Noise Definitions and Useful Websites


Energy equivalent noise level. It is a time-averaged sound level; a single-number value that expresses the time-varying sound level for the specified period as though it were a constant sound level with the same total sound energy as the time-varying level. Expressed over a set period of time (e.g. 5-min, 8-hr, 24-hr etc), known as the "time base".

Source: European Environment Agency

Percentile level

The noise level equalled or exceeded for a certain percentage of time, x, during the measuring period. Percentile levels are written Lx. For example, L10 is the level which is equalled or exceeded for 10% of the measurement time.

Source: Standards New Zealand


Day/night level or day-night average sound level. Is the time-average sound level, in decibels, over a 24 hour period (from midnight to midnight), obtained after the addition of 10 decibels to sound levels in the night (from midnight to 7.00am and from 10.00pm to midnight). A frequency weighting shall be assumed unless otherwise stated. When the day/night level is measured it is not necessary that the measurement period begin at midnight.

Source: Standards New Zealand


The maximum noise level measured with a sound level meter having a "fast" response, or an equivalent method.

Source: Standards New Zealand

Decibel (dB)

A dimensionless unit used to express the relative magnitude of sound powers and sound pressures. The number of decibels is ten times the logarithm (to base 10) of the ratio of the sound powers and twenty times the logarithm (to base 10) of the ratio of the sound pressures.

Source: Standards New Zealand

Useful websites