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In a reverberant space, the reverberation time T_R is normally defined to be the time for the sound pressure level to drop by 60 dB when the sound source is cut off (see Figure 3.20). Different reverberation times are desired for different types of spaces (see Figure 3.21). The Sabine formula is often used, T_R = T₆₀ (for 60 dB):

where V is room volume (m³), c is the speed of sound (m/s), S is wall area (m²), and is the angle‐averaged wall absorption coefficient, or

(3.74)

where S_i is ith wall area of absorption coefficient α_i.

Figure 3.20 Measurement of reverberation time T_R.

Figure 3.21 Examples of recommended reverberation times.

In practice, when the reverberation time is measured (see Figure 3.20), it is normal practice to ignore the first 5‐dB drop in sound pressure level and find the time between the 5‐dB and 35‐dB drops and multiply this time by 2 to obtain the reverberation time T_R.