Page 174 - Master Handbook of Acoustics

eight octaves from 63 Hz to 8 kHz. In general, the smoothness of the decay increases as frequency is
increased. The reason for this, as explained in Chap. 13, is that the number of modes within an octave
span increases greatly with frequency, and the greater the mode density, the smoother their average
effect. Conversely, in this example, beats in the decay are greatest at 63 and 125 Hz. If all decays

have the same character at all frequencies and that character is smooth decay, complete diffusion
prevails. In practice, decays (such as those of Fig. 11-9) with significant changes in character are
more common, especially for the 63- and 125-Hz decays.
The beat information on the low-frequency reverberation decay makes possible a judgment on the
degree of diffusion. The decays of Fig. 11-9 indicate that the diffusion of sound in this particular
studio is about as good as can be achieved by traditional means. Reverberation-time measuring

devices that yield information only on the average slope and not the shape of the decay omit
information that most consultants consider important in evaluating the diffuseness of a space.

Exponential Decay

A truly exponential decay can be viewed as a straight line on a level (logarithmic scale) versus time
plot, and the slope of the line can be described either as a decay rate in decibels per second or as a
reverberation time in seconds. The decay of the 250-Hz octave band of noise pictured in Fig. 9-2 has
two exponential slopes. The initial slope gives a reverberation time of 0.35 second and the final
slope a reverberation time of 1.22 seconds. The latter slow decay when the level is low is probably
due to a specific mode or group of modes encountering low absorption either by striking the

absorbent at grazing angles or striking where there is little absorption. This is typical of one type of
nonexponential decay, or stated more precisely, of a dual exponential decay.