Directivity of amplitude modulation sound around a wind turbine under actual meteorological conditions

Abstract

Wind turbine noise (WTN) generally has amplitude modulation (AM) components, which often cause psychological annoyance in residential areas around the power generation plant. To investigate the directivity of AM sound generated from a wind turbine, field measurements have been performed under various wind conditions. Some receiving points were set circularly around a single wind turbine, and meteorological and associated wind turbine operational data were collected along with corresponding acoustic data. The method for extracting the AM components from the analyzed sound pressure levels at 100 ms intervals is based on the ideas of the F-S method. The results revealed a distinguishable directivity pattern of the strength of the AM components contained in WTN. The magnitudes of AM sound become lower in the downwind directions and highest in the direction approximately 60 relative to the front of the nacelle. In addition, the strength distributions of the AM components at distances of up to 200 m from the turbine were examined. It was found that almost all the magnitudes of AM sound still remained above 2 dB at a distance of even 200 m, whereas the A-weighted sound pressure levels decreased with increasing propagation distance.

The relevance of AM mitigation

“…At one site a resident gave annoyance ratings at specific time periods and these corresponded with heightened modulation depths with the WTS upwind. Okada et al [11] performed field measurements under various wind conditions at receiver points in a circle around a single wind turbine. The method for assessing AM components was based on the difference between a 'fast' and 'slow' weighted sound pressure level ('F-S method') in 100 ms intervals.…”

Recent Advances in Wind Turbine Noise Research

“…The frequency of variation is the frequency at which blades pass the tower (blade-pass frequency, usually between 0.5 and 2 Hz). Recent work has shown that, although the highest noise levels are experienced in the downwind direction of a wind turbine, higher levels of AM (albeit with lower overall sound pressure levels) are experienced in a cross-wind direction, approximately 60 • from the front of the nacelle [98]. The effect of meteorological conditions on the generation of much higher than expected levels of AM (often referred to as EAM) continues to be the subject of a significant amount of continuing research effort (see [32,34,57,99]).…”