traces, etch an outline around the component to avoid problems.

It is good practice to glue or wire tie (or both) components to the board.  This way, rattles and vibrations that can stress and break wire leads are minimized.   Take one component at a time, glue it to the board (hot melt glue works great), and then solder the leads.  Cut excess wire.  Repeat the process for all the components and wires.
  

Testing

First, visually trace all the connections and junctions on both sides of the board.   Make sure there are no short/open circuits.

The second step is to test the board.  Check the board to make sure there are no DC short circuits.  Connect the board to an amplifier and speaker.  You can either use an RTA or test tones to determine the board's frequency response.  Use a volume level a bit higher than the background noise in your test environment.  If the frequency response (crossover point and slope) do not match theoretical data, you might have a short or loose connection.  Re-inspect your circuit.
  

Quick Parts Reference

Guidelines For 6-dB/Octave Crossovers

6dB/octave Low-Pass Filter

6dB/octave High-Pass Filter

Frequency 2 ohms 4 ohms 8 ohms
(Hertz) L C L C L C
80 4.1 mH 1000 µF 8.2 mH 500 µF 16 mH 250 µF
100 3.1 mH 800 µF 6.2 mH 400 µF 12 mH 200 µF
130 2.4 mH 600 µF 4.7 mH 300 µF 10 mH 150 µF
200 1.6 mH 400 µF 3.3 mH 200 µF 6.8 mH 100 µF
280 1.2 mH 300 µF 2.4 mH 150 µF 4.7 mH 75 µF
400 0.8 mH 200 µF 1.6 mH 100 µF 3.3 mH 50 µF
600 0.5 mH 136 µF 1.0 mH 68 µF 2.0 mH 33 µF
800 0.41 mH 100 µF 0.82 mH 50 µF 1.6 mH 25 µF
1000 0.31 mH 78 µF 0.62 mH 39 µF 1.2 mH 20 µF
1200 0.25 mH 66 µF 0.51 mH 33 µF 1.0 mH 16 µF
1800 0.16 mH 44 µF 0.33 mH 22 µF 0.68 mH 10 µF
4000 0.08 mH 20 µF 0.16 mH 10 µF 0.33 mH 5 µF
6000 51 µH 14 µF 0.10 mH 6.8 µF 0.20 mH 3.3 µF
9000 34 µH 9.4 µF 68 µH 4.7 µF 0.15 mH 2.2 µF
12000 25 µH 6.6 µF 51 µH 3.3 µF 100 µH 1.6 µF

 

Guidelines For 12-dB/Octave Crossovers

12dB/octave Low-Pass Filter

12dB/octave High-Pass Filter

Frequency 2 ohms 4 ohms 8 ohms
(Hertz) L C L C L C
80 5.6 mH 700 µF 11 mH 330 µF 22 mH 180 µF
100 4.5 mH 550 µF 9.1 mH 270 µF 18 mH 150 µF
130 3.5 mH 470 µF 6.8 mH 200 µF 15 mH 100 µF
200 2.3 mH 330 µF 4.7 mH 150 µF 9.1 mH 75 µF
280 1.7 mH 220 µF 3.6 mH 100 µF 6.8 mH 50 µF
400 1.1 mH 140 µF 2.2 mH 68 µF 4.7 mH 33 µF
600 0.75 mH 100 µF 1.5 mH 47 µF 3.0 mH 27 µF
800 0.56 mH 68 µF 1.0 mH 33 µF 2.0 mH 15 µF
1000 0.45 mH 55 µF 0.91 mH 27 µF 1.8 mH 13 µF
1200 0.38 mH 47 µF 0.75 mH 22 µF 1.5 mH 11 µF
1800 0.25 mH 33 µF 0.50 mH 15 µF 1.0 mH 6.8 µF
4000 0.11 mH 14 µF 0.22 mH 6.8 µF 0.47 mH 3.3 µF
6000 75 µH 10 µF 0.15 mH 4.7 µF 0.33 mH 2.2 µF
9000 50 µH 6 µF 0.10 mH 3.3 µF 0.20 mH 1.5 µF
12000 38 µH 4.7 µF 75 µH 2.2 µF 0.15 mH 1.0 µF
 

Crossover Network Design Formulas & Calculator

 This calculator will design a two-way fourth-order Butterworth crossover network for you. If you don't understand what any of the terms mean, click here for help.
 
4th Order Butterworth
Fourth-Order High Pass Filter Schematic
Fourth-Order Butterworth High Pass Filter
Butterworth Fourth-Order Low Pass Subwoofer Crossover Filter Schematic
Fourth-Order Butterworth Low Pass Filter
Bandpass Crossover Filter Schematic
Bandpass Filter
C1 = .1040 / ( Rh x f )
L1 = (.1009 x Rh ) / f
C2 = .1470 / ( Rh x f )
L2 = (.4159 x Rh ) / f
C3 = .2509 / ( RL x f )
L3 = (.2437 x RL ) / f
C4 = .0609 / ( RL x f )
L4 = (.1723 x RL ) / f
Calculate Your Crossover Here

 To use this form, enter the impedance values (at the crossover frequency) for your speakers, and then enter the frequency you want your filter to crossover at. Now, click the "Calculate Values" button, and your results will appear in the empty text boxes.

Enter Tweeter Impedance Ohms
Enter Woofer Impedance Ohms
Enter Desired Crossover Frequency  Hertz
HighpassLowpass
C1 = Microfarads
L1 =  Millihenries
C2 = Microfarads
L2 =  Millihenries
C3 =  Microfarads
L3 =  Millihenries
C4 =  Microfarads
L4 =  Millihenries
 
Equalizers/Crossovers
Equalizers give you the capability to fine tune your system. It is virtually impossible to get speakers to reproduce sound perfectly. In a multiple speaker system things are even more complex because the different drivers interact with each other. With an equalizer you can boost or cut certain frequency ranges to tailor the overall sound to whatever you desire. Usually you go for more accurate reproduction and then add some bass for a more "dynamic" sound.

An electronic crossover takes a full range signal and divides it into different frequency ranges. The most common types are 2-way or 3-way. A 2-way crossover divides the frequency range in half at some cutoff frequency. All signals below the cutoff frequency are routed to a low pass pre-amp output and the rest are sent to a high pass output. These outputs can be connected to amps to dedicate those amps to producing only those frequency ranges. A 3-way crossover is similar but splits the signal into 3 parts. You can get a single box that has an equalizer and crossover. Electronic or active crossovers act on pre-amp level signals. They use the pre-amp level output of your head unit as input and their outputs go to your amp(s). By doing this you keep the amp from trying to amplify frequencies that you do not want (like high frequencies for a sub amp). On the other hand, a passive crossover acts on the signals after they have been amplified, they are connected after the amp and before the speakers. Usually these are just simple high pass or low pass units. You connect a high pass crossover to a speaker to block bass to that speaker. Some people call these bass blockers. You use a low pass crossover with a woofer so it only plays "lows."

Number of Bands in the EQ:
The number of bands in an equalizer tells you how fine an adjustment you can make. A 10 band equalizer breaks up the audio range into 10 parts and you can adjust the levels of any of them. The Q of an equalizer tells you how wide a range each adjustment makes. Let us say a specific band is labeled as 100 Hz. A high Q high equalizer will only boost or cut frequencies right around 100 Hz and not really affect signals at say 70 Hz. A low Q equalizer generally affects a wide range of frequencies even though it may be centered at one specific one. Typically, the more bands in the EQ the higher the Q so the different bands are not affected by each other. Simple bass and treble controls have the lowest Q. Equalizers with only few bands are good for making general adjustments but bad for fine tuning. A 30 band equalizer is great for making specific adjustments and tailoring the sound exactly how you want it. A tool called an RTA (real time analyzer) is used in setting those equalizers. It gives the system a flat signal (pink noise) and shows the user what the system returns. The user adjusts the equalizer until the RTA shows the desired response. The desired response is rarely flat because a flat setting results in dull, bass shy sound that is hard and edgy. Working with an experienced installer is key here.

Slope of the Crossover:
When any crossover splits the frequencies it is not a hard split. At the crossover frequency in a 2-way crossover both outputs will have this frequency in the output albeit at a lower level. How fast the crossover transitions from one output with rising frequency to another is called the slope of the crossover. There are many interchangable terms for crossovers. A 1st order crossovers transitions at 6dB/octave or 10dB/decade. A 2nd order one will transition twice as quickly. For tweeters a minimum of a 2nd order crossover should be use in order to prevent the tweeter from seeing any bass frequencies. 4th order crossovers are common and digital crossovers of any order are possible but expensive. Use at least a 2nd order crossover to be safe. For an example of tweeter safety, say we want to use a high pass crossover frequency of 2kHz. With a 1st order crossover (or filter) at 1kHz the level is only down by 6dB and only down by 12dB at 500Hz. 500Hz is way too low for a tweeter to play so this will probably cause the tweeter to distort or blow up. Using a 2nd order filter would have the output down by 24dB which would be a signficant improvement. 3rd and 4th order filters are even better but expensive. Also analog crossovers change the phase response so try wiring your tweeter out of phase to see if it makes the sound better or worse and leave it the way it sounds better to you.

Crossovers can also be made to have different types of response near the crossover point. Butterworth filters have smooth but slow response. Chebychev filters are quicker but have some overshoot. Details of these filters is beyond the scope of this page.
 

Equalizers give you the capability to fine tune your system. It is virtually impossible to get speakers to reproduce sound perfectly. In a multiple speaker system things are even more complex because the different drivers interact with each other. With an equalizer you can boost or cut certain frequency ranges to tailor the overall sound to whatever you desire. Usually you go for more accurate reproduction and then add some bass for a more "dynamic" sound.

Things to look for:

Number of Bands in the EQ: The number of bands in an equalizer tells you how fine an adjustment you can make. A 10 band equalizer breaks up the audio range into 10 parts and you can adjust the levels of any of them. The Q of an equalizer tells you how wide a range each adjustment makes. Let us say a specific band is labeled as 100 Hz. A high Q high equalizer will only boost or cut frequencies right around 100 Hz and not really affect signals at say 70 Hz. A low Q equalizer generally affects a wide range of frequencies even though it may be centered at one specific one. Typically, the more bands in the EQ the higher the Q so the different bands are not affected by each other. Simple bass and treble controls have the lowest Q. Equalizers with only few bands are good for making general adjustments but bad for fine tuning. A 30 band equalizer is great for making specific adjustments and tailoring the sound exactly how you want it. A tool called an RTA (real time analyzer) is used in setting those equalizers. It gives the system a flat signal (pink noise) and shows the user what the system returns. The user adjusts the equalizer until the RTA shows the desired response. The desired response is rarely flat because a flat setting results in dull, bass shy sound that is hard and edgy. Working with an experienced installer is key here.

 

Mounting location

Whether an equalizer is parametric or graphic, it can be mounted in the front of the car or trunk.  EQs are mounted in the front for easy adjustment of different types of music or songs, and are meant to be used by the owner.  These are usually octave or 1/2 octave graphic equalizers or four-band parametric equalizers.   Typical locations are close to the radio or hidden in the glove box or console.

More complicated equalizers are usually adjusted once and stashed away in the trunk or other remote location.  This is usually done by a professional, who adjusts the EQ according to the user's taste.  If you don't know what you are doing you should not play with a complicated equalizer set up by a pro.  Most often than not you will end up messing up the system.  Typical mounting locations are in the trunk, back seats, or hidden inside panels.

The most important aspects of mounting locations for equalizers is noise sources and accessibility.  Since equalizers work with low-level signals, they are prone to picking up radiated noise.  RCA wires should be routed away from car's computer and power wires (especially away from power wires going to amplifiers). They also need to be accessible for adjustments.

Connections

Equalizers are very easy to connect: Since active equalizers draw very little current, power and ground wires do not have to be as massive as amplifier's wires.  EQs also need a turn-on wire from the head unit.

RCA inputs and outputs should be carefully routed to avoid noise.  The main concern is not the low current power wires from the equalizer, but power wires from the amplifiers.

Adjusting

As soon as an equalizer is installed, it should be adjusted to 0 or "flat" response (no boost, no attenuation).  An equalizer is the LAST component in a system to get adjusted. 

 
Installation Accessories
There are many accessories that are available for your car stereo. Some of them are necessary such as RCA cables and others are optional like wiring harnesses.

RCA Cables:
These are the cables used to transfer pre-amp (line level) signals. Usually, you use this type of cable for unamplified signals such as the connection between your head unit and amp or crossovers/eqs. Look for appropriate thickness and shielding. The end connectors should be sturdy to prevent breakage from stress. Some claim that very expensive cable makes a difference in sound quality. I do not think so, do not spend more than $1/foot. You may need to spend more to get cables that are more immune to noise if you have that problem in your car.

Speaker Cables:
When the line level signal from the head unit is amplified it becomes much stronger and requires thicker cable to carry it. This is even more dramatic for subwoofer wiring. Use something between 8 gauge and 14 gauge wiring for subwoofers, nothing smaller.  A lot of current has to flow through those wires. For tweeters and midranges thickness is less critical but still important because appropriate shielding is still necessary to prevent leakage and contamination from outside sources.

Power Distribution Blocks:
These make wiring up multiple components easy. You can run one thick power cable from the battery to the distribution block and from there you can use its multiple outputs for each component. Some of them come with fuses which is an added bonus. A related item is a new battery terminal which allows easy connection of your extra power wire on the battery for the car stereo.

Capacitors:
Because music is dynamic, sometimes the power requirements of your system may be very high for brief periods of time. If you have a small alternator or battery your car's electrical system may not be able to keep up with demand. A "stiffening" capacitor can help this problem by providing extra storage capacity for those high power transients. Only buy one if you have problems, like head light dimming with the bass line in music!

Wiring Harnesses:
Are you planning on replacing your factory head unit but want to be able to put it back in easily? If so, a wiring harness is for you. These are specially designed for each car and allow you to disconnect your stock radio and connect a new head unit without cutting your original wiring. The wiring harness plugs into your existing wiring and allows you to connect a new head unit to the harness.
 

Recommended tools for working on car audio and security projects.  A bit of advice:  It is better to spend a bit more money to get a nice heavy duty tool that will last you a lifetime, rather than buying a cheap tool that will break easily.

Wiring Tools

Strippers Strippers:  Used to remove insulation from wires.  Spend the extra couple dollars and get a good set ($15 - $20).  The simple adjustable strippers are the best kind.  "Automatic" strippers and other more complicated models just tend to be to big end cumbersome to use in the cramped spaces of a car.
 
Crimpers Crimpers:  A basic tool all installers should have.  Get a good Professional-grade crimper ($15 - $30) from any reputable mail order catalog such as Parts Express.
 
Fish Wire Fish wire:  A piece of home electrical solid core wire (insulated).  Cut in different lengths, it can be used to pull wires under carpet and through firewalls.  Just tape up the alarm or stereo wires you want to run to the "fish" wire and pull.  Price: $1 - $2.  Available at any electrical supplies store, or hardware store.
 
Soldering Gun Soldering gun:  Not only used for soldering.   Can also be used for "plastic surgery" (melting plastic) and heating up metals. Can be obtained pretty much at any hardware store, electronics store, or Sears for about $30 - $50.  It is highly recommended that you spend the extra money and get a high powered model (100W and above)
 
Test Light Test light:  Used to test for power (be careful not to use it on sensitive electronics, or you might fry your car's computers).   Try to get the best you can afford ($1 - 30).  Available at hardware stores, tool trucks, Sears, catalogs, even dollar stores.
 
DMM Multimeter:  Used to check voltage, current, resistance, etc.  Use it on sensitive electronics where a test light could damage car electronic modules.  Price ranges from $20 and up.  Available at Sears, some hardware stores and most electronic supplies stores such as Radio Shack.
 

  
General Installation

Mechanic's Tools Mechanic's tool set:  Will be needed to remove screws, nuts, bolts, etc.  Sets available from Sears and other stores from $50 up to $1000 or more, depending on what you get.
 
Power Screwdriver Power screwdrivers:  Will save you a lot of time, especially if you do installs on a regular basis.  Most have reversible rotation, variable speed, and clutches to control torque.  It is highly recommended to get a good quality 9-volt or higher screwdriver.  Good power screwdrivers/drills start at around $100.  Most common brands are DeWalt (personal favorite) and Makita.
 
Angled Screwdriver Angled screwdrivers:  Ideal for getting in tight corners, where normal screwdrivers will not fit.  The famous Skewdriver Pro comes even with attachments and bits and is available for about $30 at most mail order catalogs such as Parts Express and Crutchfield.
 
Door Clip Remover Door handle clip remover:  Usually a piece of flat metal with an open hole that goes behind the window crank and pushes the clip out.   Cost: $15 - $25.  Available at car parts stores and specialty tool stores.
 
Hook Hook:  Used to pull panels and components, bend the tabs on DIN radio rings for installation and removal.  Available at specialty tool stores such as Sears.  You can make your own hook by grinding the end of an old screwdriver, then bending the end after getting it red hot with a torch.
 
Panel Removal Tool Panel removal tool:  Another must have tool for any installer.  A mutation of a screwdriver, fork and pry bar, used to remove panels, and pulling snaps off.  Cost: About $15 - $20.  Available at auto parts stores and specialty tool stores.
 
Pry Tool Pry tool:  Small flat screwdrivers are a bit sharp, and easily scratch plastic.  You can either grind a small flat screwdriver, so that the edges are rounded, or better yet, make your own tool using an old antenna rod and a grinder.
 
Drill Drill:  Another essential tool.  Used for audio and security installation.  Good drills run $40 and up.  Available at any store.
Knife Knives:  A basic utility knife and carpet knife are a must have for any installer.  Get good contractor-grade knives. ($10 - $20) available at any hardware store.
 
Dremel Tool Dremel tool:  A rotary tool that has many attachments used to cut, polish and grind metal, wood, plastic, etc. It ranks high on the list of essential tools.  A kit with carrying case and attachments sells for $50 - $60. Attachments such as cutting wheels can get pretty expensive, but well worth it.
 
Metal Snips Metal Snips:  They look like scissors, but are used to cut sheet metal to enlarge speaker holes.  There are 3 types:  Left, right and straight cut.  Each runs about $15 - 25.  Available at any store where tools are sold.
File Files:  Used to smooth out cuts and enlarge holes in panels.  Get at least a round file and straight file.  Price:  $10 - 20, available at any tools store.
Hot Melt Gun Hot glue gun:  An essential tool.   Electric or battery powered.  Glue is heated and get fed via a trigger.   You can get one for $10 - $30 at any hardware or crafts store.  The glue sticks are a lot cheaper at crafts stores than in hardware stores.  Usually the bigger the gun the better.
 

  
Tweaking

Phase Tester Phase tester:  A CD is played on the system's head unit that generates a series of polarity pulses.  The tester is a small device that is placed in front of the speaker and shows if the speaker is hooked up backwards.   Prices range.  Monster cable has a polarity checker available for about $120.
 
Home-made Phase Tester Phase tester (homemade):  A cheap alternative.  All you need is a 1.5-volt battery.  If you want to get a bit fancy, you can get a small case, battery case, LED, switch, some wire and a couple alligator clips.  You can make one for about $15.
 
SPL Meter SPL meter:  Used to check sound pressure levels (how loud a car gets).  Very helpful for system tuning and adjusting.   Radio Shack sells digital SPL meters for about $60.  Better (and more expensive) brands can be obtained at other electronic supplies stores at higher prices.
 
RTA RTA:  An installer's dream machine.   Figures out frequency response of a system.  It sends a signal with equal energy at all the frequencies (pink noise), and measures the different pressure levels at certain fixed frequencies.  Price ranges from $800 (the famous PC RTA) up over $1000 (Audiocontrol, etc). Available mainly directly from the manufacturers.
 

  
Box Building/Wood Working

Circular Saw
Jigsaw
Saws:  Table saws, miter saws, etc., are nice, but can get very expensive.  A basic Jigsaw ($30 - $150) and circular saw ($30 - $200) will get you through all the wood, plexiglas and metal cutting you will need to do. Even if you get a cheap saw, make sure you get professional-grade blades that will give you fast and smooth cuts.  Available pretty much everywhere.
 
Palm Sander Sander:  An orbital or palm grip sander will simplify polishing and sanding duties greatly.  A DeWalt or similar brand sander will cost about $70 to $100.  Available at any hardware store.
 
Router Router:  Very useful tool to make moldings and trim panels.  When serious woodworking and plexiglas shaping are required.   Routers range between $50 and $200.  Available at hardware stores everywhere.
 
Power Stapler Stapler: Whether it is a manual, electric or air powered stapler, it is an essential tool when carpeting and vinyling panels and boxes.   Cost is $15 - 20 (manual), $30 - up (electric), very expensive (air).
Heat Gun Heat gun:  Can get as hot as 1500 degrees. Used for molding plastic and laminates, stripping paint and heating up heat shrink tubing, among other things.  Very useful for making custom plastic panels and bending plexiglas.  Available at paint stores, specialty stores and mail order catalogs such as Parts Express.  Heat guns run anywhere from 50 to 70 dollars, or more, depending on what nozzles you purchase with the gun.
 

If you are working on you car stereo or security system, you will most likely have to remove some panels, consoles, trim rings, etc. Factory panels are not always easy to remove. If you break a panel, you will regret not being careful. Dealers charge a fortune for parts.

Before you even think about pulling on a panel, make sure all the screws and other fasteners have been removed. If you can't figure out how to take a panel out, get help. Borrow a manual for your car at the library, ask a car stereo shop in your area or ask a local car dealer.

If the car is out in the cold, panels tend to get hard and brittle, and may break easily, particularly in old cars. Try heating the panel(s) up before you remove them with a hair dryer or heat gun.
  

Most panels are mainly held in place by screws, snaps, other panels that overlap them, and any combination of the three:

Radio Trim Rings

To make cars cosmetically appealing, manufacturers hide screws behind "dummy plates" and electronic controls. In many cars you have to pull out clocks, hazard light switches, defroster controls, etc., to get to the screws that hold the panel. If you need to remove a switch or instrument in a panel, don't just insert a flat screwdriver on the side and pry. This will bend and scratch the panel. Try pulling the desired part with a hook. If you have no other option than to pry, place a cloth on the screwdriver to prevents scratches.

Many radio trim rings use snaps, either by themselves, or in combination with screws. Double check to make sure you did not miss any screws. Pull evenly on the panel, either using a panel removal tool, or a hook. If the piece is too tight, there might be a screw somewhere you might have left out. In many cases, such as most Hondas, you don't even need to take the trim ring out at all to get to the radio, just remove a couple screws that hold the radio from behind.

Consoles

Relatively easy to remove. Ninety-nine percent are held in place by bolts and/or screws. First, take all the stuff out of pockets, boxes, compartments, ashtrays, etc. Remove all visible screws. If the console does not pull out, search for hidden screws. Many cars (especially European) use a piece of carpet to cover up screws. Cars such as Mercedes Benz have screws hidden under the ashtray. The parking brake is a common obstacle. In some cars you might have to slide the front seats all the way back and recline them to get the console out.

Dashes

Some people remove the whole dash to hide alarm components, and access electronic devices in the car. These people are experienced. Removing a whole dash takes many hours and patience. If you are not careful when reinstalling the dash, wires might get pinched and you might smoke something. Remember that the electronics around the dash control the main functions in your car, so you can't never be too careful here. Most cars have a clip that has to be pulled out in order to remove the speedometer cable from the instrument panel. Before you take anything apart, unhook the car's battery (this is good practice when you are working on your car in general). Find hidden screws and bolts by "peeling" off panels. Unhook electronic components and harnesses as you go along. Mark things if necessary for reassembly.

Seats

Most front seats are held by bolts and nuts. Some cars have extra brackets or seatbelt anchors that must also be removed. Many newer models have pieces of plastic or carpet over nuts and/or bolts holding the seats for cosmetic reasons. These pieces can be easily removed using a panel removal tool, or taking screws out (if they have any). Before you pull the seat out, be careful to unhook any wires plugged up to the seat, and take extreme care not to scratch anything while you take the seats out of the car. To make life a bit easier when remounting the seat, first slide the seat all the way up, remove the bolts on the back. Slide the seat all the way back, make sure the seat is locked in position, and then remove the remaining bolts at the front.

Rear seats are fastened in many different ways. On most cars, the base part of the seat is held in place by a metal snap going into a hole. To remove, pull on the front of the seat. Some cars have a metal or plastic tab that has to be pulled, pushed, or moved to the side, while pulling on the front of the seat. Other cars, mainly German, use bolts or screws in the front to hold the base of the seat. Many American car seats (GM) have a hook that fits into a metal brace. To remove the bottom part of the seat push hard towards the back and then up. Most Hondas use a bolt (10MM) on the back part of the seat between the bottom part of the seat and the back support (towards the middle) that has to be removed. Then the seat can be pulled up from the back. Before you pull on a seat, try to analyze what is holding it. Most seats do not need a lot of force to be removed, they all have a trick.

The back support on the rear seat is a bit more standard in the way it is fastened. At the top, there are 2, 3 or 4 pieces of metal that go into a hole. There are 2 or more bolts that hold the backrest at the bottom. Once you have removed the bottom part of the seat, take the screws or bolts out, and slide the back rest up and out. On a few cars you have to remove the rear deck and other side panels out first. If you can't figure it out, remove the panels in the other side of the backrest (trunk) and examine carefully how the seat is fastened.

Door Panels

A bit harder to remove than the rest of the panels in a car because they house window cranks, buttons, mirror controls, speakers, etc. Some cars even have seatbelts built in the doors. The first step is to remove all the screws on plain sight. Look for screws hidden behind speaker grilles, power window/lock/mirror controls, ashtrays, interior light covers, dummy plates, etc. Windows all the way down help a lot during removal and reinstallation.

If your car has manual windows, use a crank clip removal tool to get the clip out. Pull the crank out. Since clips holding the cranks are small and thin, they tend to fly away and get lost. Some cars (mostly VW) hold the crank in position with a bolt, hidden behind a plastic cover. Other cars (i.e. old AMC and Cherokees) use a crank that snaps in place. Once you have removed all the obstacles (in some cars such as Isuzu this even requires removing the speakers), try to see how the panel is ultimately held in place. There are two basic systems:

- Snaps (most cars, especially imports), which are best taken care of with a panel removal tool. Sometimes snaps break from the panel and stay on the car. Remove them from the door with a panel removal tool, and reattach the snaps to the door panel before reinstallation. Once you get everything loose, most panels need to be pulled out at the bottom and then up.

- Hooks (some Fords, i.e. Thunderbird and GM, i.e. Camaro), in which the panel has to be pulled up first and then out.

Rear Decks

Rear decks are not fun to take out. Most involve removing the back seat and backrest, side panels, seatbelts, speakers, etc.
The best way to remove a rear deck is to follow these guidelines: Remove snaps using a panel removal tool. Remove third brake light casing, if necessary. Remove other obstructions such as speaker grilles, speakers, seats, panels, seatbelts, etc.

Trunk Panels

Manufacturers do not take much time trying to hide screws and snaps on the trunk/hatch. That makes trunk panels fairly easy to remove. Most are held by snaps, screws, or a combination of both. Again, the procedure is to remove any visible screws and snaps. Search for hidden screws under dummy plates, access doors and light bulb covers. On some hatchbacks, speaker grilles, speakers, seatbelts, even the back seats need to be removed to clear the way for the panels to come out.

Kick Panels

Probably the easiest to remove, due to their small size. Most manufacturers use bolts and/or snaps. In some cases, such as old BMWs, the speaker grilles hold the kick panels. The most annoying obstruction is generally the hood latch popper.
  

Repairing broken panels

Even the pros break a panel or a snap every once in a while (professional installers are very good at repairing broken panels). If you cracked a panel, there might still be hope. A hot melt gun is a must have here.

Since most panels are made out of plastic, it is fairly easy to fix cracks and breaks. One of the best techniques is to cut a piece of metal from a paper clip, and dig it in the plastic for support. Here's how to do it: First place the panel to be fixed upside down on a flat surface (over a cloth, so that it does not get scratched). Cut a piece of metal from a paper clip (about an inch long or so). Place the piece over the crack (again, on the back side of the panel) and hold it in place with a flat screwdriver or pliers (NEVER with your fingers). Use a soldering gun to heat the metal, applying a bit of pressure so that the clip melts its way in the plastic as it gets hot. It is better if you start on one side, and then work your way to the other side of the crack, don't try do it all at once. Be very careful not to push the clip all the way through to the other side of the plastic, you don't want anything showing on the front side of the panel. It is highly recommended that you practice a couple times on a piece of scrap plastic before you attempt the actual panel. When you are done with the soldering gun, clean the tip with a wire brush. The left over burnt plastic will not let it hold solder very good.

Another technique, which can be used in addition to the one previously mentioned or by itself, is to use a hot glue gun and pieces of either plastic or wood: Prepare the panel in the same way as before, but instead of placing a clip over it, spread some hot melt over the area, then place a small piece of wood or plastic, and add some more hot melt. Let cool down a couple minutes, and add glue on top as many times as needed. Make sure that the panel will fit in the car before you do this. Hot glue can also be used to attach broken snaps, and to build custom panels.
If you do break a panel and can't fix it, try a junkyard before you go to a dealer.

 

Calculating Impedance Loads

 This calculator is intended to help you determine the impedance of a speaker or subwoofer wiring configuration. There are many ways you can connect multiple speakers together. The most common and best way is to connect them in parallel. You do this by connecting the positive speaker wire from the amp to the positive terminal on the first subwoofer. In the picture below, one channel is shown.

Speakers Wired in Parallel
 
Then, you connect the positive from that first subwoofer to the positive of the next subwoofer. You can connect as many speakers together like this as your amplifier can handle. Before you attempt this, be sure of what your amplifier can handle. If it says it can handle 2 ohms stereo, don't connect more than four 8 ohm woofers or two 4 ohm woofers per channel. If you ignore this warning, you may destroy your amp/speakers, or cause a fire, so unless you are absolutely sure your amplifier is capable of handling lower impedance loads, don't try it!
 
Formula for Similar Impedances
 

The formula above is for multiple subwoofers with the same impedance.

Example: You have three eight ohm woofers. Inserting the variables in the formula gives you the ratio 8/3. Eight divided by 3 gives you approximately 2.66 ohms. If your subwoofers were four ohm versions, you would get an answer of 4/3 or 1.33 ohms. This would cause a problem for most amplifiers. There are some high current competition amplifiers that can handle these lower impedances, but in general most amps won't run with this load. If you are not sure, contact the manufacturer of your amplifier. This also the same way you wire an amplifier in mono except in a mono wiring schematic you usually use the positive of one channel and the negative of the other channel. When you use a mono configuration, the impedance load at the amp is half as much.
 

Explanation of Terms
Rt is total impedance.
 
Calculate
 To use this form, enter the impedance of one driver and the quantity of drivers you intend to use. Then just click the "Calculate Values" button, and the answers will appear in the empty text boxes.
Enter Impedance Ohms
Enter Number of Drivers 
Stereo  Mono

 Ohms
 
 

When we listen to a home stereo, we have ideal conditions: A quiet environment, speakers pointing to the "sweet spot" on the same axis, etc.   By properly aligning the speakers, and designing a good crossover, a sweet spot can be achieved on a car with as good quality sound, staging and imaging as a high-end home stereo.

There is still one thing that is different between a car and a home stereo listening room: The background noise. There are all kinds of exterior (road noise, rain hitting the windshield, etc.) and interior (rattles) noises that draw attention away from the music in a car. To make up for the road noise, we simply turn the stereo up louder.

Even though it is impossible to eliminate the noise completely in a car. There are products that will decrease the noise floor a great deal, particularly on non-luxury cars.  Reducing the noise in a car will make a big difference in the audio system's performance and overall ride comfort.
   

Liners

Tar mats and similar products such as Dynamat are used to reduce resonances in metal panels. A car lined with a mat will have a much lower road noise. To add a mat liner to a car, seats, carpet, door panels, etc. have to be removed. With the help of a heat gun, and a small wallpaper roller, the material can be laid over door panels, floors, wheel wells, etc. A cheaper alternative to Dynamat, as mentioned in the rec.audio.car FAQ is a product used by roofing contractors called “Ice Guard”, which has an adhesive backing and works the same way.

Sprays

There are products such as Rockford Fosgate’s Noise Killer Blue which are sprayed to the panels. They are used in places where a liner can’t be applied such as inside doors, trunks, etc. Most of those products are applied in the same way as paint: Either sprayed or with a brush. There are two types of sprays: Some need an air compressor and a spray nozzle and the others already come in a spray bottle such as Stinger's RoadKill.

An alternative is rubberized undercoating which can be obtained at any major car parts store. It comes in a spray can and is easy to apply. The only drawback is that it is very sticky and messy. Could be used for the inside of the doors or places where it won't come in contact with carpet or fabric.

Expandable insulation spray foam is used in homes to seal around pipes and fill up holes in basements. In a car, it can be used in irregular surfaces where tar mats can’t be applied, such as the trunk, trunk lid, etc. To apply, clear the area from fabric, panels, etc. Once the foam dries (about four hours), cut excess off with a long knife.

Adhesive Strips

Used for home door insulation. A strip of foam with an adhesive material on one side, used to seal between the door and the door jamb to keep air from escaping the house. Apply between panels, behind license plates, etc. Quick, inexpensive and easy way to get rid of annoying rattles.

Another product that can be placed between panels to cover larger areas is carpet padding, available at any carpet store.
  

Damping a Door: Step by Step Instructions

A combination of three products will be used in this case: A spray noise damping material, spray rubberized undercoating, and a tar mat. This allows for maximum noise isolation.

Step 1
rattles_step1.jpg (8638 bytes) After carefully removing the door panel, parts not to be sprayed were protected by masking tape and paper. Parts inside the door, such as lock mechanisms, window rails and power window motor were protected using aluminum foil.
The inside surface of the door was prepared by drying moisture and cleaning the surface with a solvent.
The inside of the door was sprayed with one can of Stinger Road Kill, being careful to apply an even coat throughout the door.
  
Step 2
rattles_step2.jpg (8539 bytes) Since doors always get wet and RoadKill is a water based product, rubberized undercoating was applied to seal it off.
After letting the door dry for 24 hours, the rubberized undercoating was applied over the RoadKill Spray.
On parts of the car that don't get wet, this is not really necessary. An alternative is to apply only undercoating inside the doors.
  
Step 3
rattles_step3.jpg (12893 bytes) Masking tape, paper and foil were removed.
The surface of the door was cleaned to ensure good adhesion of the tar mat material.
Without removing the adhesive backing, the mat was measured and cut in the approximate shape of the door.  The mat was then heated with a heat gun to make it more malleable and help the adhesive stick better.
  
Step 4
rattles_step4.jpg (10429 bytes) The backing of the mat was removed to expose the adhesive. After lightly placing the mat over the door, cuts were made to accommodate wiring, and lock mechanisms that hook up to the door panel.
A 1" wallpaper roller was used to make a good bond between the mat and the door. The heat gun was used to help shape the mat to the door contours.
  
Step 5
rattles_step5.jpg (9130 bytes) The factory water shield was placed over the mat.
This is not necessary when the mat covers the door completely, but it is better to do it this way to assure that no water gets in the door panel through holes, protecting electronic components, and giving the door a factory appearance.
The controls in the door panel were connected, and the door panel was reinstalled.
 

Step 1:  Car Preparation

Protect upholstery, carpet and panels by lining up with plastic and masking tape.   If any resin gets to a seat or carpet, there is no way to get it out.  Take the extra time to make sure any potential spill won't cause damages to your car.  Make sure you will not interfere with clutch, hood release or other mechanical parts.
  

Step 2:  Making a Mold (Back Piece)

If you are mounting the pod to a flat surface, such as a door panel, then all you have to do is to make a template out of cardboard.  Use the template to transfer the contour of the area to a piece of particleboard or MDF.  This is the back of your pod.

If you are not lucky enough to have a flat surface to work with, you need to make a fiberglass back piece.  For example, let's say you are building the pods to go in the corners of the floor, between the firewall and kick panels:

  • Line the area to be molded with aluminum foil.  This way, the mat won't stick to your car or liner.
  • Cut a piece of fiberglass mat/cloth.  It should be at least a couple inches bigger than the final pod size.
  • You can either apply the resin to the mat with a paint brush, or dip the mat in the resin.
  • Place the wet mat on the surface, let dry to hold shape.
  • If the mat won't stay, or sticks to your gloves, use a paint brush.  Sometimes masking or duct tape will help keeping the mat in position for curing.  Fine metal mesh or chicken wire could help hold the fiberglass in place for curing when building complicated shapes.
  • Let the piece cure.  If once the piece has cured it is not hard enough, you might need to add one or two more layers of fiberglass.  You can do this on a workbench.   Remember, you are only trying to get the shape here, the back piece does not have to be rock-solid at this point.

You now have the back piece of your pod.  Other options is to cover a factory panel with cloth, add resin to the cloth for hardening, and use the factory panel as part of your kick panel.
  

Step 3:  Baffle (Front Piece) Fabrication

The best material to work with is MDF.  If the enclosure is for low energy applications (such as mids and tweeters), a couple layers of 1/4" plywood would work.

With a jig saw, drill and router, you can build the baffle and mold it to accommodate your speakers.  Carefully plan the layout, speaker mounting configuration and grilles.
  

Step 4:  Speaker Positioning/Aiming

Once you have the baffle, connect wires to the speakers.  Mount the speaker(s) on the baffle.  Using metal braces, pieces of wood, etc, connect baffle and back part together to make a "skeleton".  Metal braces are sometimes better because it is easier to re-aim.  At this point, you don't care what it looks like. If you want, add some cloth to create some kind of a box effect.   Don't worry about the back part of the speakers being semi-exposed.

Start by aiming each pod to the opposite side, at ear height.   From this starting point, play around with different aiming angles for best results.  If you are competing, make sure good results are achieved from both front seats.  If you don't care about passenger's side much, optimize aiming at driver's side.  This is the most important part of the whole process, and may take weeks of critical listening to get ideal angling.  Keep in mind that at this stage you want to optimize staging and imaging, not sound quality.  The speakers will sound a lot better once the pods are closed off.
    

Step 5:  Joining the Baffle with the Mold

Once speakers are aimed for best sound, remove speakers from baffle.   Trim bottom part to desired size.  There are different techniques to shape and wrap baffle and mold.  A lot of people wrap the front and back "skeleton" with fleece cloth.  Resin is applied to the cloth and left to harden.  This is good for concave pods, but for rounded pods you might need to try a different approach:   Fill in areas with a material you can remove later such as paper towels or foil.   Apply first layer of fiberglass.  It doesn't have to be perfect, just cover the intended volume with no major protrusions.  Let it harden. 

Several layers will need to be added afterwards.  How many depends on how hard you want the enclosure to be, and whether you are using fiberglass mat or cloth (cloth is thinner).
  

Step 6:  Smoothing the Pods

Once you have a nice strong surface, add auto body filler (i.e. Bondo®) to round surface off.  Let dry and sand.  This process will have to be repeated at least twice, depending on finish desired and what you are using to cover up the pod.  At the beginning, power tools can be used for sanding, but last steps might require hand sanding.
  

Step 7:  Finishing

After you have a smooth finish, cover up the pod with vinyl, carpet, etc.  Build grilles out out wood and metal mesh, run wires in and seal with silicone or Liquid Nails (glue), fill enclosure with polyfill if desired and mount speakers.

Make sure you safely secure the pods to your car.  Best option here is to use hidden metal braces, or run screws from the inside of the pod to the car.   Enjoy!

 

Do you really need to use fiberglass?

Working with fiberglass is a very messy and time consuming process:   Prepare area, lay fiberglass, wait for it to dry, sand/cut if necessary, lay more fiberglass, wait, sand, an so on.  Once you are done with fiberglass, repeat the process with Bondo (car body filler) for finishing:  Apply, wait, sand, reapply, wait, sand.  It might take several days, even weeks to do a nice set of kick panels, subwoofer box or amplifier rack.

If possible, try to determine if you can use an alternate material such as wood and then  shape using Bondo.  Keep in mind that fiberglass is strong when bent.  Straight fiberglass panels have to be very thick (read: time and money) for adequate rigidity.  Sometimes a combination of wood, MDF or particleboard (for large flat sections) and fiberglass (for round, odd sections) works best.

Materials

  1. Fiberglass mat or cloth, resin and hardener.
  2. Bondo (body filler), hardener.
  3. Box of disposable gloves, respirator, protective clothing.
  4. Paint brush, plastic sheeting, aluminum foil, mold release or WD40.
  5. Tools such as sander, multi-purpose shears, screws, etc.

For small projects, such as small amplifier racks or small kick pods, you can buy all the supplies at a car parts store such as Trak Auto.  Products can be found at the "body repair" aisle.  For bigger projects, supplies can get pretty expensive.  Boat supply stores sell products in larger quantities, but at lower overall prices.

Safety

Fumes and dust particles are a very important concern when working with fiberglass.  Get a respirator or a dust mask designed to work with fiberglass.   Wear gloves at all times when handling fiberglass and resin, or sanding.   Protect ALL exposed skin, especially when sanding.

Work in an open area!  Resin/hardener mixture fumes are bad for your health.  If you work with resin indoors, the smell will remain in the area for days.  Do not handle fiberglass mat or sand dry fiberglass indoors.  It causes rashes and itching.

Read instructions and warning labels carefully.

Car Preparation

Before any work starts with fiberglass, plan the whole project.   Look ahead into how you are mounting speakers, components, fastening the panels, panel finish, etc.

Once resin falls on carpet, upholstery, or other parts in your car, there is no way to get it out.  Cover areas to be worked thoroughly.  If possible, remove panels, seats, carpeting, etc in case an accident does occur.

Cover area to be molded with fiberglass with aluminum foil.   Fiberglass can be laid over the foil and once it dries, foil can be easily peeled off.

Making a Mold

If you are creating a shape in "mid air", you need to make a mold first.  There are different options available.  Some people like to make a frame out of aluminum foil and/or chicken wire.  Other people use modeling clay or shape dried spray expanding foam.

Another option is to make a "skeleton", shape it with cloth and then fiberglass over it:  Make a top and bottom part out of fiberglass, wood, plastic, existing car panels, etc.  Join both with wood or metal braces.   To fill the gaps, glue or staple sweatshirt material or pantyhose.  Apply resin to the cloth or pantyhose.  Once they dry, lay fiberglass over it.

The third option is to use an existing shape, such as a spare tire hole in a trunk.  After removing factory panels and carpeting, apply mold release, aluminum foil or WD-40 to surface (to avoid fiberglass from sticking).  Lay fiberglass, and let dry.

First Layer

First, mix resin and hardener.  Only mix what you will need.   It takes a lot of practice to get the resin/hardener ratio right.  Too much hardener and it will dry right away, too little and it could take several hours.   Temperature in work area also influences drying time.  The hotter the temperature is, the quicker resin will dry.  Also, keep in mind that resin will get warn when drying.

Cut fiberglass mat to size.  It is better to cut a bigger size than what you need for the first layer.  You can always trim excess off when dry.

There are two ways to "wet" the fiberglass mat:  By dipping it in the resin/hardener mixture, or by applying resin with a brush.  In most cases, it is easier to dip the mat.

Once you have a wet mat on your hands, place it on the area that will "shape it".  If you are a beginner, this might be a bit tricky.   The mat will tend to stick to gloves and other stuff you don't want it to.   Spraying some WD-40 on your gloves will help a bit solving this problem.  This first layer would become the foundation of the piece you are building.

Additional Layers

Once the first layer is dry, remove it from the car.  In most cases there is no need to work inside the car for subsequent layers.

Cut and sand excess fiberglass from fist layer, clean dust.   Add next layers in same fashion as fist layer.  Try not to have any gaps or bubbles between layers.  You can use a cheap 1" brush to help get rid of bubbles.  Do not worry about imperfections at this stage, you just want a rough shape with no major protrusions.  All gaps and imperfections will be fixed at the last stage.

Shape and use of the object will determine amount of layers required.  For kick panels, 3 to 4 layers is usually enough.  Subwoofers boxes require more layers.

Bondo Stage

Once you have a defined shape, no major holes and a pretty sturdy piece, you need to smooth out by sanding rough edges.

Bondo is very similar to fiberglass.  Just add a few drops of hardener and drying process begins.  Spread Bondo over you panel.  Try to fill in gaps and valleys.  Do not worry about smoothing it out much.   Once Bondo dries, sand.  Repeat the process as many times as necessary:  Add Bondo, let dry, sand.

On the first steps, a power sander can be used to quickly remove excess material.  On finishing stages, manual sanding might be required, depending on finish desired.

Finishing

Finish smoothness depends on what material you are using to cover the piece up.  Carpet is very forgiving when it comes to imperfections.  Vinyl is less forgiving, you need a pretty smooth surface (a couple extra steps of Bondo might be required).  If you are finishing with paint, then you do need a perfectly smooth surface.

 

It is fairly simple to build custom panels to cover up amplifiers, processors, and other components like the pros do. All you need is to follow the step by step methods presented here, some woodworking skills, and some basic tools such as a circular saw, jigsaw, router, sander, drill, hot melt gun, a very sharp knife (carpet knives work great), stapler, etc.

Most panels follow the same construction process, no matter what materials you use: Making a template, building the frame (base), raising (if necessary), filling, smoothing and covering the panel.  For example if you are making a door panel, you would first make a frame out of wood, then round off the edges by filling them with Bondo and covering up the whole thing with vinyl.
  

Step 1: Making a template

One of the easiest (and cheapest) methods is to use cardboard for the template. For example, you are making a one-piece panel for the trunk, that will go around a subwoofer and a couple of amps. Cut a (straight) end of the cardboard box and place it next to an amp. Keep cutting pieces of cardboard and gluing them with hot melt over each other to make a big template consisting of glued pieces of cardboard. This way, instead of figuring out the shape of one big piece, you will have to figure out the contour of a small part of the whole panel at a time.

Once you come up with a template, make sure you have no gaps. If you have to bend the template to get it in and out, you will have trouble with the final panel, since the panel will most likely not bend. You might want to split the design in two or more panels. Plan ahead of time what material you are going to use to cover up the panel, and subtract the thickness of the material from the panel. For example, if you are using vinyl, you might want to reduce the outline of the panel by 1/8", but if you use carpet, maybe 3/8" will work better.

Place the template over the material you are cutting. Trace the outline with a pencil.  Cut using a circular saw on the flat parts, and a jigsaw on the curved parts. Use good saw blades to get smooth cuts (this will save you some work later).

After the panel is cut, take it back to the car and make sure that it fits, check that whatever material you are using to cover it up fits in between the gaps. If you took the time making a good template, then the panel should be a pretty good fit.

Step 2: Raising the panel

If you are making a flat panel, then go to step three, if not, keep reading. On this panel we are making as an example, we are planning to raise edges around the panel to outline the amps. Make a square "ring" around a hole and screw it to the main panel. Then, make another ring (this one smaller outside), and screw it on top of the first ring. Keep going until you get to the desired height and shape. (It is a lot easier to do various 1/4" levels, rather than trying to shape one thick piece of wood. You only have to get an approximate shape. The gaps will be filled on the next step.   Fiberglass can also be used here.

Step 3: Filling and smoothing out edges

On the previous panel, there are gaps in edges of the layers of rings. Fill the gaps using a material such as Bondo. Try to get a shape as close as possible to what you want. Once the filling material is dry, sand the panel down.   Additional layers might need to be applied for a good finish.

If you are using a thin material to cover up the panel, such as vinyl or grille cloth, then you need a very smooth surface. On the other hand, if you are using carpet, or padded vinyl, then you don't have to worry about sanding the panel down too much.

Step 4: Covering the panel

Most of the time, spray glue is the adhesive of choice. You can get a can of 3M (or similar) spray adhesive at any hardware store for about $10. Cut the covering material (i.e. vinyl) around the shape of the panel, leaving at least a two-inch overlap. Place the covering material upside down and spray the glue over it. Also spray the panel. Let sit for about a minute, and place the panel on the covering material, stretching the fabric over the panel. Use a clean rag on the surface to make sure the whole top of the panel is in contact with the fabric. Flip the panel over and spray glue on the overlap, and edges of the panel. Cut the overlap as you go to fit edges and corners. It is good practice to staple the fabric on the underside, since glue will sometimes not hold very good when the car gets hot.

Figure on your initial design how you are going to hold the panel. You do not want to have screws messing up your panel's finish.  Pressure fitting requires a bit of skill, but is the cleanest way to go.
  

Materials

If you are using the panel just to cover up an install, the you don't need a 3/4" particleboard panel, adding more weight to your car. Plywood will do just fine. On the other hand, if you are building a sub box, you don't want to use anything that will bend. Common panel materials are particleboard, MDF, plywood, various metals, Plexiglas and other plastics, even fiberglass.

To fill gaps and smooth out surfaces, you usually want a material that will adhere to the panel extremely well, and will be hard. Most installers use Bondo (car body filler), epoxy (for small fixes), and fiberglass.

There are many materials used to cover up panels. The most popular are carpet, vinyl, leather and various types of fabrics ranging from speaker cloth to velour. You can also add padding under the cover material.

There are many materials and finishes available. It just takes practice to see what material is best for each application. Even though there is no single way to build a panel, following the above guidelines will give you an idea of the basic process. The rest you will just have to figure out by trial and error. Even though whole panels can be build using simple tools such as handsaws and sandpaper, power tools will greatly reduce fabrication time, especially when sanding.

 

You have finally hooked up all your sources, processors, amplifiers and speakers.  Now it is time for one of the most critical aspects of the installation: Fine tuning your system (tweaking).  Tweaking is a very long process, especially if you have many channels of amplification.  Take your time to get everything set for optimum performance.  Professionals take days, even weeks to set a system up.

1. Get rid of noise

Make sure your system is 100 percent noise free (see the "alternator noise" section for more help).
  

2. Check speaker polarity

To make sure all your speakers are in phase, unhook the speaker you want to test at the amp (both wires preferably). Using a 1.5 volt battery (any size), touch the positive terminal of the battery to the positive wire going to the speaker, then do the same for the negative wire.  Have a friend look at the speaker.  If the speaker pops out, the polarity is correct.  If the speaker pops in, the speaker is hooked up backwards (out of phase).  To fix this, simply reverse the wires when hooking the speaker back to the amplifier.  A word of caution here: DO NOT hold the battery power to the speaker for more than 1 second, all you want to do is to see if it pops in or out.  You will damage the speaker if you hold constant power to it.   Do not use a higher voltage.  Also, do not try this test on tweeters, you could fry the voice coils.  If there are crossovers with capacitors along the line, this test will not work (capacitors block DC voltage).  Bypass the caps momentarily.

A much more elegant and quicker way to do this is by using a commercially available polarity checker, which uses a test CD.  All you have to do is pop the CD in the head unit and hold the polarity tester in front of each speaker.   The advantage here is that you can test for absolute polarity of the system on all the speakers, including tweeters.  Polarity checkers are available from various companies such as Monster Cable.  Retail for the Monster Cable polarity checker is about $120.

Sometimes, when speakers are not mounted close to each other (i.e., mids on the doors and tweeters up in the dash), reversing the polarity on tweeters or mids makes the system sound better because it makes up for phase differences due to distance.   Try different combinations and see what sounds better.
  

3.  Get a clean signal

The third step is to set all your sources and processors "flat".  Turn the loudness off.  Set the bass, mid and treble controls on the radio to 0.  Set all EQ bands to 0dB.  Defeat all bass and treble boosts, etc.  Set the gains on all the amps and processors to the middle.  Balance and fader should also be in the middle.  By now your stereo should sound pretty good.   If not, check your installation.  EQs are not designed to compensate for installation flaws.
  

4.  Setting Gains for max. power and min. distortion

Start with a high level signal at the first components of the chain.   This will reduce noise and give you more headroom.  Try to start with a head unit that has a high voltage signal.  With everything still flat, set the amplifier and processor gains. Pegging the gains on amplifiers or any other processor all the way up will most likely introduce clipping (distortion) in your system, which damages speakers.

RCA.gif (1168 bytes)The best and quickest way to set gains is to use an oscilloscope.  By using a scope, you will be able to get the maximum possible power without distortion.  Make a probe adapter using a male and female RCA ends (see figure).  Splice a wire in the positive (center) and one in the negative (outside).  Insulate exposed wires independently.  To probe a channel, simply unplug the RCA from the component, plug the RCA to one end of the "probe" and plug the probe to the component.  Hook up the scope's probe to the two wires you spliced.

clipped.gif (1600 bytes)Once you probe is hooked up, you need to pop a test CD with different test tones such as the Autosound 2000 amplifier setting CD.  Make sure the tones are at 0dB reference.  Use a frequency in the middle range of the crossover.   For example if there is a crossover before the amplifier that lets frequencies from 100 to 3000 Hz pass, use a 1000 Hz test signal.  For subs try about 40Hz.

Start with the head unit.  Raise the volume up until you see clipping.  Set the head unit at the maximum volume before clipping and leave it there for the remaining of the gain setting procedure (if it is too loud, turn the gains on the amp(s) down or unhook the speakers).
Try the output of the next component down the line.  Again, turn the gain control up until you get clipping.  Keep setting controls until you reach the amplifier outputs.   Be careful not to fry the voice coils on the speakers.  A sine wave requires a lot of effort for a speaker to reproduce.  Even tough the speaker's impedance will affect amplifier output, it is wise to sometimes unhook the speakers for testing.  If you change a component, it would be wise to readjust the system's gains.

If you don't have access to an oscilloscope, you can do the adjustments using test tones and your ears.  First, listen to a test CD with tones containing distortion, so that you know what it sounds like.  Then follow the same procedure as mentioned above, but use your ears to check for clipping:  Start turning the gain up.   When you hear distortion (clipping), turn the gain back down a little bit.
  

5. Adjusting processors to smooth out frequency response

First, tweak only using gains and crossover settings, do not be tempted to adjust the equalization yet.  Since you already set your gains for maximum output, if you have to re-adjust, turn gains down on the components that are loudest, do not boost gains up.

If you have access to an RTA, your time spent tweaking will be greatly reduced.  Simply adjust the crossovers and gains (do not exceed settings from step 4) trying to make the response as flat as possible.

The second best option is to get a SPL meter (Radio Shack sells them for less than $60) and a test CD producing tones (ideally every 1/3 octave).  By recording the sound pressure level readings from the meter at each frequency, you can draw a response curve the same way an RTA does. Since SPL meters don't have an unwheighed frequency response, set your meter to slow "C" weighing and add the following values to each measurement: +7dB @ 20Hz, +4dB @ 25Hz, +3dB @ 31.5Hz, +2dB @ 40Hz, +1.25dB @ 50Hz, +1dB @ 63Hz, +0.5dB @ 80Hz, +0.25dB @ 100Hz on the lower end, and +0.25dB @ 2.5kHz, +0.5dB @ 3.15kHz, +1dB @ 4kHz, +1.25dB @ 5kHZ, +2dB @ 6.3kHZ, +3dB @ 8kHz, +4dB @ 10kHz, +7dB @ 12.5kHZ, +9dB @ 16kHz and 11.5dB @ 20kHz on the upper end.  If you can't get either the RTA or the SPL meter, you will just have to rely on your ears.   Keep tweaking and measuring until you are happy with the results.

A perfect flat curve measured with an RTA or SPL meter will not necessarily sound great.  Next step is to use your ears to fine tune the system.   Choose different types of music.  Even if you don't like to listen to jazz or classical music, they are a great resource to set systems.  Remember that at this point you are NOT listening to music, you are listening to your system.

Music should appear to come from the front of the vehicle.  The singers/band should seem to be up and in front of you. Classical music is very good for this because of all the different instruments that are used, covering pretty much the entire audio spectrum. The system should be completely transparent. The whole purpose of the system is to give you the illusion that the music is coming from a live band, not from a bunch of paper and plastic cones moving back and forth.
  

6. Equalization

Once you are very happy with your results (this could take days, even weeks), and firmly believe that you can't make the system sound any better without using equalization, then you can start EQ-ing.  A bit of advise: Mark all your settings before you go any further (you will be very sorry if you don't have a reference in case you mess up).  Try not to boost frequencies up on the EQ, only lower down the peaks.  If you have "holes" in your system, then you might have a problem with speaker location, or crossover points/slopes. 

Use either the RTA, SPL meter or any other medium you have available, to adjust the equalization and other processors (i.e bass/treble enhancement, etc).  Grab your CDs and hit the road again.  Take into consideration that your system will "sound" different sitting in a garage and on the road, due to road noise (this is were you wish you would have added damping material to your car and taken care of all the rattles).  Have knowledgeable people listen to your system and give you their opinion. Most of the time they will catch something you missed. Another good idea is to have a "reference system" (a high-end home or car audio system from a friend or relative) to compare your car stereo to.  Once again, the process will last many hours until you are satisfied with the results.

Finally, recheck the output of the amplifier(s) at different frequencies (preferably all the frequencies affected by the equalizer) using an oscilloscope.  This is to ensure that you did not introduce any clipping when boosting frequencies with the EQ.  If there is clipping, turn the volume down on the radio until you see no clipping.  That is the maximum volume setting of your system and you should never exceed it.

 
Head Units
Ideally, your head unit would be used to provide a signal to your amplifiers that is line level and you would not use its internal amplifiers (if any). They usually do not have the power and strength to drive speakers both loudly and cleanly. The line level signal is cleaner than the speaker level outputs on the head unit because it is does not go through the internal amplifiers in the head unit. That being said there are occasions where you would use the head unit's internal power.  One situation is when you are on a budget and are building your system over time.  The head unit can be used to drive speakers (but not subwoofers) until you can get an amp.  The other situation is when you are building a system where the benefits of an amp are not important to you.  Read my planning page for more details about what is right for you.

No head unit typically has more than about 60watts of total output power because more would require a real DC-DC power supply (which does not fit in a head unit easily). Using the head unit power can be a temporary solution until a separate amplifier can be purchased, just make sure you do not try to power any subwoofers or insensitive component sets with the head unit's built in power. According to Car Audio and Electronics magazine, most head units use the same chips for the internal amplifiers so they all produce about the same low power. The best they have measured is about 14watts into 4 channels at 1% distortion. Their power level at a better lower distortion figure (like 0.1%) is significantly lower.

Note about using factory head units:
Many people ask me about using the factory head unit that came with their car with external aftermarket amps. Typically you cannot get a clean signal from the head unit because factory heads do not have line level (RCA) pre-amp outputs to drive an amp. You can use a speaker level to line level converter but the sound is still going through the factory head's internal amps. Some people are willing to sacrifice some sound quality in order to keep their factory head. Also, if your factory system uses an external amp you may be able to find an adapter so you can use an aftermarket amp instead.

Usability:
Your head unit is the part of the car stereo that you interact with most so it is important to get one that "feels" good to you. Always look at a head unit in a store display and use it for awhile. Try to flip through radio stations and tracks on a CD to see if it is quick and easy. If you have problems with small buttons, imagine what it will be like when you are driving! Since many models in the same price range are similar in features and sound quality, usability is often the deciding factor between models.

Power:
Even though I just said not to use the built-in power of a head unit I know sometimes it is necessary. Bear in mind that the power specifications given by most manufacturers for head units are not accurate. They often use terms like "music power" or "peak power" which have little real meaning because there is no standard definition of those terms. If the power is quoted in "RMS" terms then it is usually accurate. However, there is still one other place of misconception. Often manufacturers will quote power as "30watts x 4 RMS". The "RMS" seems to mean it is a true indication of power but they are implying that all 4 channels can produce 30watts rms AT THE SAME TIME. With a head unit, this is almost always not true. Because of the small power supplies in head units they can rarely output more than 15-60 watts TOTAL. This means that the power to each channel at maximum loading would only be 1/4 of that total. Some manufacturers are better than others about giving accurate specifications and a few models are available with sophisticated power supplies which have higher power output but they are VERY expensive. If you're paying less than $800 for a head unit (and most of us are!) then your head unit will not put out much power. I have written a more comprehensive explanation of power amplifier specs as well. Speakers which are not producing bass do not draw nearly as much power so you can get away with using the head unit to power them but use passive high pass crossovers (bass blockers) and they will play even louder and cleaner. Bear in mind that the distortion may be higher from the head unit than an external amp however.

Cassette vs. CD:
This choice is mostly a matter of preference. If you do not have many cassettes then an in-dash CD player is probably right for you. If you need the capability to listen to cassettes and CDs then a cassette head unit with changer controls should be adequate. Be aware that many in-dash CD head units can control a CD changer as well so you can use both. An in-dash CD is convenient for changing discs quickly while on the road. Because of size of most CD changers they are usually mounted in the trunk or under the seats although there are some newer models which are small enough to fit in glove compartments. Under the seats or in the trunk are not easy places to get to while you're driving!

Theft Protection:
Detachable faces are the most common theft prevention scheme in head units today. There are two flavors, fully detachable and partially detachable. With a fully detachable face all the controls on the front come off leaving behind a blank panel, whereas a partially detachable face leaves some features on the head unit but the head unit is still useless without the face. Fully detachable faces are larger and bulkier to carry around than partially detachable ones but leave nothing behind to be seen. Another option is Eclipse's ESN system. With these head units when you first apply power to them you must supply a CD which the unit remembers as the "reference" CD. Thereafter if the unit ever loses power you must insert the "reference" CD before it will work again. Only you know what the "reference CD" is so the head unit is useless to a thief. Eclipse also tracks the units they repair. More than once a stolen head unit was returned to them for service because it was not working. Upon verifying the head unit was stolen they can apprehend the thief as the person who returned the stolen head unit for service. I still wouldn't count on the thief to know that Eclipse does this though so I stick with a conventional fully detachable face. A new twist from Kenwood flips the face around when you turn off the power so the thief can't see the head unit. I think it would work even better if the face then went back into the head unit, giving the appearance that the unit is a detachable face head unit with its face removed.

Pre-amp outputs:
These are must for any serious head unit. These outputs allow you to run an amplifier directly without need for any conversion. This is the cleanest output of the head unit. Some units have multiple outputs and sometimes ones that are crossed over. Look for the amount and type that you need for your system but keep in mind future expansion. One is sufficient but having two allow you fade, or adjust the levels of multiple amplifiers right from the head unit. Some head units now offer 4 volt outputs instead of the usual 1-2 volts. This can be very beneficial since cars have a lot of electrical noise in them. The 4 volt output is less susceptible to noise, however, you must be certain that the amplifier or crossover being connected to the output can handle 4 volts or you will not be able to use the extra voltage. If your head unit does not have pre-amp level (RCA type) outputs you can buy an adapter which will convert your speaker level outputs to line level. They range in price from $12 on up but since I have not used them I do not know how much difference there is among them. Another option is to use an amplifier that accepts speaker level signals directly but those are not as easy to find.

Other features:
There are many other minor differences in features between head units. Choose the one that appeals to you most. Switch able illumination is nice if you want the head unit's display to match the other instrumentation in your car. Dolby Noise reduction and full logic tape controls are nice as well. Finally, a remote control can be useful or can be a waste depending on whether you use it. A remote control mounted in the steering wheel can be very convenient though.  Some CD heads come with a buffer to minimize effects from bumps.  This can be useful but in my experience if you mount the head unit securely it will not skip much anyway and using the anti-skip buffer can have a slight negative effect on sound quality because of the way the buffer is implemented
 

Head Units

Fortunately, most units follow the same size standards (DIN).   In many cars, once the factory radio is removed the aftermarket radio will fit in the hole.  In many other cars, a kit is needed if the factory hole is too big, or not deep enough.  In some cases the dash has to be cut.  Any car stereo store should have kits required for installation.

Even though not necessary, it is recommended to use a wiring harness when installing an aftermarket radio.  The harness is wired up to the radio, an plugs directly in the factory plug, making a good and easy connection.  Since the factory plugs are not cut, the manufacturer's warranty is not voided on the vehicle, and the factory radio can be reinstalled when it is time to sell the car.
  

Radio Mounting

Aftermarket radios can mainly be mounted in two ways:

ISO mounting is when the radio can be screwed to existing factory radio brackets, such as in most Japanese cars.

Ring mounting: Most aftermarket radios come with a metal ring that gets mounted to the factory radio hole or aftermarket kit via bendable tabs.  In many cars, dash and trim rings have to be filed to enlarge the radio hole.   Once the ring is installed, the radio slides in and is held by snaps.  In most cases, special tools are required to remove the radio.

Using the Factory Head Unit

Adding amplifiers to factory head units or head units without RCA outputs can be easily achieved with a high-level to low level adapter.  The adapter reduces the level of the signal coming from the head unit's speaker outputs to lower levels that are acceptable for amplifier inputs.  Some amplifiers have this adapter built in for convenience.  The drawback of using speaker outputs is that the signal is not as clear as it would be coming straight from a set of RCA wires.  If the factory unit has distortion on the output, the distortion will be passed along to the amplifier.

Replacing the Factory Head Unit

Many cars with high-end factory systems such as Volkswagen's Atkiv Speakers, GM's Delco-Bose, etc. have amplifiers that require an interface kit to match signal levels, or are best completely rewired.  These kits are usually expensive.   To bypass amplified speakers sometimes existing wiring can be used.  In other cases wires have to be run to each speaker.  Factory amplifiers such as in some Fords use a 5-volt turn on wire instead of the usual 12v.  Even though factory amplifiers can be hooked up to aftermarket radios directly, they can be prone to noise.   Consult a professional before tackling one of these projects.

Getting Better AM/FM Reception

Believe it or not, factory tuners are usually better than aftermarket units.  The most important part of the tuner is definitely the antenna.  If you have a bad or broken antenna, you tuner will not pick up the stations as it should.  If the antenna has to be replaced or upgraded, make sure it is the same length as the original.  The length of the antenna greatly affects reception.  Lower frequencies (AM) are best caught with a long antenna, while higher frequencies (FM) need a shorter antenna.   Car manufacturers compromise a bit, giving you a length that would work best while receiving both AM and FM frequencies.  If you get a short antenna, such as the 1 foot rubber antennas, the FM reception will be poor and AM will be almost non-existent.

Troubleshooting: If you have poor reception, do this simple test:  Try a couple FM stations and a couple AM stations.  If you have no AM at all, but you get FM, then the problem is most likely the antenna.  If your radio can't get neither, then the problem is either a broken or disconnected cable or a bad tuner.  Plug a test antenna to the radio to make sure the problem is not the radio itself.

 
Check speaker polarity

To make sure all your speakers are in phase, unhook the speaker you want to test at the amp (both wires preferably). Using a 1.5 volt battery (any size), touch the positive terminal of the battery to the positive wire going to the speaker, then do the same for the negative wire.  Have a friend look at the speaker.  If the speaker pops out, the polarity is correct.  If the speaker pops in, the speaker is hooked up backwards (out of phase).  To fix this, simply reverse the wires when hooking the speaker back to the amplifier.  A word of caution here: DO NOT hold the battery power to the speaker for more than 1 second, all you want to do is to see if it pops in or out.  You will damage the speaker if you hold constant power to it.   Do not use a higher voltage.  Also, do not try this test on tweeters, you could fry the voice coils.  If there are crossovers with capacitors along the line, this test will not work (capacitors block DC voltage).  Bypass the caps momentarily.

A much more elegant and quicker way to do this is by using a commercially available polarity checker, which uses a test CD.  All you have to do is pop the CD in the head unit and hold the polarity tester in front of each speaker.   The advantage here is that you can test for absolute polarity of the system on all the speakers, including tweeters.  Polarity checkers are available from various companies such as Monster Cable.  Retail for the Monster Cable polarity checker is about $120.

Sometimes, when speakers are not mounted close to each other (i.e., mids on the doors and tweeters up in the dash), reversing the polarity on tweeters or mids makes the system sound better because it makes up for phase differences due to distance.   Try different combinations and see what sounds better.

 

Calculating Driver Reference Efficiency

 This calculator is intended to help you determine the reference efficiency of any loudspeaker. There is no standard for stating the efficiency of a loudspeaker, so some manufacturers will give a rating at 1kHz for a subwoofer. This makes the speaker seem like it is a lot more efficient, but it is unfair when comparing to another subwoofer with an efficiency rated at 100 Hz (and not many subwoofers are used at 1kHz anyway). This formula will give you a reference number that will allow you to compare subwoofers on an equal basis.
 

Speaker Reference Efficiency
 

This will give you the efficiency as a decimal. To convert it to a percentage, multiply by 100. Common percentages range from 0.3 to about 1.6.
 

Explanation of Terms
Vas is in cubic feet.
 
Calculate
 To use this form, enter the values for Fs, Vas, and Qes. Then just click the "Calculate Values" button, and the answers will appear in the empty text boxes.
Enter Fs Hz
Enter Vas Ft3
Enter Qes
 decimal
 percentage
 
 

The speakers you use will have the final say in how your system will sound. There are many types of speakers available. A single speaker can be used to reproduce the full range of sounds but it is not ideal. If the speaker is too large it will have problems reproducing high frequencies which require rapid movement of the speaker. If it is too small it will have problems reproducing low frequencies which require large amounts of air to be moved. Because a single speaker cannot reproduce all sounds accurately multiple speakers are used each of which reproduces sound in the frequency range it was designed for. A speaker called a tweeter reproduces high frequencies generally above 2 kHz. Tweeters are small and lightweight so they can respond quickly. Very little power is required for powering tweeters because they are very efficient. Woofers are the exact opposite because they usually require large amounts of power to really move air. Woofers are meant to produce sound at frequencies below 250 Hz and often just below 100 Hz (in the case of subwoofers). Because a woofer must move large amounts of air they are usually large with typical sizes of 10", 12", 15" and even 18"! On the other hand tweeters are usually very small ranging in size from 1/2" to 2" in size. Typically, tweeters larger than 1" in size cannot respond quickly enough to sound good and are too directional. In between are midrange speakers which handle the frequencies between the woofers and tweeters. Further division can be done but is usually unnecessary and just complicates the crossover which must separate the full audio signal into multiple parts for each speaker.

Things to look for:

Power Handling: Just as with amplifiers, RMS or continuous power is important here. Some manufacturers will claim very high power handling figures but they are usually for very short peaks only. Granted music is not continuous but the continuous power handling gives you a much better impression of how much power a speaker can really handle. For tweeters and midranges, power handling is not as important since it does not take much power for them to play loudly. For woofers though a rough match should be made between the woofer and the amp driving it.

Sensitivity:This is a very important spec for a speaker. It gives you an idea of how loud a speaker will play given a certain input power. If a speaker is insensitive then it will require more power to play at the same volume level than a speaker that is more sensitive. Figures between 85 dB and 95 dB at 1 watt RMS at 1 meter are common. If you use anything outside of this range you may have problems matching the output levels of the speakers relative to each other. If you're going to run speakers off of a head unit then try to get speakers with higher input sensitivities since head units typically do not have much power.

Physical Size: You must pay attention to the size of the speakers you choose. Tweeters are very small but need to mounted where they fire nearly directly at you or they may not be heard properly. Some tweeters have better off axis response than others. If you will not be on axis with the tweeter when you audition tweeters in a store listen to how their sound changes as you move around them to see if they will work in your car. Midranges should fit in the door or dash spaces provided or you will have to do some cutting or fabrication. In general the larger the woofer the larger the enclosure required to hold it. Some woofers are better optimized for small enclosures than others (Kicker Solobaric, JL Audio W6 for example). Make sure you have enough room in your trunk or hatchback for the woofer. Kickpanels for midranges and tweeters or coaxials typically offer better imaging than locations in the door however the soundstage is sometimes lower than when you have the tweeters mounted high in the doors or on the A pillars.

Enclosures for Woofers:Because woofers move a lot of air they generate a back wave behind them.  If you mount a woofer in free space without an enclosure you will get almost no bass because the back wave will cancel out the sound from the front of the woofer.  There are many types of enclosures for woofers to handle this backwave. A popular one is a ported box. This enclosure has the woofer mounted in box with a hole in it and a port (tube) attached to the hole. The port is made a specific size and depth to cause a "bump" or rise in the frequency response at that point. This makes the overall system more efficient but can cause the bass to be somewhat "boomy" or less "tight" depending on how its done. A newer technique is a bandpass enclosure. The woofer is mounted inside the box and fires into another chamber within the box that is ported to the outside. Again, this increases efficiency greatly but only at a certain frequency. This effect can make the system very loud and boomy. Another method employs mounting the woofer (which needs to be a free air type in this case) to the rear deck of the car and using the trunk as a big box. This method is subject to many variables but can work well if done properly. Another benefit of this method is that you do not lose space from a large enclosure box. The oldest and most popular type is a sealed enclosure. This method simply has the woofer firing into the car and the back wave is suppressed inside the box. This method usually produces tight accurate bass but is not as efficient. Also this method typically requires a large box to work well. Finally because of the lower efficiency of this design more powerful amps and woofers are needed to play loudly. When any of these enclosures are created using the specs of the woofer as a guide you can create the type of bass response that you desire.

 

A trick that professional installers use to get more power out of amplifiers is to wire up speakers in different ways, playing with resistances to achieve a desired total impedance "seen" by the amplifier. Even though speakers are active loads (resistance changes with frequency), it is accepted to treat speakers as resistors with a fixed resistance value (usually 4 ohms).

By combining speakers in different ways, maximum amplifier output can be obtained.  For example if a 2-channel amplifier is rated to deliver a maximum output of 400 watts at 2 ohms mono (bridged), then by hooking up two 4 ohm subwoofers in parallel, a total load of 2 ohms is "seen" by the amplifier, obtaining optimum power.

Parallel Resistance

Parallel Wiring Diagram

People commonly hook up two or more speakers to the same channel out of an amplifier in parallel. This is achieved by hooking up the negative wire from the amp to all the negative connections of the speakers, and the positive to all the positive connections of the speakers. By doing this, the load seen by the amplifier is lower. For example, if two 4-ohm speakers are wired-up in parallel, then their total resistance will be half, or 2 ohms. If three speakers are wired up in parallel, and they all have the same resistance value, then the total load would be a third of the value of each speaker's resistance. Here's a formula to calculate parallel total resistance for two speakers:

Parallel Resistance Formula 1

For more than two speakers, use the following formula:

Parallel Resistance Formula 2

So what are the advantages and disadvantages of this scheme? First, if one of the speakers burns out, then the other one(s) keep playing. If the amplifier is not designed to receive lower loads provided by hooking the speakers up in this fashion, you might end up destroying your amplifier. Check your manual or consult an expert.
  

Series Resistance

Series Wiring Diagram

Speakers are hooked up in series to decrease total load to an amplifier. To hook up speakers in series, connect the positive terminal of the amplifier to positive of one speaker, then hook up negative of that speaker to positive of next speaker, and so on. Then hook up negative of last speaker to negative of the amp. It is a lot easier to calculate total resistance for speakers hooked up in series. This is easily done by adding up all the individual resistances:

Series Resistance Formula

The disadvantage of hooking up speakers in series other than getting less power out of an amplifier, is that if one of the speakers burns up, the other one(s) stop working.

 
Speakers
The speakers you use will have the final say in how your system will sound. There are many types of speakers available. A single speaker can be used to reproduce the full range of sounds but it is not ideal. If the speaker is too large it will have problems reproducing high frequencies which require rapid movement of the speaker. If it is too small it will have problems reproducing low frequencies which require large amounts of air to be moved. Because a single speaker cannot reproduce all sounds accurately multiple speakers are used each of which reproduces sound in the frequency range it was designed for. A speaker called a tweeter reproduces high frequencies generally above 2 kHz. Tweeters are small and lightweight so they can respond quickly. Very little power is required for powering tweeters because they are very efficient. Woofers are the exact opposite because they usually require large amounts of power to really move air. Woofers are meant to produce sound at frequencies below 250 Hz and often just below 100 Hz (in the case of subwoofers). Because a woofer must move large amounts of air they are usually large with typical sizes of 10", 12", 15" and even 18"! On the other hand tweeters are usually very small ranging in size from 1/2" to 2" in size. Typically, tweeters larger than 1" in size cannot respond quickly enough to sound good and are too directional. In between are midrange speakers which handle the frequencies between the woofers and tweeters. Further division can be done but is usually unnecessary and just complicates the crossover which must separate the full audio signal into multiple parts for each speaker.

Power Handling:
Just as with amplifiers, RMS or continuous power is important here. Some manufacturers will claim very high power handling figures but they are usually for very short peaks only. Granted music is not continuous but the continuous power handling gives you a much better impression of how much power a speaker can really handle. For tweeters and midranges, power handling is not as important since it does not take much power for them to play loudly. For woofers though a rough match should be made between the woofer and the amp driving it.

Sensitivity:
This is a very important spec for a speaker. It gives you an idea of how loud a speaker will play given a certain input power. If a speaker is insensitive then it will require more power to play at the same volume level than a speaker that is more sensitive. Figures between 85 dB and 95 dB at 1 watt RMS at 1 meter are common. If you use anything outside of this range you may have problems matching the output levels of the speakers relative to each other. If you're going to run speakers off of a head unit then try to get speakers with higher input sensitivities since head units typically do not have much power.

Physical Size:
You must pay attention to the size of the speakers you choose. Tweeters are very small but need to mounted where they fire nearly directly at you or they may not be heard properly. Some tweeters have better off axis response than others. If you will not be on axis with the tweeter when you audition tweeters in a store listen to how their sound changes as you move around them to see if they will work in your car. Midranges should fit in the door or dash spaces provided or you will have to do some cutting or fabrication. In general the larger the woofer the larger the enclosure required to hold it. Some woofers are better optimized for small enclosures than others (Kicker Solobaric, JL Audio W6 for example). Make sure you have enough room in your trunk or hatchback for the woofer. Kickpanels for midranges and tweeters or coaxials typically offer better imaging than locations in the door however the soundstage is sometimes lower than when you have the tweeters mounted high in the doors or on the A pillars.

Enclosures for Woofers:
Because woofers move a lot of air they generate a back wave behind them.  If you mount a woofer in free space without an enclosure you will get almost no bass because the back wave will cancel out the sound from the front of the woofer.  There are many types of enclosures for woofers to handle this backwave. A popular one is a ported box. This enclosure has the woofer mounted in box with a hole in it and a port (tube) attached to the hole. The port is made a specific size and depth to cause a "bump" or rise in the frequency response at that point. This makes the overall system more efficient but can cause the bass to be somewhat "boomy" or less "tight" depending on how its done. A newer technique is a bandpass enclosure. The woofer is mounted inside the box and fires into another chamber within the box that is ported to the outside. Again, this increases efficiency greatly but only at a certain frequency. This effect can make the system very loud and boomy. Another method employs mounting the woofer (which needs to be a free air type in this case) to the rear deck of the car and using the trunk as a big box. This method is subject to many variables but can work well if done properly. Another benefit of this method is that you do not lose space from a large enclosure box. The oldest and most popular type is a sealed enclosure. This method simply has the woofer firing into the car and the back wave is suppressed inside the box. This method usually produces tight accurate bass but is not as efficient. Also this method typically requires a large box to work well. Finally because of the lower efficiency of this design more powerful amps and woofers are needed to play loudly. When any of these enclosures are created using the specs of the woofer as a guide you can create the type of bass response that you desire.
 

Speaker installation is very critical for performance. Whether you spent $50 or $1000 on a set of speakers, if they are not properly installed, the sound will not be up to par.

What makes a good installation? Well, certainly mounting speakers in most factory locations, such as on the bottom of the doors pointing at your legs, are not acceptable. In this cases a new mounting location might need to be improvised.

Distance

The first thing to consider is distance. If the left speaker is only a couple feet away from your ears, while the right speaker is several feet away from you, then the sound will arrive at different times giving you poor sound. The left speaker will sound louder since it is closer.

The best solution is to figure out a location where the difference between the distance of the right speaker to your ears and left speaker (also known as path length difference), are minimal. This is where kick panels shine, making it the preferred location for many audiophiles and competitors alike.

The other solution, which can get expensive, depending on the gear you get, is delays. By adding a delay to the left speaker, the sound can be doctored to arrive from both sides at the same time. This is only a patch, and does not sound as well as equally spaced speakers, but is the second best alternative.

Multiple Speaker Placement

If you have a system with two or more speakers per side, you need to carefully try different locations to obtain the best possible sound in your car.

Let's take a 2-way system with a tweeter and woofer per side as an example. The woofers are mounted in the factory location at the bottom of the door. The tweeters are mounted high up on the front corner of the door panel. Looking at the speakers from the driver's seat, you can see that there are 4 speakers all aimed towards different orientations and all at a different distance to your ears. This interaction of sound waves at different frequencies arriving at your eras at different times seldom sounds good. The best thing to do is mount the woofer and tweeter on each side as close as possible to each other. This is also why kick panels are used so much these days.

Professional installers do use some tricks such as inverting the tweeters' polarity when mounted for example on top of the dash while the woofers are low. Achieving good sound with unconventional mounting schemes is very, very hard and is only achieved after plenty of time has been spent trying different configurations.

Aiming

Our ears can distinguish the direction of sound more easily at higher frequencies.  This means that aiming the mids, and most importantly, tweeters towards your ears play a critical role in sound imaging.  Midbases are not so critical, but should be also aimed towards the listener's ears if possible.

To figure out the best aiming angle involves many hours -even days- of work.  To start, try to aim the speakers towards the center of the car.  Play around with different angles until you obtain the best "sweet spot".

Subwoofers should be mounted up front for best sound. Since this is not possible in most cars, mounting subs in the back is not such a bad thing, since most people can't distinguish where bass comes from. If you have good midbases going down to 60 Hz or less and subs picking up the signal below 60 Hz, then the bass will seem to come from the front.

Enclosures

Everyone is aware that subwoofers need a properly designed enclosure to give top performance. How about midbases and mids? They also do sound much better if they are installed in enclosures. The best sounding and easier to build enclosure type for midbases and mids is sealed.

Mounting Speakers

If you are using speakers that fit into a factory location, make sure there are no gaps or holes.  Sometimes building a wood or fiberglass baffle helps reduce holes and gives you much better sound.  Always be careful when using power tools around speakers.  Warranties usually don't cover holes in speakers.

For unconventional speaker locations, sometimes metal has to be cut.   If you have the resources, plasma cutters and pneumatics tools work great.   For most mortals that do not have these tools, a pair of metal snips (left and right cut) will do the job.

 

What is Alternator Noise?

Alternator noise is a high pitched whine caused by the car's electrical system. When the engine spins the alternator around, the alternator induces an AC voltage that is converted to DC and used to charge the car's electrical system. It acts pretty much as an inverted electric motor (motion is put in, and voltage comes out). The problem is that a small amount of AC voltage remains in the system. Frequencies change accordingly to the engine's RPMs. If the engine spins faster, noise frequency is higher. That is why you would hear alternator noise coming from mids and tweeters, but not subwoofers, since subwoofers only play low frequencies.
  

What causes alternator noise?

1. Induced noise through RCA's:

When a wire has current through it, a magnetic field circles around it (i.e electromagnets). Conversely, if there is a magnetic field perpendicular to a wire, current will be induced. If you have your RCA wires going from the radio or equalizer to the amp running in parallel to your power wires, an AC current will be induced and added to the sound signal. The sound signal travelling to the amp is a low voltage signal (in the mV range - thousands of a volt). The induced signal will be amplified along with the music.

Avoiding this problem is very simple: DON'T run power and RCA wires together. If there are points in which they do have to cross, try to place them perpendicular to each other. Run the power wire from the battery to the amp on one side of the car, and the RCA wires along the other side of the car. On most cars it is better to run RCA's on the passenger's side, and power wires on the driver's side. Note that noise may be also be induced by factory harnesses, car computers and other electronic equipment.

2. Ground loops:

Your car's electrical system (and your stereo) use the car metal chassis as a ground (there is always current flowing through your car's metal parts). If your battery and alternator are (typically) under the hood, and you are installing an amplifier all the way back in the trunk, then current flows through that power wire you ran from the battery to the amp, and back through the metal chassis to complete the circuit.

Theoretically the car's metal has no resistance, and it should not matter where you tie grounds for amplifiers, radio, battery and alternator. They all should "look" like the same point, right? Well, the metal in your car does have resistance, and there is a potential difference from the front of the car, where the battery is to the middle of the car, where the radio is, and to the back of the car, where most amplifiers are. The potential difference of the grounds makes the whole system act as an antenna, where they pick up noise. Measure voltages at battery, amplifiers and radio. There should be very little difference between the measured voltages. If there is a difference more than 1/2 volt, then you might have noise problems.

To fix this problem, make sure that the amplifiers have a good ground first. Use at least 10 Gauge wires for the grounds (and power). If you have 2 or more amplifiers, DO NOT go from the ground terminal of one amp to the other and then from there to ground, most likely you will have noise. Ground each amplifier independently. Same thing if you have added stiffening capacitors, go to a separate ground for the cap.
  

Troubleshooting

If you installed everything using the above guidelines and you still have noise, then try to figure out what is causing the noise (a very LONG and tedious process). First, double check grounds at amplifiers, crossovers, radio, etc. Make sure AM/FM antenna has a good ground. Try to figure out what is causing the noise. For example, if you have crossovers, equalizers, etc, bypass them by hooking RCA wires straight from the radio to the amplifier. If noise went away, you know problem is maybe RCA wires or grounds hooked up to crossovers/equalizers. If you have more that one amplifier and have noise only on one amplifier try switching RCA wires around. If noise stays the same, then problem is the amplifier, if it switches, noise is coming from previous components up the line. As said before, it is very hard to find out what is causing alternator noise.

Don't get one of those noise filter boxes unless you have completely figured out that the head unit or equalizer are causing the noise. 99.9% of the time you will be wasting your money in buying noise filters.

If you have tried everything in the world, and still have that annoying noise, contact your nearest car stereo shop. Some of them will be reluctant to fix something not installed by them, or maybe will charge you a lot for something you could not figure out that only took a couple of minutes for them to fix, so shop around first.

 

Loud Speakers 101

The Loudspeakers 101 contains a tutorial on loudspeaker system design and construction. You will also find the JavaScript enclosure designers and crossover calculators here. 80

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