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. Note 1: Ambient noise level is measured with a sound level meter. Note 2: Ambient noise level is usually measured in dB above a reference pressure level of 0. 00002 Pa, i.e., 20 Pa (micropascals) in SI units. A pascal is a newton per square meter. Note 3: In the centimeter-gram-second system of units, the reference level for measuring ambient noise level is 0. 0002 dyn/cm2. Synonym room noise level.