Page 218 - Master Handbook of Acoustics

modal excitation pattern will be somewhat different. For example, the 12 modes in the 63-Hz octave
will all be highly energized, but each to a somewhat different level the instant the noise is stopped.
This helps address, but does not completely solve, the effect that room normal modes have on the
measured reverberation time in small rooms.

Mode Decay Variations

To illustrate this discussion, let’s consider measurements in a real room. The room is a rectangular
studio for voice recording having the dimensions 20 ft 6 in × 15 ft × 9 ft 6 in, with a volume of 2,921
3
ft . The measuring equipment is that described in Fig. 11-5, and the technique is that described
previously. Four successive 63-Hz octave decays from the graphic-level recorder are shown in Fig.
11-8A. These decays are not identical, and differences can be attributed to the random nature of the

noise signal. In particular, the fluctuations in the decays result from beats between closely spaced
modes. Because the excitation level of the modes is constantly shifting, the form and degree of the
beat pattern shifts from one decay to another depending on where the random excitation happens to be
the instant the noise is stopped.

3
FIGURE 11-8 Decays of random noise recorded in a small studio having a volume of 2,921 ft . (A)
Four successive 63-Hz octave decays recorded under identical conditions. (B) Four successive 500-
Hz octave decays also recorded under identical conditions. The differences noted result from the