Noise, by its very nature, can be highly variable - both in amplitude and character. Between the lowest sounds at the limit of audibility and upper limit of comfort, the intensity ranges from 1 pico-watt up to 1 milli-watt per square metre. Compared to other common forms of energy, such as heat and light, these levels are surprisingly low.
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.
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.