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"
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
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.
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.
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.
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.
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
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
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