Page 271 - Master Handbook of Acoustics

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volume of room, ft or m 3

The value of the air attenuation coefficient m varies with humidity. With humidity between 40 and
3
60%, the values of m at 2, 4, and 8 kHz are: 0.003, 0.008, and 0.025 sabins/ft and 0.009, 0.025, and
3
0.080 sabins/m , respectively.
3
For example, a church seating 2,000 people has a volume of 500,000 ft . At 2 kHz and 50%
3
relative humidity, the air absorption is 0.003 sabins/ft . This yields 1,500 sabins of absorption at 2
kHz.

Panel (Diaphragmatic) Absorbers

The absorption of sound at lower audible frequencies can be effectively achieved by resonant (or
reactive) absorbers. Glass fiber and acoustical tiles are common forms of porous absorbers in which
the sound energy is dissipated as heat in the interstices of the fibers. However, the absorption of glass
fiber and other fibrous absorbers at low audio frequencies 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, resonant absorbers are often used to obtain absorption at low
frequencies.
A mass suspended from a spring will vibrate at its natural frequency. Panels designed with an air
cavity behind them act similarly. The mass of the panel and the springiness of the air in the cavity are
together resonant at some particular frequency. Sound is absorbed as the panel is flexed because of

the damping caused by frictional heat losses of the material within the panel. (Similarly, a mass on a
spring will stop oscillating because of damping.) Absorption provided by panel absorbers is usually
relatively modest because the resonant motion also radiates some sound energy. Panels made of limp
materials with high damping provide greater absorption.
Damping increases as the velocity of the panel increases, and velocity is highest at the resonant
frequency. There the absorption of sound is maximal at the frequency at which the structure is

resonant. As noted, the enclosed and sealed air cavity behind the panel acts as a spring; the greater
the depth of the airspace, the less stiff the spring. Likewise a smaller airspace acts as a stiff spring.
The frequency of resonance for a flat, unperforated panel can be estimated from:

where f =
0
frequency of resonance, Hz
m =
2
surface density of panel, lb/ft or kg/m 2
d =
depth of airspace, in or m

Note: In metric units, change 170 to 60.