Page 275 - Master Handbook of Acoustics

Some music rooms owe their acoustical excellence to the low-frequency absorption offered by
extensive paneled walls. Plywood or tongue-and-groove flooring or sub flooring 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 an important role in the construction of homes, studios, control
rooms, and other spaces. Drywall absorbs sound by a flexural, diaphragmatic action, working as a
resonant system. Drywall is particularly important in the absorption of low-frequency sound. 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 reverberate
conditions. Drywall of 1/2-in thickness on studs spaced at 16 in offers an absorption coefficient of
0.29 at 125 Hz and even higher at 63 Hz (which would be of interest in music recording studios).
Drywall absorption in small audio rooms must be recognized and its low-frequency absorption
included in calculations. This is sometimes difficult because different thicknesses of drywall are

used, and the frequency of peak absorption varies according to thickness and airspace. A 1/2-in
2
2
drywall has a surface mass of 2.1 lb/ft and double 5/8-in drywall has a surface mass of 5.3 lb/ft .
With an airspace of 3-3/4-in, the 1/2-in panel resonates at 60.6 Hz, and the double 5/8-in panel
resonates at 38.1 Hz.
We have noted that porous materials commonly show their greatest absorption in the high-

frequency region. Vibrating panel arrangements show their best absorption in the low frequencies. In
treating small listening rooms and studios we find that structures giving good low-frequency
absorption are invaluable in controlling room modes.
Panel sound absorbers are quite simple to build. An example of a panel absorber to be mounted on

a flat wall or ceiling surface is shown in Fig. 12-24. A 1/4- or 1/16-in plywood panel is fastened to a
wooden framework to give the desired spacing from the wall. A glass or mineral fiber blanket of 1 to
1-1/2 in is glued to the wall surface. An airspace of 1/4 or 1/2 in should be maintained between the
absorbent and the rear surface of the plywood panel.