How is Noise Measured?

The units used in New Zealand for measuring and assessing sounds in the environment are set out in NZS 6801: 2008 Acoustics - Measurement of environmental sound. In summary, these units are:

Lmax The single highest sampled level of sound. Used in night-time emission limits as a means of ensuring sleep protection. Short duration, high-level sounds such as audible warning devices, pressure relief valves have a significant effect on Lmax values.

L10 or L10 The level of sound exceeded for no more than 10% of the monitoring period. This level of sound therefore equates to an average maximum sound and is used widely in emission limits as the L10 correlates well with the subjective reaction to sound

L90 or L90 The level of sound exceeded for 90% of the monitoring period. This level of sound can be used to define the background sound level, and is influenced by constant sources such as industrial equipment and constant background city sounds, eg from air handling equipment. Noise emission limits are not generally specified in terms of an L90 level.

Leq,T or Leq,T The time-averaged sound level (or equivalent sound level) over the measurement period, T, that has the same mean square sound pressure level as the time-varying sound level under consideration. Commonly referred to as an 'energy average' measure of sound exposure.

All the above units are measured using 'dBA' values.