Page 212 - Master Handbook of Acoustics

Some values of m in sabins per foot are: 0.003 at 2 kHz, 0.008 at 4 kHz, and 0.025 at 8 kHz; and in
sabins per meter, 0.009, 0.025, and 0.080, respectively. The value of m depends on relative humidity
of air, and the values given apply to RH between 40% and 60%. Air absorption increases at low
humidity.

Measuring Reverberation Time

There are many approaches to measuring the reverberation time of a room, and many instruments are
available to serve those who have an interest in the effects of reverberation. For example, sound
contractors need to know the approximate reverberation time of the spaces in which they are to install

a sound-reinforcement system, and measuring it may be more conclusive than calculating it.
Measurements can be more accurate than calculations because of inherent uncertainty in absorption
coefficients. Acoustical consultants called upon to correct a problem space or verify a carefully
designed and newly constructed space, generally lean toward the method of recording multiple sound
decays. These sound decays give a wealth of detail meaningful to the practiced eye.

Impulse Sources

Both impulse sources and steady-state sources can be used to examine a room’s response. Any sound
source used to excite an enclosure must have enough energy throughout the spectrum to ensure decays
sufficiently raised above the noise floor to give the required accuracy. Examples of impulse sound
sources are powerful electrical spark discharges, pistols firing blanks, and balloon bursts. For large
spaces, even small cannons have been used as impulse sources to provide adequate energy,
especially at lower frequencies. Whatever the actual method of producing an impulse, the impulse

decay, the room’s response measured over time, can be used to examine the temporal characteristics
of sound in a room. For example, the more diffuse the room’s sound field, the smoother the decay
response. Conversely, for example, a room with echoes will yield a nonuniform decay response as
energy is concentrated at the times when echoes appear.
The impulse decays for a small studio room are shown in Fig. 11-4. The sound source was an air

pistol that ruptures paper discs. The peak sound-pressure level at 1-m distance is 144 dB, and the
duration of the major pulse is less than 1 msec. Such a pulse is ideal for recording echograms.