BRUEL & KJAER 2204 - Influence of Background Noise

ly a separate indicating instrument may be used connected to the OUTPUT
of the Sound Level Meter.
When the sound comes from all directions it may be necessary to use a
Random Incidence Corrector and an Extension Connector in order to obtain
a better omnidirectivity.
Equipped with an Extension Connector and a 1/2'' microphone the instru-
ment meets all the requirements of the I.E.C. recommendation for precision
sound level meters as well as the American standard, provided that the
system is properly calibrated, e.g. with a pistonphone or sound level cali-
brator. See p. 45.
Influence of Background Noise.
If it is required to measure the sound emitted from a particular piece of
equipment, e.g. an electric motor, best results would be obtained by removing
the motor and carrying out the measurements in an anechoic room with no
disturbing background noise. However, this is not always possible, and the
measurements have to be done with the background noise present.
If the background sound level with the motor switched off is much lower than
the sound level with the motor running, no correction is required, but if the
Number of
dB to
be
subtracted
from
total
reading
7
6
5
3
2
0
0
1\
1\
\
1\
\
\.
\
~
f'
......... ~'..
r----. .....
.......
........
-
2 3 4 5 6
7
Fig. 6.8. Background correction.
r--
-
~
8
9 10 dB
Difference between
total reading and
background reading
t'6"J72~
43