Noise, by its very nature, can be highly variable - both in amplitude and character.
The EPA has issued a Noise Measurement Procedures Manual to define methods to be used for measuring, estimating, calculating and assessing sound pressure levels.
Noise Measurement Procedures Manual (1Mb)
Noise amplitude is generally represented as decibels (dB) on a logarithmic scale where the limit of hearing is about 0 dB and the upper limit of comfort is about 100 dB. As a refinement, the frequency response can be adjusted to that of the human ear, in which case the level is quoted in dB(A) where A refers to the 'A-weighting' idealised frequency response of the undamaged human ear.
Although noise is quantified in terms of dB(A), its content can have an additional and profound effect on both the subjective loudness and the degree of annoyance/nuisance that can be attributed to the noise. These effects are referred to as 'intrusive or dominant characteristics' and include tonality, impulsiveness, modulation and low frequency. These characteristics are defined in the Noise Measurement Procedures Manual for the purpose of adjusting measured dB(A) levels to allow for the increased loudness and/or annoyance caused by such characteristics. This adjustment scheme is not perfect. Very complex sounds, such as animal growls or sounds that include many discrete tones, may not be adequately evaluated.
In a given time interval, the noise level can change over a significant range and it is common to describe the 'average' sound level as the equivalent noise level (denoted Leq) – being the level of a continuous noise that has the same sound energy as the noise under investigation. The Leq is thus the dB(A) representation of the average acoustic energy, not the average of the dB(A) level.
It is also useful to measure the statistical distribution of noise levels. The statistical distribution can be used to segregate the contribution of different noise sources. It is common to describe noise in terms of its L-statistics, Ln, where Ln is the noise exceeded n% of the time. The most commonly used are the L90, which is representative of the lower noise levels, and L10, towards the upper levels, is often used for measuring the contribution of traffic noise.