The Brechfa Forest West Wind Farm Order 2013

PART 4 NOISE GUIDANCE NOTES

These notes form part of Requirements 32 to 36. They further explain these Requirements and specify the methods to be employed in the assessment of complaints about noise emissions from the authorised development.

Reference to ETSU-R-97 refers to the publication entitled “The Assessment and Rating of Noise from Wind Farms” (1997) published by the Energy Technology Support Unit (ETSU) for the Department of Trade and Industry (DTI).

Note 1

(a) Values of the L_A90,10min noise statistic shall be measured at the complainant’s property using a sound level meter of EN 60651/BS EN 60804 Type 1, or EN 61672 Class 1 quality (or the replacement thereof) set to measure using a fast time A-weighted response as specified in BS EN 60651/BS EN 60804 or BS EN 61672-1 (or the equivalent UK adopted standard in force at the time of the measurements). This shall be calibrated in accordance with the procedure specified in BS 4142:1997 (or its replacement). These measurements shall be made in such a way that the requirements of Note 3 shall also be satisfied.

(b) The microphone should be mounted at 1.2-1.5 m above ground level, fitted with a two layer windshield (or suitable alternative approved in writing by the relevant planning authority), and placed outside the complainant’s dwelling. Measurements should be made in “free-field” conditions. To achieve this, the microphone should be placed at least 3.5 m away from the building facade or any reflecting surface except the ground at a location that shall be approved in writing by the relevant planning authority.

(c) The L_A90,10min measurements shall be synchronised with measurements of the 10-minute arithmetic mean average wind speed and with operational data, including power generation information for each wind turbine, from the turbine control systems of the authorised development.

(d) The undertaker shall continuously log arithmetic mean wind speed and arithmetic mean wind direction data in 10 minute periods from the hub height anemometer located on the site permanent mast unless otherwise requested by the relevant planning authority to enable compliance with the Requirements to be evaluated. The mean wind speed data shall be ‘standardised’ to a reference height of 10 metres as described in ETSU-R-97 at page 120 using a reference roughness length of 0.05 metres. It is this standardised 10m height wind speed data which is correlated with the noise measurements of Note 2(a) in the manner described in Note 2(c).

Note 2

(a) The noise measurements shall be made so as to provide not less than 20 valid data points as defined in Note 2 paragraph (b). Such measurements shall provide valid data points for the range of wind speeds, wind directions, times of day and power generation requested by the relevant planning authority. In specifying such conditions the relevant planning authority shall have regard to those conditions which were most likely to have prevailed during times when the complainant alleges there was disturbance due to noise. At its request the undertaker shall provide within 28 days of the completion of the measurements all of the data collected under Requirement 35 to the relevant planning authority.

(b) Valid data points are those that remain after all periods of rainfall have been excluded. Rainfall shall be assessed by use of a rain gauge that shall log the occurrence of rainfall in each 10 minute period concurrent with the measurement periods set out in Note 1(c) and is situated in the vicinity of the sound level meter.

(c) A least squares, “best fit” curve of a maximum 2^nd order polynomial or otherwise as may be agreed with the relevant planning authority shall be fitted between the standardised mean wind speed (as defined in Note 1 paragraph (d)) plotted against the measured L_A90,10min noise level. The noise level at each integer speed shall be derived from this best-fit curve.

Note 3

Where, in the opinion of the relevant planning authority, noise immissions at the location or locations where assessment measurements are being undertaken contain a tonal component, the following rating procedure shall be used—

(a) For each 10-minute interval for which L_A90,10min data have been obtained as provided for in Note 1, a tonal assessment shall be performed on noise immissions during 2-minutes of each 10-minute period. The 2-minute periods shall be regularly spaced at 10-minute intervals provided that uninterrupted clean data are available. Where clean data are not available, the first available uninterrupted clean 2 minute period out of the affected overall 10 minute period shall be selected. Any such deviations from standard procedure as described in Section 2.1 on pages 104 – 109 of ETSU-R-97 shall be reported.

(b) For each of the 2-minute samples the margin above or below the audibility criterion of the tone level difference, L_tm (Delta L_tm), shall be calculated by comparison with the audibility criterion given in Section 2.1 on pages 104-109 of ETSU-R-97.

(c) The margin above audibility shall be plotted against wind speed for each of the 2-minute samples. For samples for which the tones were below the audibility criterion or no tone was identified, a value of zero audibility shall be substituted.

(d) A linear regression shall then be performed to establish the margin above audibility at the assessed wind speed for each integer wind speed. If there is no apparent trend with wind speed then a simple arithmetic average shall be used.

(e) The tonal penalty shall be derived from the margin above audibility of the tone according to the figure below. The rating level at each wind speed shall be calculated as the arithmetic sum of the authorised development noise level, as determined from the best-fit curve described in Note 2, and the penalty for tonal noise.

Note 4

If the authorised development noise level (including the application of any tonal penalty as per Note 3) is above the limit set out in the Requirements, measurements of the influence of background noise shall be made to determine whether or not there is a breach of Requirement. This may be achieved by repeating the steps in Note 1 & 2 with all of the wind turbines switched off in order to determine the background noise, L₃, at the assessed wind speed. The wind turbine noise at this wind speed, L₁, is then calculated as follows, where L₂ is the measured authorised development noise level at the assessed wind speed with turbines running but without the addition of any tonal penalty—

The authorised development noise level is re-calculated by adding the tonal penalty (if any) to the authorised development noise.