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Noise from offshore wind turbines 1 Noise emission measurements for offshore wind turbines: Measurement ProcedureMeasurement of noise emission from offshore wind turbines cannot in general be made with the standardized and generally accepted method in IEC 61400-11:2002 [1]. Most often the waves are so high that it is not safe to leave a microphone on a reflecting plate floating on the surface of the sea, and the noise produced by even small waves hitting the reflecting plate may exceed the noise level produced by the wind turbine. Instead an alternative method has been proposed where the microphone moved from a position on a plate on the ground to a position 3 to 5 m above the sea surface. The direct sound from the wind turbine and the reflected sound from the surface of the sea are almost equal, and when the frequency band considered is wide enough, the reflection causes a doubling of the energy of the direct sound and a 3 dB increase in sound pressure level. Actually, this principle lies behind the standards for measurement of sound power level in a free field above a reflecting plane, e.g. the ISO 3740-series. The principle does not apply to determination of tone levels. Thus the proposed alternative method cannot be expected to give conclusive results regarding the tonality of the wind turbine noise. The method follows the general line of action in Statutory order No. 304 of 14th May 1991 "Noise from Wind Turbines" from the Danish Environmental Protection Agency [2] and in IEC 61400-11:2002 [1].The main exceptions from the standard are described below. The measurements are made with a microphone mounted onboard a vessel. The wind speed is determined from the power produced by the wind turbine and a calibrated power curve rather than a simple wind speed measurement. The sound power level as well as other acoustic parameters are determined at integer wind speeds from 6 to 10 m/s and not only at 8 m/s. The tonality is not considered. 1.1 Microphone positionThe microphone is placed 3 to 5 m above the surface of the sea, free of reflecting obstacles like ship cabin, hull, etc. A position 1 m to 2 m from the vessel is preferred, but for practical reasons a position nearer the bulwark rail of the vessel is acceptable. The microphone axis shall point towards the hub of the wind turbine. To obtain an adequate signal-to-noise ratio, the microphone shall be fitted with extra wind noise reducing equipment, e.g. a standard spherical windshield 10 cm in diameter supplied with a secondary wind shield as described in [3]. In Figure 1. the mounting of the microphone with a secondary windshield used for the wind turbine noise measurements is shown. Figure 1 Mounting of microphone on board the vessel. The microphone is supplied with an extra wind shield.
1.2 Anemometer positionAn anemometer is placed onboard the same vessel as the microphone and at a height of 10 m above sea level. The anemometer is only used for the background noise measurements. The anemometer is expected to give the correct wind speed at 10 m height due to the small roughness length of the surface, and hence the normalization suggested in [1] is not necessary.
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