Wind Turbine Sound Propagation in the Atmospheric Boundary Layer

Abstract: Wind turbines have grown both in size and number in the past decades. The taller turbines has made it possible to place them in forest areas which is fortunate for a country like Sweden with lots of forest. An issue with wind turbines is the sound they produce. The sound mainly comes from the rotor blades when they pass through the air. The sound heard some distance away from the turbine is sometimes masked by ambient background noise such as wind induced sound in the vegetation, but this is not always the case. Noise concerns among some people living in the vicinity of wind turbines are sometimes raised. Sound propagation models are used to predict the wind turbine sound level at certain distance. It is important that these models are accurate. Sound propagation is greatly influenced by the meteorological conditions. These conditions change over the day and year and vary a lot depending on the terrain conditions. In the past, large meteorological propagation effects have been found for sound sources close to the ground. Higher elevated sources like wind turbines have not been studied as much. One reason for this is that wind turbines are a relatively new sound source. In this thesis the meteorological influence on the wind turbine sound propagation is studied. Continuous simultaneous acoustic and meteorological measurements are performed at two different wind turbine sites during two years to capture all variations in the weather. The two sites are covered by forest, one is flat and the other has shifting terrain. The sites are representative for many locations in Sweden and around the world. The differences between the measured and expected wind turbine sound levels are established for different meteorological categories. The median of all deviations within each meteorological category is then compared. During no snow cover conditions the variation of the median under different meteorological conditions is 6 dBA and during snow cover the variation of the median is 14 dBA. The variations are due to the combined effect of refraction, ground conditions and terrain shape. The deviations from an expected value are seen for all octave bands from 63 Hz to 1000 Hz but are found to most distinct at low frequencies of around 125Hz. Meteorological effects starts to be important somewhere between 400 m and 1000 m from wind turbines.The characteristic "swish" sound from wind turbines are also studied in this thesis. The swish sound or as it is also called, the amplitude modulated sound, is found to be more common under some meteorological conditions such as temperature inversions and downwind conditions. A metric for detection of amplitude modulation duration and strength is proposed. Amplitude modulation, is according to some, the reason why wind turbine sound is perceived as more annoying than other typical environmental sounds at the same sound level. The swishes probably increase the probability to hear the wind turbine sound in presence of other background noise.