Plan topics Noise management in mixed-use urban environments
What is noise?
Noise is defined as ‘unwanted’ sound. Humans are very sensitive to sound and can hear sounds that vary in acoustic energy from the whining of a mosquito’s wings to the racket of a jackhammer. Sound levels vary so greatly in magnitude that it is impractical to describe their sound pressures in familiar linear units. Sound pressures are therefore measured on a logarithmic ‘decibel’ (dB) scale. A ‘weighting’ of the sound level is widely used in measurements of environmental sound to ensure electronic instruments match the sensitivity of the human ear across the audible spectrum. Hence the notation ‘dBA’.
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If the dBA scale is applied to everyday activities then 0 dBA would be so quiet that the human ear would have problems detecting any noise. A quiet bedroom by contrast would generally have a noise level of a little under 40 dBA and a busy office 60 dBA. At around 65 dBA verbal communication starts to become difficult. A heavy truck travelling along a road close to where the noise is being measured may produce 90 dBA, a level of noise that may present a hazard to hearing if people are continuously exposed to it. Older style, or large jet aircraft may produce more than 100 dBA, while 120 dBA represents the threshold of pain.
Noise from land use activities is measured and assessed for its potential impact using the measurement units Lmax, L10, L90, and Leq,T (all expressed in dBA values). These units are defined in NZS 6801: 1999 Acoustics – Measurement of Sound.
Maximum permissible noise levels are specified in terms of the L10 criteria in most district plans and in the 1991 New Zealand Standards (NZS) dealing with noise (eg NZS 6802: 1991 Assessment of environmental sound). The L10 unit is used as it is seen as a good predictor of human annoyance at sounds in the environment. However, in New Zealand there has recently been a move to specify environmental sound using the Leq,T unit (which is more aligned with international practice).
In addition, Lmax limits are applied during night-time in residential areas to control loud single events that can interrupt sleep. While noise limits may vary slightly among the towns and cities of New Zealand , these types of noise rules are widely adopted and represent current practice.
How is noise measured?
The units used in New Zealand for measuring and assessing sounds in the environment are set out in NZS 6801: 1999 Acoustics – Measurement of 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.
