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Sound contains different frequency components which are constantly changing. For comparison with noise limits, a standard method must be used to represent varying sound as a noise level.

Sound sources cause changes in air pressure which are detected by our ears. These changes in pressure can also be measured by a sound level meter. The pressure changes are expressed in decibels, which is written as 'dB'.

Noise is often defined as unwanted sound. The figure below shows typical noise levels for various situations and activities, which may be used for comparison with noise levels quoted in this web site.

Noise can occur across a whole range of frequencies from low frequency rumbles such as thunder, to high frequency squeaks and squeals like alarm bells. Measured noise levels include all frequencies, but as our hearing is less sensitive to lower frequencies the measured levels are adjusted to correspond to human hearing. This adjustment is called “A weighting” and is identified by the letter A.

Noise levels fluctuate and therefore it is necessary to consider both average (LAeq(t)) and maximum (LAFmax) values. For construction noise, average values are assessed over a time period (t) between fifteen minutes and one hour, as appropriate to the particular activities (typically 15 minutes). For road traffic noise the time period used in New Zealand is 24 hours. The following figure provides an illustration of the average and maximum levels. The LAeq(t) is obtained from an ‘energy’ average of the decibel values; this results in a higher average level than normal arithmetic averaging. 

For a LAFmax measurement, the ‘maximum’ recorded will depend on the response time of the sound level meter. A sound level meter ‘Fast’ response is standardised as one-eighth of a second, and is identified by the letter F. In summary, all noise levels are measured in ‘dB’ and the following parameters are used to describe transport noise in NZ:

Issue

 

Metric

Road traffic noise

 

LAeq(24h)

Road construction noise

average

LAeq(15min)

 

maximum

LAFmax

For further information contact environment@nzta.govt.nz.

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