Page 221 - Master Handbook of Acoustics

versus frequency; this is sometimes called the reverberation characteristic.
This reverberation-time analysis is quite different from a real-time analysis that shows level
versus frequency. For example, consider a room that exhibits excessive reverberation time at low

frequencies, causing poor bass clarity. If the problem is mistakenly addressed through equalization,
by rolling off low frequencies, a real-time analysis might show a flat low-frequency response. But
reverberation time measurements would show that reverberation time is still too long at low
frequencies and bass clarity is still poor; the problem has not been addressed. That is because
equalization will not change the reverberation characteristic. To solve the problem of frequency
balance, acoustical treatment must be applied to change the reverberation characteristic.

To measure the reverberation characteristic, a pink-noise source is band-pass filtered to produce
octave-band noise at various center frequencies. For example, these center frequencies may be used:
63; 125; 250; 500; 1,000; 2,000; 4,000; and 8,000 Hz. The source is activated to produce a steady-
state level, then turned off. The RT is the time required for the level to drop 60 dB. As noted,
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because of dynamic-range constraints, the first portion of the decay can be extrapolated to produce the
full measurement. The entire measurement is conveniently performed by recording the steady-state
noise and the decay. Because of fluctuations in level at low frequencies, measurements must be
repeated. Because only the time decay of a narrow band is relevant, the frequency response of the
source loudspeaker and microphone are relatively unimportant. Also, because of room modes,
measurements at low frequencies are affected by position of the source and the microphone.

As noted, the results are plotted as reverberation time versus frequency. Figure 11-10 shows an
example of a room’s reverberation characteristic before and after room treatment. As shown, the
room exhibited a significant rise in reverberation time in the upper bass and lower midrange. After
treatment, reverberation time was much flatter, with a desirable moderate increase in reverberation
time at low frequencies. Generally, rooms are preferred with a smooth characteristic, with somewhat

higher RT at low frequencies. Undue bumps in the reverberation-time characteristic can be reduced
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by adding absorption treatment in that frequency range. A practical example of room treatment to
obtain desirable reverberation times is given at the end of this chapter.