Page 219 - Master Handbook of Acoustics

differences in random-noise excitation the instant the source is turned off to start the decay.

Even though the four decays are similar, fitting a straight line to evaluate the reverberation time of
each can be affected by the beat pattern. For this reason, it is good practice to record perhaps five
decays for each octave for each microphone position in a room. With eight octaves (63 Hz to 8 kHz),
five decays per octave, and three microphone positions, this means 120 separate decay readings for

each room, which is laborious. However, this approach yields a statistically significant view of the
variation with frequency. A handheld reverberation-time measuring device could accomplish this
with less work, but it may not give detail of the shape of each decay. There is much information in
each decay, and acoustical flaws can often be identified from aberrant decay shapes.
Four decays at 500 Hz are also shown in Fig. 11-8B for the same room and the same microphone
position. The 500-Hz octave (354 to 707 Hz) embraces about 2,500 room modes. With such a high

modal density, the 500-Hz octave decay is much smoother than the 63-Hz octave with only a dozen
modes. Even so, the irregularities for the 500-Hz decay of Fig. 11-8B result from the same cause.
Remembering that some modes die away faster than others, the decays in Fig. 11-8 for both octaves
are composites of all modal decays included.

Frequency Effect

Figure 11-9 shows decays for octave bands of noise from 63 Hz to 8 kHz as measured in the 2,921-ft 3
voice studio. The greatest fluctuations are in the two lowest bands, the least in the two highest. This is
what we would expect, given that the higher the octave band, the greater the number of normal modes
included, and the greater the statistical smoothing. However, we should not necessarily expect the
same decay rate because reverberation time is different for different frequencies. In this particular
voice studio, a uniform reverberation time with frequency was the primary design goal, which was

approximated in practice.