BRUEL & KJAER 1022 Instructions And Applications page 31

must be small in comparison to the time constant determining the compres-
sor speed. Under normal circumstances these conditions are easily fulfilled.
To give reliable measurements the room to be used need not be fully
anechoic as the regulating effect of the compressor will compensate for any
minor reflections set-up. However, for correct operation of the regulation
circuit, the reverberation time of the room must not be too long and a low
scanning speed should be used for the frequency sweep.
In Fig.5.7. is a recording showing the frequency response of a micropho-
ne recorded by employing the previously outlined system.
Measurement of Reverberation Time
One of the more important factors that determines the acoustical quality
of a room is its reverberation time and the 1022 has been designed with
certain special features to aid such measurements. The compressor circuit
serves to keep the sound level in the room constant with frequency while
frequency modulation of the signal ensures that standing waves and inter-
ference phenomena are avoided. The latter feature helps to produce smooth
slopes to recorded decay curves whereas if pure sine waves were radiated in
the room, standing waves might produce uneveness.
Various set-ups can be used to measure reverberation time using the 1022
as an integral part. An arrangement whereby the decay curves are recorded
automatically using the Level Recorder 2305 is discussed here. The set-up
can be seen in Fig.5.8. The Frequency Spectrometer 2113 is chosen as
amplifier for the microphone since selective reception in 1/3 or 1/1 octave
bands will reduce background noise and increase the dynamic range of the
decay curves.
Decay curves throughout the complete audio range can be recorded auto-
matically at 1/3 octave intervals using this combination of equipment and
can be made on both frequency calibrated or nonfrequency calibrated pa-
per. In order to record the decay of sound in the room the sound source
must be disconnected at definite intervals by means of the oscillator stop.
Only the decay curve is required so the pen should be lifted from the paper
between decay curves and, as selective reception is used, the filters of the
Spectrometer also need to be switched between curves. The oscillator stop,
pen lift and the filter switching can all be performed simultaneously by
remote control from the two-channel selector of the Level Recorder. The
necessary connections between the different instruments are shown in Fig.
5.9 a. whilst the electrical circuit produced can be seen in Fig.5.9 b.
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