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. |
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
- Make sure you have Java turned on in your browser.
- Enter required information.
- Click "Calculate" to get the answers.
- The calculators will give accurate results only for
rectangular shaped objects.
Calculators
- 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."
- 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".
- Conversion Factors:
Metric to English units, Cubic inches to cu ft, etc.
Box Dimensions Calculator
Solid Object Volume Calculator
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.
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.
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.
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. |
 |

Typical Sixth-Order Bandpass Enclosure Design |
This style of enclosure is patented by the Bose Corporation, information provided is for personal use only. |
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. |
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. |
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 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. |

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. |
 |

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. |
|
| 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™). |
| If you want to calculate square vents, the formula below will give you the value of R to use in the formula above. |
| In the formula above, a is the area of your square vent (height x width), and (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 |
 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 |
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.
|
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.  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. |
- 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 |
 |
 Typical Sealed Enclosure Design |
The following formulas will allow you to design a sealed speaker enclosure for any Qtc value you desire. |
= (Qtc / Qts)2 - 1 |
Vb = Vas /  |
Fc = (Qtc x Fs) / Qts |
 |
 |
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.
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.
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
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 =  |
|
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.
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.

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
The
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
The
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
 |
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). |
 |
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 =  |
|
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 |
|
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 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 |
|
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?
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.
|
 |
|
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.
|
 |
|
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.
|
 |
|
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.
|
 |
|
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.
|
 |
|
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.
|
 |
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:
- 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.
- 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 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
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. |
|
Instructions
- Make sure you have Java turned on in your browser.
- Enter high and low pass speaker impedances.
- Enter desired crossover frequency.
- On the second-order crossover calculator you must
select type of crossover.
- 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
- Phase shift on a first-order crossover is 90
degrees.
|
Second Order (12db/octave) Two-Way
Crossover
- 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
- Phase shift on a third-order crossover is
270 degrees (-90 degrees).
|
Fourth order (24dB/octave) Two-Way
Crossover
- The phase shift on a fourth-order crossover
is 360 degrees = 0 degrees (no phase shift).
|
Zobel Circuit (Impedance
Stabilization)
- 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)
- 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

- 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).
- 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.
- 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
- 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.
- 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.
- 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.
- 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.
- 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 |

|

|
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 |

|

|
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
|
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: 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: 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: 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: 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: 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. |
 |
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 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 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 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 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: 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: 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: 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: Another essential
tool. Used for audio and security installation. Good drills
run $40 and up. Available at any store.
|
 |
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: 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: 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.
|
 |
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 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: 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. |
 |
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: 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: 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

 |
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. |
 |
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: 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. |
 |
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: 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.
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 Fosgates Noise
Killer Blue which are sprayed to the panels. They are used in places where a
liner cant 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 cant 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 |
 |
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 |
 |
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 |
 |
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 |
 |
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 |
 |
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
- Fiberglass mat or cloth, resin and hardener.
- Bondo (body filler), hardener.
- Box of disposable gloves, respirator, protective
clothing.
- Paint brush, plastic sheeting, aluminum foil, mold
release or WD40.
- 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.
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.
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.
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
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:

For more than two speakers, use the following
formula:
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
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:
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
|