Page 175 - Master Handbook of Acoustics

FIGURE 9-2 A typical double-slope decay, showing evidence of a lack of diffuse sound conditions.
The slower decaying final slope is probably due to modes that encounter lower absorption.

Another type of nonexponential decay is illustrated in Fig. 9-3; the response deviates from a
straight-line slope. This is a decay of an octave band of noise centered on 250 Hz in a 400-seat
chapel, poorly isolated from an adjoining room. Decays taken in the presence of acoustically coupled

spaces are characteristically concave upward (such as in Fig. 9-3) and often the deviations from the
straight line are even greater. When the decay traces are nonexponential, that is, when they depart
from a straight line in a level versus time plot, we conclude that true diffuse conditions do not
prevail.