Page 165 - The Master Handbook Of Acoustics

140 CHAPTER SEVEN

Measurement Procedure

Every time the switch is closed the room is filled with a very loud
wideband pink noise “sh-h-h-h” sound. This is usually loud enough to
require the use of ear protectors for everybody in the room. Opening
the switch, the sound in the room decays. The microphone, at its
selected position, picks up this decay, which is recorded on magnetic
tape for later analysis and study.
Signal-to-noise ratio determines the length of the reverberatory
decay available for study. As mentioned previously, it is rarely possi-
ble to realize the entire 60-dB decay involved in the definition of
RT60, nor is it necessary. It is quite possible, however, to get 45- to
50-dB decays with the equipment shown in Fig. 7-6 by the simple
expedient of double filtering. For example, the octave filter centered
on 500 Hz in the sound level meter is used both in recording and in
later playback for analysis.
The analysis procedure outlined in the lower part of Fig. 7-6 uses the
same magnetic recorder and B&K 2215 sound-level meter, with
the addition of a B&K 2305 graphic-level recorder. The line output of the
tape recorder is connected to the front end of the sound-level meter cir-
cuit through a 40-dB attenuating pad. To do this, the microphone of the
sound-level meter is removed and a special fitting is screwed in its
place. The output of the sound-level meter is connected directly to the
graphic-level recorder input, completing the equipment interconnec-
tion. The appropriate octave filters are switched in as the played-back
decay is recorded on the level recorder. The paper drive provides for
spreading out the time dimension at adjustable rates. The graphic-level
recorder offers a 50-dB recording range for the tracing pen on the paper.

Analysis of Decay Traces
An octave slice of pink noise viewed on a cathode-ray oscilloscope
shows a trace that looks very much like a sine wave except that it is
constantly shifting in amplitude and phase, which was the definition
of random noise in Chap. 5. This characteristic of random noise has its
effect on the shape of the reverberatory decay trace. Consider what this
constantly shifting random noise signal does to the normal modes of a
room (Chap. 15). When the axial, tangential, and oblique resonant
modes are considered, they are quite close together on the frequency