15
mation can be made using Figure 3-4 and measurements of the background
noise alone and the sound source with the background noise. Simply take the
difference between these measurements and locate this difference value of the
abscissa of Figure 3-4. Move in a vertical direction from this point to the intersect
of the curve and then horizontally to the corresponding ordinate point. This ordi-
nate point indicates a value to be subtracted from the total or sound source with
background noise measurement. The difference obtained represents the sound
source level. It should be noted that a difference of 3 dB or less between sound
source and background noise will provide only an indication of this sound source
level, not an accurate measurement.
3.9
Dual Sound Sources
When separate measurements are made on two sound sources, the combined
effect cannot be predicated by direct addition. For example: If two machines are
emitting equal noise levels of 80 dB, the combined level will be 83 dB, not 160 dB.
If the level of one source is 10 dB or greater than the other, the lower level source
can be disregarded as it will be "masked" by the higher level source. Information
listed in Table 3-2 can be used for calculating the combined effect of the two
sources. If more than two sound levels are involved, combine the highest two first,
then combine the total with the next highest remaining sound level. Continue this
procedure until all noteworthy levels are combined.
Figure 3-4. Background Noise Correction
10
9
8
7
6
5
4
3
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0
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