Sound-measuring instruments do not distinguish between the noise of principal interest and any background noise present. Background noise corrections are needed to determine the contribution of a specific source or a group of sources.
The simplified procedure is as follows: the noise level is measured with the unknown source(s) in operation; then with the other conditions unchanged, the unknown source is stopped and the background level measured. (If several different sources are being measured, they can be turned off in different combinations.) Analysts must evaluate the difference between the readings on an energy basis, not simply by taking numerical differences.
Table 6.4.3 provides a convenient means for adding or subtracting dB values, accurate enough for most work. Background noise corrections subtract the background from the total noise. For example, if the total noise measures 93 dB and the background noise measures 89 dB, the level for the unknown source is 90.7 dB. Using these tables, analysts can also add dB values; the values for more than one source are measured separately, then combined in pairs. Three sources, of 85, 87, and 89 dB, add to give 92 dB. In both addition and subtraction, the dB values must be obtained in the same manner, including weighting.
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