The group suggests limits set relative to background noise over a range wind speeds up to 12m/s, measured at a height of ten metres. However, in particularly quiet areas a margin above background would be unduly restrictive. Separate noise limits should apply for day-time and for night-time, with day-time noise limits derived from background noise data taken during quiet periods of the day and night-time limits derived from background noise data collected at night.
The group agreed that the LA90, ten minute descriptor should be used for all background noise. This is a weighted measurement, based on the response of the human ear, exceeded for 90% of the time, based on measurements over a ten minute period. The recommended limit is 5dB(A) above day and night-time background levels and it is suggested that the absolute lower limit should be 43 dB(A) during the night (because it is presumed people are in their homes) and 35 to 40 dB(A) during the day.
Modifications to this strategy are proposed if the occupier of the nearest dwelling has a financial involvement in the wind farm. Simpler measurement options can be used if appropriate. Penalties for tones are also included in the recommendations. Tonal penalties are zero if the incremental tone noise is less than 2 dB; 2 dB, rising linearly to 5 dB, for tonal increments up to 6.5 dB; 5 dB for higher levels.
The group, made up of a cross section of people involved in the assessment and control of environmental impact from noise of wind turbines, was set up by the UK's Department for Trade and Industry to define a framework for measuring and rating noise from wind plant. It was also to provide indicative noise levels for encouraging best practice in wind turbine design and wind plant development. The aim was to grant a reasonable degree of protection to wind farm neighbours and it has now presented a series of recommendations to cover a range of scenarios.