Directivity of sound from wind turbines : A study on the horizontal sound radiation pattern from a wind turbine

Sammanfattning: In the present paper, a study on the directivity of sound from a wind turbine has been conducted. The aim of the study is to investigate the horizontal sound radiation pattern through a field study compared to a noise prediction. The benefit of the results may be used to optimize the output effect from the wind turbine while the guidelines for noise levels at nearby residential areas still are met. The complete directivity pattern around the wind turbine was investigated by performing emission measurements around the wind turbine from a method described in IEC 61400-11 Wind turbine generator systems – Part 11: Acoustic noise measurement technique. Furthermore, the dominant sound source from the wind turbine, the turbulent boundary layer trailing edge noise, and the frequency range where it is dominating has also been scrutinized. The results show that the dipole character of the trailing edge noise has an impact on the entire horizontal radiation pattern from the wind turbine. From a field study it was found that there was a distinguishable directivity of the sound. On a distance of 125 m from the wind turbine the sound pressure level in the crosswind direction of the wind turbine is close to 3 dBA less than the sound pressure level in the downwind direction of the wind turbine when the wind speed is 8 m/s at a height of 10 m. The difference between other directions compared to the downwind direction is less significant. This could be utilized to optimize the power output, however the difference in sound level is relatively small but the advantage for power output have to be quantified before a conclusion of the benefits can be made.