A new study from industry group RenewableUK, prepared by the National Aerospace Laboratory of the Netherlands, provides some interesting insight into the issue of noise from large wind turbines.
Large wind turbines are found to make two types of noise: a fairly quiet and continuous swishing noise caused by the flow of air over the blades and emanating mostly from the trailing edge of the blade and a thumping or louder swishing noise that typically occurs for only a few minutes at a time but which can be very annoying to some individuals. The former is known as Normal Amplitude Modulation and is a typical component of turbine operation. The latter is known as Enhanced Amplitude Modulation and is the major subject of the study.
EAM has increased low-frequency content compared to NAM and may be associated with specific directions from the spinning blades. Modelling conducted for the study indicates that EAM may be associated with aerodynamic stalling of parts of the airflow over the turbine blade. This may be caused by wind shear and changes in wind direction across the diameter of the turbine blades. The stalls are capable of causing generation of sound waves that transmit a thumping and/or above-average swishing sound.
The good news from the study is that, if the analysis is correct, the software that controls the turbine may be used to eliminate the local stalling on the blades and hence eliminate the EAM.
More research is needed to confirm these findings but elimination of the sound profile that some individuals find most annoying would go a long way to addressing the reasonable concerns of turbine neighbours.
A summary, and a link to the full report (mostly a very technical acoustical engineering analysis), are available at http://www.renewableuk.com/en/news/press-releases.cfm/renewableuk-publishes-new-research-on-wind-energy-acoustics