Page 173 - Master Handbook of Acoustics

FIGURE 9-1 The recorder tracing of a slowly swept sine-wave sound-transmission response of a
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12,000-ft studio. Fluctuations of this magnitude, which characterize many studios, are evidence of
nondiffusive conditions.

The response of Fig. 9-1 is typical of even the best studios. Such variations in response are
evidence of a sound field that is not perfectly diffuse. A steady-state response such as this taken in an
anechoic room would still show variations, but of lower amplitude. A very live room, such as a
reverberation chamber, would show even greater variations.
Fixed measurements are one way to obtain the steady-state response of a room. Another method of
appraising room diffusion is to rotate a highly directional microphone in various planes while holding

the loudspeaker frequency response constant and recording the microphone’s output to the constant
excitation of the room. This method is used with some success in large spaces, but the method is ill
adapted to smaller rooms, where diffusion problems are greatest. In principle, however, in a totally
homogeneous sound field, a highly directional microphone pointed in any direction should pick up a
constant signal. Both fixed- and rotating-measurement methods reveal the same deviations from a

truly homogeneous sound field. We see that the criteria of negligible frequency and spatial
irregularities are not met in any practical room.

Decay Beats

By referring ahead to Fig. 11-9, we can compare the smoothness of the reverberation decay for the