Noise From Wind Turbines
G.P. van den Berg, University of Gronigan, The Netherlands
"Sound from wind turbines involves a number of sound production mechanisms related to different interactions between the turbine blades and the air. An important contribution to the low frequency part of the sound spectrum is due to the sudden variation in air flow which the blade encounters when it passes the tower. The angle of attack of the incoming air suddenly deviates from the angle that is optimised for mean flow.
Hitherto, low frequency sound from wind turbines has not been shown to be a major factor contributing to annoyance. This seems reasonable as the blade passing frequency is of the order of 1Hz, where the human auditory system is relatively insensitive.
This argument, however, obscures a very relevant effect: the blade passing frequency modulates well audible higher frequency sounds and thus creates periodic sound: "blade swish". This effect is stronger at night because in a stable atmosphere there is a greater difference between rotor average and near tower wind speed.
Measurements have shown that additional turbines can interact to amplify further this effect. Theoretically, the resulting fluctuations in sound level will be clearly perceptible to human hearing. This is confirmed by residents near wind turbines with the same common observation: often late in the afternoon or in the evening the turbine sound acquires a distinct "beating" character, the rhythm of which is in agreement with the blade passing frequency.
It is clear from the observations that this is associated to a change toward higher atmospheric stability. The effect of stronger fluctuations on annoyance has not been investigated as such, although it is highly relevant because (i) the effect is stronger for modern (i.e. tall) turbines and (ii) more people in Europe will be living close to these wind turbines as a result in the growth of wind energy projects."