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The most widely accepted document for wind farm noise assessment is "THE ASSESSMENT AND RATING OF NOISE FROM WIND FARMS" - ETSU-R-97 September 1996, published by the UK's Department of Trade and Industry [DTI].  This document is recommended for the assessment of wind turbine noise in PPS 22 "Renewable Energy".

Summarising, the ETSU "method" is to

  1. Measure the background noise level in terms of LA90,10 min., at nearby properties during the quiet daytime and nighttime periods. Data from the main "working" part of the week is excluded

  2. Plot the LA90 measurements against simultaneous wind speed measurements made at the proposed wind farm; perform a regression analysis to produce a single line of wind speed versus background noise level. This procedure is performed separately for the quiet daytime periods and for the nighttime.

  3. The calculated level of wind farm noise [LA90,10 min.] is then compared against the measured background noise level, at any given wind speed, i.e. the wind turbine predicted levels for a wind speed of say 8 m/s are compared against the background noise level measured when the wind speed is 8 m/s.

  4. The wind farm noise [LA90] is permitted to be 5 dB above the regression line for background noise [LA90]. For example at a given wind speed the background level may be 38 LA90, then the permitted wind farm noise level at that wind speed would be 43 LA90.

  5. However, the lowest nighttime limit for wind farm noise is set at 43 LA90, similarly for the quiet daytime period, the noise limit is not set lower than 35 LA90 [possibly 40 LA90]. For example if the nighttime background noise level at a given wind speed was say 20 LA90, the permissible wind farm noise limit would have been 25 LA90 [20+5]; however, because of the above "cutoffs" the permissible level is in fact set at 43 LA90.

  6. Allowance is made for tonal noise following the recommendations of an International Standard.

This method of assessment is broadly based on BS.4142. The recommendations for nighttime noise are based on the World Heath Organisation Criteria 12 - Noise.

It should be noted that there is provision within ETSU-R-97 for a simplified assessment based on predictions alone if the turbine "...noise is limited to an LA90,10min of 35 dB(A) up to wind speeds of 10 m/s at 10m height."   The ETSU document considers that compliance with this condition alone would offer sufficient protection of amenity and background noise surveys would be unnecessary.

Finally ETSU-R-97 contains no recommendations on a prediction methodology for wind turbine noise, but an article in the Institute of Acoustics Bulletin in March/April 2009 set out effectively what has become the established practice.

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