Noise from wind farms is contentious: people who live nearby complain of annoyance, and yet broadband measurements of infrasound seem to indicate the noise is generally not above audibility criteria. The paradox can be resolved by supposing that wind farms generate a strong tonal signal at the blade passing frequency, 0.8 Hz, and that this infrasound, with a wavelength of 400 m, can constructively interfere if two or more wind turbines operate in synchrony and the path lengths differ by a multiple of 400 m. Coherent infrasound at 0.8 Hz could propagate many kilometres, would tend to carry many harmonics due to the rapid changes within its waveform, and the high harmonics in the 20–30 Hz band have the potential to be heard by human ears. The existence of coherent infrasound from wind turbines has not been specifically recognised, but evidence of the phenomenon can be discerned in two anomalies contained in data from recent infrasound monitoring of wind farms in South Australia. This paper interprets the anomalies in terms of a model which suggests that wind farms produce enhanced sound pressure levels when the blades of multiple machines become mutually entrained and the sound from them becomes coherent. The inference is that acoustic measures, which assume wind turbine signals are stationary, may not be accurate indicators of peak noise levels.
John Curtin School of Medical Research, Australian National University, Canberra
Acoustics Australia, Vol. 42, No.3, December 2014, pp. 212-218
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