Page 172 - Master Handbook of Acoustics

Evaluating Diffusion in a Room

To obtain the frequency response of an amplifier, a variable-frequency signal is input and the output
is observed to measure the response. The same approach can be applied to a room’s playback system
by reproducing a variable-frequency signal through a loudspeaker and measuring the acoustical signal
picked up by a microphone located in the room. However, the acoustical response of playback
systems in rooms is never as flat as the response of electrical devices. These deviations are partly
due to nondiffusive conditions in the room. As noted, diffusion is welcome because it helps envelop

the listener in the sound field. On the other hand, too much diffusion can make it difficult to localize
the sound source.

Steady-State Measurements

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Figure 9-1 shows the steady-state response of a small studio with a volume of 12,000 ft . In this
example, the loudspeaker was placed in one lower tri-corner of the room, and the microphone was at

the upper diagonal tri-corner about 1 ft from each of the three surfaces. These positions were chosen
because all room modes terminate in the corners and all modes should be represented in the trace.
The fluctuations in this response cover a range of about 35 dB over the linear 30- to 250-Hz sweep.
The nulls are very narrow, and the narrow peaks show evidence of being single modes because the
mode bandwidth of this room is close to 4 Hz. The wider peaks are the combined effect of several
adjacent modes. The rise from 30 to 50 Hz is due primarily to loudspeaker response, and the 9-dB
peak between 50 and 150 Hz is due to radiating into a one-quarter space; therefore, these are testing

artifacts that are not part of the room response.