Simpson 886-2, Operator'S Manual, Page: 16 / 24

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3.10 Background Noise
Ideally, any sound source being measured should be isolated from effects due to
extraneous or background noise. If the background noise is 10 dB or more below
the sound source level desired, the effect from the background noise can be
considered negligible. In instances where the ratio is less than 10 dB, an ap-
proximation 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
ordinate 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.
3.11 Dual Sound Sources
When separate measurements are made on two sound sources, the combined
effect cannot be found 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, then 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.
Table 3-2. Combining Noise Sources
Difference Between No. Of dB to be Added to
Levels in dB Higher Level
0 3.0
2 2.1
4 1.5
6 1.0
8 0.6
10 0.4
4. OPERATING INSTRUCTIONS
This section contains instructions required for the operation of the 886-2 Sound
Level Meter. Follow these instructions carefully to obtain accurate and useful
data.
If noise levels exceed 115 dB(A) (SLOW response) immediate use of protective
ear muffs or plugs is required. According to the Occupational Safety and Health
!