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Research into aerodynamic modulation of wind turbine noise
In January 2007 a multi-discipline team from the Acoustics Research Centre, in collaboration with the Hayes McKenzie Partnership, was awarded a contract to investigate Aerodynamic Modulation of sound from wind turbines. The work was commissioned by BERR (Department for Business, Enterprise and Regulatory Reform, formerly DTI) and Defra (Department for Environment, Food and Rural Affairs), and was completed in July 2007. The team comprised: Dr Andy Moorhouse (Acoustics), Dr. Sabine von Hunerbein (Meteorology and Acoustics), Dr. Mags Adams (Sociology), Ben Piper (Acoustics), and Malcolm Hayes (consultant acoustician).
Our final report is published on the BERR website. The executive summary is reproduced below:
The study described in this report has been commissioned by Defra, BERR (formerly DTI) and CLG. It follows on from a report by the Hayes McKenzie Partnership to DTI in 2005 in which reports of low frequency noise emission from windfarms were investigated. Their report concluded that the complaints were not caused by low frequency noise, but by amplitude modulation of aerodynamic noise (AM) from the wind turbines. The term AM indicates aerodynamic noise from wind turbines, but with a greater than normal degree of regular fluctuation at blade passing frequency, typically once per second. The aims of this current study are to ascertain the prevalence of AM on UK wind farm sites, to try to gain a better understanding of the likely causes, and to establish whether further research into AM is required. The study was carried out in four parts, a survey of local authorities with windfarms in their areas, further investigation of sites for which AM was identified as a factor, a literature review and a survey of wind turbine manufacturers.
The survey of local authorities was in two parts, a scoping survey aimed at identifying problem sites, and a detailed survey to establish whether AM could have been a factor in causing complaints. The response to both parts of the survey was 100%, although full information was not available for all sites at the detailed stage. The results showed that 27 of the 133 windfarm sites operational across the UK at the time of the survey had attracted noise complaints at some point. An estimated total of 239 formal complaints have been received about UK windfarm sites since 1991, 152 of which were from a single site. The estimated total number of complainants is 81 over the same sixteen year period. This shows that in terms of the number of people affected, wind farm noise is a small-scale problem compared with other types of noise; for example the number of complaints about industrial noise exceeds those about windfarms by around three orders of magnitude. In only one case was the windfarm considered by the local authority to be causing a statutory nuisance. Again, this indicates that, despite press articles to the contrary, the incidence of windfarm noise and AM in the UK is low.
AM was considered to be a factor in four of the sites, and a possible factor in another eight. Regarding the four sites, analysis of meteorological data suggests that the conditions for AM would prevail between about 7% and 15% of the time. AM would not therefore be present most days, although it could occur for several days running over some periods. Complaints have subsided for three out of these four sites, in one case as a result of remedial treatment in the form of a wind turbine control system. In the remaining case, which is a recent installation, investigations are ongoing.
The literature review indicated that, although there has been much research into the general area of aerodynamic noise it is a highly complex field, and whilst general principles are understood there are still unanswered questions. Regarding the specific phenomenon of AM there has been little research and the causes are still the subject of debate. AM is not fully predictable at current state of the art. The survey of wind turbine manufacturers revealed that, although there was considerable interest, few have any experience of AM.
The low incidence of AM and the low numbers of people adversely affected make it difficult to justify further research funding in preference to other more widespread noise issues. On the other hand, since AM cannot be fully predicted at present, and its causes are not fully understood we consider that it might be prudent to carry out further research to improve understanding in this area.