Boxes and Baffles for
Better Bass
The car obviously belonged
to a young guy -- late teens or maybe early twenties. It wasn't so much that it was one of
those over-powered land yachts that had seen better days, so beloved of the
high-testosterone set. It was that I could see from half a block away a pair of over-sized
raw speakers perched on the rear deck, ready to do some serious pounding. As I got close
to the car, I spotted an equalizer in the dash with the bass cranked all the way up, and I
could easily imagine what the whole rig sounded like.
Horrible.
I don't have anything against loud music or lots of bass --
I indulge in both fairly frequently, although less than when I was this kid's age. But if
those were what he was trying to produce, he was going about it exactly the wrong way, and
his setup was almost guaranteed to produce more distortion than music.
Sound is a series of air pressure differences that radiate
outward at a constant rate from whatever is creating them. When they hit our eardrums,
these waves cause them to move in and out slightly, and our brain interprets these
movements as sound. A speaker consists of a diaphragm attached to an electromagnetic
assembly that causes it to move back and forth in step with the varying electrical
strength of an audio signal
The diaphragm alternately compresses the air in front of it
as it moves forward, and decompresses (or rarefies) it as it moves back; these pressure
differences then radiate into a room -- or car -- as sound.
Sound behaves differently in different parts of the audio
range. In the treble, the waves tend to be fairly directional, radiating forward in lines
that get straighter the higher you go. For that reason, it's best to sit directly in front
of your speakers for best high-frequency performance, and for good stereo imaging.
Every speaker produces as much energy to the rear as to the
front, but that doesn't matter very much at high frequencies. Even if the sound is being
produced by a raw speaker -- that is, not mounted in a box -- the rear-firing sound will
be directed away from the listener; by the time it bounces off a surface behind the
speaker and returns to the front, its energy is likely to be so dissipated that it will
have little effect.
Not so with bass, which is very susceptible to interference
and selective cancellation. Imagine two identical speakers side by side playing the same
music, but wired so that when one diaphragm is moving out the other is moving in. They are
out of phase, and when one is compressing the air, the other is decompressing it. Because
the speakers are close together, these waves cancel each other out, and very little
low-frequency energy emerges.
Exactly the same thing happens when you run a speaker in
the open air. Unlike treble, which is directional, bass tends to be omnidirectional -- it
moves away from the speaker in a spherical wave. Unless care is taken to contain it, the
wave from the rear of the diaphragm will emerge with about the same energy as the front
wave, but since the motion creates a compression in front and a decompression behind, the
two cancel each other.
In home speakers, the usual solution is to put the speaker
in a box so the rear wave can be kept away from the front. Either it is absorbed inside
the box, or it's inverted in phase and allowed to escape through a duct or port, to
reinforce the front wave.
Another technique is the use of an "infinite
baffle" -- a board in which the speaker is mounted, and whose dimensions are large
enough that the rear wave loses enough energy in traveling around it to be harmless.
Mounting speakers in walls so that the rear energy radiates outside or into another room
is one version of this, but not practical for many homes.
But the infinite baffle idea often works in cars. The
speakers can be mounted in holes in the rear deck, with the back wave radiating into the
trunk. Or there are enclosures designed for use in cars that take up less space than home
speaker boxes, but still produce adequate bass.
The way not to go is to try to overcome physics by
cranking up the bass with an equalizer. In the first place, it probably won't work, but
will just produce bigger sound waves to interfere with each other. And in the process,
it's likely to drive the power amplifiers into gross distortion, which normally occurs at
frequencies such speaker can readily reproduce.
...Ian G. Masters
ian@mastersonaudio.com
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