Noise Pollution Level (NPL)

A noise measurement procedure recently introduced in the U.S. in an attempt to relate various earlier studies of community noise. The measurement is conceived so that it combines the Ambient Noise Level with the degree of steadiness in time of the noise (assuming that the less steady it is, the more distracting and annoying it becomes). The basic definition is:

L_NP = L_eq + kσ

where L_eq is the Equivalent Energy Level measured in dBA or PNdB, k is a constant which is provisionally given the value 2.56, and σ is the standard deviation of instantaneous levels in time. This measurement system applies to any environment, unlike those specifically concerned with aircraft and traffic. As a result, however, it is incapable of determining whether the noise being measured is wanted or unwanted sound.

See also: Noise Level, Noise Pollution. Compare: Traffic Noise Index.

Ref.: D.W. Robinson, "Towards a Unified System of Noise Assessment," Journal of Sound and Vibration, vol. 14, 1971, pp. 279-98.