The relevance of AM mitigation

Abstract

The rhythmic character of wind turbine sound contributes significantly to the annoyance of the sound. In field studies the most often mentioned description refers to this rhythmic character (swishing, lashing, swooshing). A rhythmic variation of wind turbine sound has the frequency at which a blade passes the tower and is produced in two different ways. 'Normal' Amplitude Modulation (NAM) is caused by an increase in sound level when a blade moves towards a listener. 'Other' Amplitude Modulation (OAM) is most likely caused by flow separation at the blade tips. Long term measurements show that AM can be measured for a considerably percentage of time in a wide area around a wind turbine or wind farm. It occurs predominantly in two directions relative to the wind turbine(s): either in or close to the rotor plane or approximately perpendicular to the rotor plane. The two directions agree with what is expected for NAM and OAM, respectively. Although there are numerous studies on the modelling of wind turbine sound, as yet there are no methods to predict the presence or magnitude of OAM sound and hardly any effort to reduce it.