BRUEL & KJAER 4220 - Ambient Pressure and Volume Corrections

indication coincides with the S.P.L. (RMS Sound Pressure Level) pro·
duced by the Pistonphone. See Applications, for the case of B & K equip-
ment.
( 4) Stop the Piston phone as soon as the calibration has taken place in order
to conserve the batteries.
Ambient Pressure and Volume Corrections.
The S.P .L. produced by the Pistonphone is equal to the calibration value
indicated on the Calibration Chart. However, a correction must be made to
this value, if the ambient pressure is not the normal atmospheric pressure
(760 mm Hg = 1013 millibar = 29.9 inches Hg), and also, if the total volume
of the coupling cavity, including the equivalent volume of the microphone, is
not the same as in the reference case, where a B & K condenser microphone
with normal protecting grid is inserted.
The ambient pressure correction is immediately read off the barometer
supplied with the pistonphone, as described above.
The volume correction is, according to the pistonphone formula, if V' is the
total volume of the coupler cavity: 20 log1o ~, dB, to be added to the calibra-
tion value which corresponds to the volume V = 19.6 cm3.
For example, in the case of a pistonphone calibrated to 123.9 dB, and
assuming the pointer of the barometer indicating - o.2 dB and the piston-
vhone fitted with a special coupler to a certain microphone so that the total
volume is V' = 24.1 c.c., the S.P.L. produced will be in dB:
+
19.6
S.P.L. = 123. 9- o.2 20 log
24
_
1
= 123.9- o.2 -1.8 = 121.9
Volume
corrections for use with different types of standard microphones
directly fitted to the Pistonphone are given in the following table:
Type
Without Protecting
grid I
'Vith Protecting Grid
WE 640 AA + o.3 dB
I
+ o.42dB
MR 103
+ o.3 dB + o.42 dB
+ o.3 dB
B & K 4131, 4132
(with
coupler adapting 0
ring DB 0111)
15