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Hole in Sub-Woofer Cabinet
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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Update: June 2012
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