BRUEL & KJAER 1022 Instructions And Applications page 34

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73 ~ ~ r o r r c m - · l : l l l i l i l ! l l l l l i l l l i ! l l l l l l l l l l l l l l l l l l l l l l i l ! l l l ~ l ~ l ~ i ~ l l !
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"' T=ot5sec.!=J:=I=l=
Mod
Freq
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10
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oo
QP 0123 10 100 1000 10000
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Fig.5.11. Recording of decay curves, compressor loop used
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BrUel
&
ICjmr
f=
Potentiometer Range : 50 315C/s
~Rectifier :
RMS
f=
Lower Limiting Freq. 50
r=
Writing Speed: 400
f=:
Paper Speed : · 100 T• D.45sec.
~Drive
Shaft Speed : 12
~ Loop
Length: 490mm
~Measuring
room: Information Deportment
f= Dote: 20-1-61 B.E. B.
1000t/s
4000C7s ~ 100~C/s
__
T• O .Jsec.
f 618H'
Fig.5.12. Decay curves at 10 mm intervals recorded on a 490 mm loop
divided into four sections marked "75 dB 10 mm/sec.", "75 dB 30 mm/
sec.", "50 dB 10 mm/sec.", and "50 dB 30 mm/sec.". When one of these
four combinations of RANGE POTENTIOMETER and PAPER SPEED has
been employed during the measurements, the reverberation time can be read
directly in seconds.
1. The Protractor is held so that the printing is readable. The proper section
is chosen and its left limiting line (thick diagonal) is placed on top of the
portion of the recorded decay curve to be measured, and in such a man-
ner that the centre of the Protractor coincides with one of the horizontal
lines on the recording paper. See Fig.5.13.
2. Reverberation time in seconds is then read on the scale at the point
through which the horizontal line passes.
The decay curves should preferably be approximated into a straight line
making it easier to determine the average slope.
If paper speeds other than 10 and 30 mm/sec. have been used, the deter-
mined reverberation times should be multiplied or divided by factors of 10.
34