Ambient noise level
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In atmospheric sound transmission or noise pollution, ambient noise level is the sound pressure level at a given location, normally specified as a reference level to study a new intrusive sound source. Ambient sound levels are often measured in order to map sound conditions over a spatial regime to understand their variation with locale. In this case the product of the investigation is a sound level contour map. Alternatively ambient noise levels may be measured to provide a reference point for analyzing an intrusive sound to a given environment. For example, sometimes aircraft noise is studied by measuring ambient sound without presence of any overflights, and then studying the noise addition by measurement or computer simulation of overflight events. Or roadway noise is measured as ambient sound, prior to introducing a hypothetical noise barrier intended to reduce that ambient noise level.
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. Most frequently ambient noise levels are measured using a frequency weighting filter, the most common being the A-weighting scale, such that resulting measurements are denoted dBA, or decibels on the A-weighting scale.
Ambient noise level is sometimes called background noise level, reference sound level or room noise level. In practical applications ambient noise levels are often reported using a day night weighting scheme such as Community noise equivalent level, (CNEL) in order to account for greater noise sensitivity in evening and nighttime hours.
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This article contains material from the Federal Standard 1037C (in support of MIL-STD-188), which, as a work of the United States Government, is in the public domain.
