Page 293 - The Master Handbook Of Acoustics

268 CHAPTER THIRTEEN

The character of the decay will be independent of the direc-
tional characteristics of the measuring microphone.

These six factors are observation oriented. A professional physicist
specializing in acoustics might stress fundamental and basic factors in
his definition of a diffuse sound field such as energy density, energy
flow, superposition of an infinite number of plane progressive waves,
and so on. The six characteristics suggested by Randall and Ward
point us to practical ways of obtaining solid evidence for judging the
diffuseness of the sound field of a given room.

Evaluating Diffusion in a Room

There is nothing quite as upsetting as viewing one’s first attempt at
measuring the “frequency response” of a room. To obtain the fre-
quency response of an amplifier, a variable-frequency signal is put in
the front end and the output observed to see how flat the response is.
The same general approach can be applied to a room by injecting the
variable frequency signal into “the front end” by means of a loud-
speaker and noting the “output” picked up by a microphone located
elsewhere in the room.

Steady-State Measurements

Figure 13-1 is a graphic-level recorder tracing of the steady-state response
of a studio having a volume of 12,000 cubic feet. In this case, the loud-
speaker was in one lower tricorner of the room, and the microphone was
at the upper diagonal tricorner about one foot from each of the three sur-
faces. These positions were chosen because all room modes terminate in
the corners and all modes should be represented in the trace. The fluctua-
tions 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 evi-
dence 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 (due to radiating into ⁄4 space)
1
should not be charged against the room. The rest is primarily room effect.
The response of Fig. 13-1 is typical of even the best studios. Such
variations in response are, of course, evidence of a sound field that is