BRUEL & KJAER 1022 Instructions And Applications page 33

Frequency Calibrated Paper
When 50 mm wide paper is made into a paper loop 495 mm long as in
Fig.5.10. (i.e. two chart lengths minus 5 mm where 5 mm is the distance
between two holes) it is possible to obtain curves spaced 1/3 octave apart
and synchronised with the frequency calibrated paper as shown in Fig.5.11.
The centre frequency of a particular filter is represented by the small black
squares at the top of the paper (QP 0424). It is possible to keep the sound
pressure level fairly constant by means of a compressor circuit as shown in
Fig.4.1. so ensuring that the decay curves commence at the same level.
Overlapping junction.
Fig.5.10. Making up of paper loop
Non-frequency Calibrated Paper
When more spacing than 5 mm is required between decay curves the
paper loop can be made 490 mm long to give 10 mm spacing as in Fig.5.12.
In such circumstances, however, only the lined paper QP 0402 can be used
and it is necessary to keep check of the frequency at which each particular
curve is taken.
Waxed paper and a stylus is preferable for reverberation measurements
due to the high paper speeds needed. If only a few reverberation curves are
needed automatic recording may not be necessary. The Oscillator Stop
push-button can then be used and the filter switching done by hand.
Use of the Protractor SC 2361
The Protractor has been designed to facilitate the determination of reverbe-
ration time from recorded decay curves on the 50 mm width paper. It is
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