Page 230 - The Master Handbook Of Acoustics

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ABSORPTION OF SOUND

tutes a vacuum that tends to suck sound energy into this “sound sink”
from surrounding areas. The bass trap’s effect, then, is greater than its
opening area would suggest because of this “vacuum cleaner” effect.
The bass trap effect, like the drape spaced from a reflective wall,
occurs not only at a quarter-wavelength depth, but also at odd multi-
ples of a quarter wavelength. Great trap depths are required for very
low bass frequencies. For example, a quarter wavelength for 40 Hz is
7 ft. Unused spaces above control room ceilings and between inner
walls and outer shells are often used for trap space.

Diaphragmatic Absorbers

The absorption of sound at the lower audible frequencies can be
achieved by porous absorbers or by resonant (or reactive) absorbers.
Glass fiber and acoustical tiles are common forms of porous absorption
in which the sound energy is dissipated as heat in the interstices of the
fibers. The absorption of commercial forms of glass fiber and other
fibrous absorbers at low audio frequencies, however, is quite poor. To
absorb well, the thickness of the porous material must be comparable
to the wavelength of the sound. At 100 Hz, the wavelength is 11.3 ft,
and using any porous absorber approaching this thickness would be
impractical. For this reason we turn our attention to the resonant type
of absorber to obtain absorption at low frequencies.
Some of the great chamber music rooms owe their acoustical excel-
lence to the low-frequency absorption offered by extensive paneled
walls. Plywood or tongue-and-groove flooring or subflooring vibrates
as a diaphragm and contributes to low-frequency absorption. Drywall
construction on walls and the ceiling does the same thing. All such
components of absorption must be included in the acoustical design of
a room, large or small.
Drywall or gypsum board plays a very important part in the con-
struction of homes, studios, control rooms, and other spaces. It also
plays an important part in the absorption of low-frequency sound in
these spaces. Usually, such low-frequency absorption is welcome, but in
larger spaces designed for music, drywall surfaces can absorb so much
low-frequency sound as to prevent the achievement of the desired rever-
1
beratory conditions. Drywall of 2-inch thickness on studs spaced 16
inches offers an absorption coefficient of 0.29 at 125 Hz and even higher