94
Technical Reference
Infl uence of Background Noise
When measuring a certain sound in a certain location, all other sound present
at that location except the measurement target sound are background noise
(also called background). Since the sound level meter will display the com-
bination of target sound and background noise, the amount of background
noise must be taken into consideration when determining the level of the
target sound.
If the difference between the meter reading in absence of the target sound
and the reading with the target sound is more than 10 dB, the infl uence of
background noise is small and can be disregarded. If the difference is less
than 10 dB, the values shown in the table below may be used for compensa-
tion, to estimate the level of the target sound.
If for example the measured noise level when operating a machine is 70 dB,
and the background noise level when the machine is not operating is 63 dB,
the compensation value for the difference of 7 dB is -1 dB. Therefore the
noise level of the machine can be taken to be 70 dB + (-1 dB) = 69 dB.
The above principle for compensating the infl uence of the background noise
assumes that both the background noise and the target sound are approxi-
mately constant. If the background noise fl uctuates, and especially if it is
close in level to the target sound, compensation is diffi cult and will often be
meaningless.
4
5
6
7
8
9
-2
-1
Background noise compensation
Display reading difference with
and without target sound (dB)
Compensation value (dB)
Summary of Contents for NA-42
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