Noise from wind power plants. A study in anticipation of the recommendation from the Swedish Environmental Protection Agency

Noise from wind turbines are today treated as industrial noise sources according to the guidelines for external industry noise set by Naturvaardsverket (the Swedish Environmental Protection Agency) in RR 1978:5. A praxis has been established with recommended limit 40 dBA equivalent continuous sound pressure level outside dwellings day, evening and night. Naturvaardsverket is planning new guidelines specific for wind turbine noise. A draft was presented at an information meeting 13th May 2005. Special requirements, which in some cases may be far-reaching, are planned for wind turbines. The purpose of this investigation is to illustrate the fairness of the planned requirements. Application of the recommended prediction model for sound propagation above a sea surface in the draft of Naturvaardsverket may lead to serious consequences for the planning of wind power plants near the coast. Research with measurements on sound propagation above water is at present made by the Royal Institute of Technology in Kalmarsund in Sweden. The results of these measurements, which probably will be completed during the spring 2006, should be waited for before a prediction model is recommended. If the model would be valid for sound propagation from wind turbines at sea, there should be some reports on complaint on noise from offshore based wind power plants. We have not been able to locate such complaints in Sweden (Bockstigen), in Denmark (Middelgrunden, Nystedts havmoellepark and Horns rev) or in the Netherlands. For Middelgrund and Nysted, the sound level calculated with Naturvaardsverkets model at 4,5 km and 7 km respectively is around 48 dBA. According to Swedish studies, such a level is annoying to many people. Two methods to set out limits for wind turbine noise are used internationally. In the first an absolute limit for the equivalent continuous sound pressure level is set. In the other, the sound pressure level is related to the background noise level. Naturvaardsverket is planning to set an absolute limit in combination with a stronger requirement in locations with topographical unfavourable circumstances. The reason is that at such locations, the background may be very low, while the wind turbine radiates clearly audible sound. The opposite phenomenon should also be considered. At some places the wind turbine noise may be masked by unusually high background noise, for instance near the shores of the sea or in the vicinity of a waterfall. Of the two methods for formulation of limits, the method based on background noise seems to lead to fewer complaints. In many countries, wind turbines are regarded and treated the same as industrial noise sources. A wind power plant is in Sweden the outdoor sound source with the strongest requirements in praxis. There are no concealed or direct health effects of wind turbine sound. Any effects depend on the reactions of the listeners to what they hear. Infrasound levels from today's wind turbines, with the rotor upwind of the tower, are so low that they are below the threshold of sensitivity. This is true also for extra sensitive persons and even in the plant area