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Car Audio - How Do Speakers Work
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
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
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
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.
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.
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