by Larry Klein
Car Speaker Enclosures
Q. I've been told that speakers mounted on the rear package shelf of a car sound better when they have enclosures. I just bought a pair of expensive three-way 6 x 9-inch speakers meant for rear-deck mounting, but nothing is said about enclosures in the instructions. What do you suggest?
CHARLES SALVA; Los Angeles, CA
A. It's no secret that a speaker works best in an enclosure designed for its specific electromechanical and acoustical characteristics. Unless a car stereo speaker comes with specific instructions for a rear housing or enclosure, assume that it is not designed to operate with one.
The basic purpose of any enclosure is to separate the pressure (sound) waves emitted by the front of a speaker cone from those produced by its rear. If the high-pressure wave produced when the cone's front pushes the air were to meet the low-pressure wave created simultaneously at the rear of the cone, then the two would cancel and no sound would be propagated into the environment. A very small baffle is sufficient to prevent cancellation of the shorter, high-frequency sound waves, but for the longer, low frequencies a larger barrier-such as an enclosure-is needed for adequate separation of the two pressure areas.
An enclosure is a mixed blessing, however, because the air inside the box pneumatically loads the rear of the speaker cone and raises its resonance. A speaker system's resonance pretty much determines its bass performance. In the frequency area above system resonance, a speaker responds more or less linearly. At the resonance frequency, a speaker tends to respond excessively.
And below the resonance, there is little or no acoustic output.
This electro-acoustical fact of life can become a problem when a woofer with a free-air cone resonance of 70 Hz or so is installed in a small box. The air loading of the box shifts the resonance upward into a frequency area (100 Hz or so) that causes bass notes and male voices to become boomy and unnaturally emphasized, while bass frequencies below the resonance are no longer reproduced.
There are two common solutions to the problem: (1) Start with a woofer with a very low free-air resonance so that the enclosure loading simply moves it to a desirable frequency-say, 45 to 60 Hz. (2) Tune the enclosure with a port or vent so that the acoustic resonance of the box interacts with the mechanical resonance of the speaker in such a way as to smooth and extend the bass performance of the system.
(Knowledgeable readers will recognize the first type of speaker as an acoustic-suspension system and the second as a bass-reflex or vented system.) A third type of enclosure design, which for practical reasons is now seldom used in the home, is the "infinite-baffle" system. It employs a housing that is large enough--if not truly "infinite"--to leave the basic resonance of the speaker relatively unaffected by the enclosure. In mono days, it was frequently suggested that an infinite baffle could be achieved without a very large enclosure by mounting a speaker in a cutout in a wall between rooms or in the door of a large clothes closet. Today the need for symmetrical stereo speaker installations makes such ideas somewhat impractical in the home. Most car trunks, however, are large enough to serve as quasi-infinite acoustic baffles for 6 x 9-inch woofers or multi-element drivers installed in the rear package shelf, and such units are designed to work properly with the rear loading presented by a typical trunk.
So why do some car stereo installers go to the trouble and expense of constructing and mounting separate rear-deck speaker enclosures? Assuming that the installers know what they are doing-and that the enclosures are not just part of an elaborate sales hype-it is possible to get improved bass response by careful matching of special drivers to enclosures. By special drivers, I mean low-resonance woofers originally intended for use with an enclosure.
The high-quality car speakers sold by many different companies for rear deck/trunk installations are not de signed for use in a sub-enclosure. For the reasons I've explained, any attempt to use them in such housings will produce worse, not better, bass response.
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Source: Stereo Review (USA magazine)