Assessment of non-steady sound
Most sounds are not steady, so that the sound pressure level fluctuates with time. A measurement is therefore meaningless unless we know whether it represents a minimum, maximum or some kind of time-averaged level. Various parameters have been derived to measure sounds of differing characters, and the most relevant to this report are as follows:
- Leq,T: The equivalent continuous noise level is used widely to measure noise that varies with time. It is defined as the notional steady noise level that would contain the same acoustic energy as the varying noise. Because the averaging process used is logarithmic, the Leq,T level tends to be dominated by the higher noise levels measured.
- L90,T: This is the sound pressure level exceeded for 90% of the measurement period T. It is an indication of noise levels during the quieter periods of measurement, and is widely used to measure background noise.
- Lmax: This is the maximum level measured, and is used to assess sleep disturbance from intermittent sources such as aircraft and train noise. Lmax is normally defined as the maximum reading given by a sound level meter set to “Fast” response (with a 0.125 second time constant).