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Name: Joseph
Status: Other
Grade: Other
Location: N/A
Country: United States
Date: October 2007


Question:
What is the use of a hole in the cabinet of a subwoofer? Is it for limiting the diffracting effect of the sound? What makes the speaker produce a louder bass through the hole?



Replies:
The membrane of a subwoofer moves a lot. Without the hole in the back of the cabinet, it would compress the air in the cabinet. This would interfere with the movement of the speaker cone, just in the same way that it is difficult to compress a sealed plastic bag. Allowing the air pressure to equalize on both sides of the cone allows it to move farther (more volume to the sound) and with greater fidelity.

Richard Barrans, Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming


Hi Joseph,

The "hole" you see in some types of woofer cabinets is an inherent part of the type of low frequency cabinet design called a "bass reflex" system. There are several common types of basic woofer cabinet designs... Infinite Baffle (a completely sealed cabinet), Air Suspension (a variant of the Infinite baffle), Transmission Line, and Bass reflex.

In any speaker system, the sound radiating from the front of the speaker cone, is exactly out of phase from the sound radiating from the back. This is obvious is you think about it, because when the cone moves outward, it briefly compresses the air in front, but the back side creates a momentary partial vacuum as the cone momentarily pulls away from the air behind it. Thus, the sound from the front will tend to cancel the sound from the rear. In an Infinite Baffle (or sealed box) system, the back of the speaker is sealed off, to avoid sound cancellation, and thus the sound from the rear is wasted.

In a Bass Reflex system, the cabinet is designed so that at a certain low range of frequencies, the sound coming from the "hole" or "port" is delayed slightly so that what comes out of the port is now in phase with the sound coming from the front of the speaker.

This does several things. First, the sound from the port, reinforces the sound from the front of the speaker at very low frequencies, and increases the low bass output. Second, this design, if done right, extends the low frequency response to almost an octave lower in frequency than would be possible with a sealed enclosure. Third, the system efficiency is increased; that means louder bass for the same amplifier power.

Notice I said "if done right". Design of a good bass reflex enclosure is VERY tricky. It is not so simple as punching a hole in the speaker cabinet! If it is not done correctly, the result is usually a speaker that produces "one-note bass"; that is, it produces annoying, loud bass only over a very narrow range of frequencies. In fact, most low-cost bass reflex woofers have terrible frequency response, and tend to produce a very unnatural, boomy sound that may be good for impressing young children or rattling windows, but are very inaccurate sound reproducers. A correctly designed bass reflex system, on the other hand, can sound very good indeed, producing "tight", natural, non-boomy bass that is a pleasure to listen to.

By the way, as a minor point, most people inaccurately refer to "woofers" as "sub-woofers". A woofer is the normal bass speaker, of which there are two (left and right) in a stereo system. A sub-woofer is an additional ultra low frequency speaker that reproduces only the very low frequencies below that which normal woofers are able reproduce. Typically, because very low bass notes do not seem to be directional, there is a single subwoofer that works for both the left and right stereo channels. Most of what many people call "sub- woofers" are simply standard woofers.

Regards,

Bob Wilson



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