Instruction manual

121
Technical Notes
Background Noise
When measuring a certain sound in a certain location, all other sounds present at that
location except the measurement target sound are background noise (also called ambient
noise or dark noise). Since the sound pressure level meter will display the combination 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 influence of background noise is small and
may be disregarded. If the difference is less than 10 dB, the values shown in the table
below may be used for compensation, to estimate the level of the target sound.
If for example the measured sound pressure level when operating a machine is 70 dB, and
the background sound pressure level when the machine is not operating is 63 dB, the
compensation value for the difference of 7 dB is -1 dB. Therefore the sound pressure level
of the machine can be taken to be 70 dB + (-1 dB) = 69 dB.
The above principle for compensating the influence of the background noise assumes that
both the background noise and the target sound are approximately constant. If the back-
ground noise fluctuates, and especially if it is close in level to the target sound, compensa-
tion is difficult and will often be meaningless.