Study on the Empirical Formula for the Sound Directivity around a Wind Turbine

Noise emitted from wind turbines is composed of aerodynamic and mechanical sound and has directional radiation characteristics. To investigate the horizontal sound directivity around a wind turbine under various wind conditions, field measurements of noise emitted from two different wind turbines has been performed. Wind turbine operational data were corrected at 1 s intervals along with corresponding acoustic data. An empirical formula for the average directivity correction was proposed on the basis of the measured A-weighted sound pressure levels in 1/1-octave bands at the receiving points set circularly around the wind turbine. The results showed that the directivity pattern of the A-weighted sound pressure level for two different wind turbines is almost the same, whereas the frequency dependence of the sound directivity is different for the individual wind turbines. The correlation between wind turbine noise and the rotor rotational speed is extremely strong compared with that with the wind speed at hub height. Additionally, the preliminary results of comparison between the rotor speed estimated by using the analyzed amplitude modulation and those actual values are presented.