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Wind Energy Noise emissions relation to their capacity. A Vestas V 90 reaches its maximum noise emission at 7.5 m/s, an E 82 at 9 m/s”, explains Bunk. For noise assessors, this is the decisive factor. These noise emission values are guaranteed by the manufacturers. When the noise assessors predict the noise emissions of a wind farm or come to take measurements after its completion, they measure how many times that level is reached per year, and how much of that noise could reach people living nearby. However, in many European countries the measurements are only conducted at a standard height of ten metres, not at hub height. The research continues For the AM phenomenon on the other hand, there is not yet a standard process to measure the noise, which makes comparisons difficult. “In the UK, AM was a big issue last year, which led to some measurements being taken, including under laboratory conditions, to determine the cause of the anomaly. Experts believe that it is probably caused by stalling, because the angle of attack of the blades is hard into the wind”, says Sylvia Broneske of the acoustics consultancy Hayes McKenzie Partnership Ltd. In response to protests, the industry association RenewableUK commissioned Temple Group to conduct a study (Turbine Amplitude Modulation: Research to Improve Understanding as to its Cause and Effect) that made use of data from the Danish institute Risø DTU. Temple Group came to the conclusion that AM should be differentiated into normal and abnormal sounds. The normal swishing sound is caused by air friction, turbulence or the airflow being blocked by the tower. The abnormal noises on the other hand could be caused by an enormous number of possible combinations. These could result from the location, the height of the hub or the atmospheric stratification of the wind. In the vicinity of the turbine, the noise reaches at most 5 dBA and is hardly an irritation, according to the report. This conclusion is based on a study by the University of Salford from 2007 that looked at 113 wind farms and only detected AM at four of them. “For us, AM is no longer a big issue, even though public interest remains high. Some people seem to confuse noise pollution and normal noise. This has to be looked into project by project”, says Robert Norris of RenewableUK. Until recently, local authorities adhered to guidelines from 1997. The ageing document was replaced in May 2013 by the IOA Good Practice Guide developed by the Institute of Acoustics and a working group. According to these new guidelines, wind turbines can emit 35 to 40 dBA during the day and up to 43 dBA at night, and may exceed these figures by up to 5 dBA depending on the ambient noise level. Despite numerous objections, the new guidelines do not include a minimum distance to residential buildings or standardised methods for measurement. A case for psychologists The idea that people might be confusing general noise with noise pollution also emerges from a new study by the German Foundation for the Environment (Deutsche Bundesstiftung Umwelt). The study conducted interviews with local residents and took measurements. The swishing could be heard at nine Whether the noise levels during operation of a wind turbine are perceived as disturbing or not, does not depend on the actual noise emission itself. Photo: Torsten Thomas 44 Sun & Wind Energy 8+9/2014


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