Assessment of wind turbine noise and residual noise in immission areas around wind power plants

In wind turbine noise problem, not only the noise level caused by wind turbines, but also the noise emergence, the increase of the noise level caused by wind turbines over the residual noise (background noise without any specific noises) is important. To investigate this matter, the noise levels around 17 wind farms across Japan under operating and ceasing conditions were measured. In addition, residual noises in four kinds of regions (rural, coastal, urban residential, and industrial districts) were measured. As a result, the actual condition of the noise emergency caused by wind turbines and the difference of residual noise between daytime and nighttime have been known for respective types of districts. For the assessment of the noise levels under both conditions with/without wind turbine noise should be evaluated in terms of equivalent level (LAeq). When measuring LAeq, however, it is necessary to strictly eliminate extraneous noises such as pass-by noise of road vehicles, fly-over noise of aircrafts and creatures' sounds. This procedure is very troublesome and therefore we examined the method estimating LAeq statistically from the 90% noise level (LA90) which is very robust against transient extraneous noise.