Combine Noise Levels

Decibels are logarithmic, so none of this works the way intuition expects. Two identical 80 dB machines make 83 dB, not 160. A source 10 dB below another adds almost nothing. And the arithmetic mean of several readings is simply the wrong average — energy has to be averaged, not the levels.

Per ISO 11201 a measurement less than 3 dB above the background cannot be corrected reliably; between 3 and 10 dB apply the correction shown; above 10 dB the background is negligible.