Amplitude Modulation of Wind Turbine Noise: A Review of the Evidence

Introduction

Many of the complaints in the UK relating to wind farm noise appear to be due to the amplitude modulation (AM) of the aerodynamic noise from the blades, sometimes referred to as “swish” or “thump”. The mechanism of this noise is not known though various possible reasons have been put forward. Although the prevalence of complaints about AM is relatively small it is not clear whether this is because it does not occur often or whether it is because housing is not in the right place to observe it. Furthermore the fact that the mechanism is unknown means that it is not possible to predict when it will occur. 

A report by Salford University commissioned by BERR1 concluded that “the incidence of AM and the number of people affected is probably too small at present to make a compelling case for further research funding in preference to other types of noise which affect many more people.” Consequently BERR decided not to fund further research. However, the opinion of the Noise Working Group that advised BERR (then the DTI) was that, although the incidence was small it might become greater with larger turbines and “a greater understanding of the effects and causes relating to AM [is] required to ensure that this phenomenon can be managed.” Specifically the first stage objective was to “Identify up to 10 potential sites which could be used to carry out objective noise measurements”. The government decided not to accept the groups advice. 

This paper does not pretend to present any substantial new evidence but I have tried to review the current situation in order to provide some direction for future work. 

Where “upwind” and “downwind” are insufficient descriptions, references to observer positions in this paper are made by compass bearings on the assumption that the wind direction is from the North. The turbine is assumed to rotate clockwise as viewed from upwind. The phrase “axis of the turbine” in this context means on the north-south line passing through the centre of the turbine.

Conclusions

It seems probable that there are two distinct mechanisms in operation to create AM. The first is swish which is a function of the observers position relative to one turbine. The second is thump which is due to turbine blades passing through uneven air velocities as they rotate. In the second case the uneven air may be due to interaction of other turbines, excessive wind shear or topography. These two mechanisms are entirely separate though it is possible that they interact. If this is the case there is little that can be done about swish but further research into thump would help to avoid excessive AM in future developments.