Ambient noise level
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In telecommunications, ambient noise level or room noise level is the level of acoustic noise existing at a given location, such as in a room, in a compartment, or at a place out of doors. Ambient noise level is measured with a sound level meter. It is usually measured in dB above a reference pressure level of 0.00002 Pa, i.e., 20 μPa (micropascals) in SI units. A pascal is a newton per square meter. the centimeter-gram-second system of units, the reference level for measuring ambient noise level is 0.0002 dyn/cm2.
Source: from Federal Standard 1037C and from MIL-STD-188