It is well-known that wind farm noise is dominated by low-frequency energy at large distances from the wind farm, where the high frequency noise has been more attenuated than low-frequency noise. It has also been found that wind farm noise is highly variable with time due to the influence of atmospheric factors such as atmospheric turbulence, wake turbulence from upstream turbines and wind shear, as well as effects that can be
attributed to blade rotation. Nevertheless, many standards that are used to determine wind farm compliance are based on overall A-weighted levels which have been averaged over a period of time. Therefore the aim of the work described in this paper is to investigate the time dependent nature of unweighted wind farm noise and its perceptibility, with a focus on infrasound. Measurements were carried out during shutdown and operational conditions and results show that wind farm infrasound could be detectable by the human ear although not perceived as sound.
Outdoor and indoor measurements were carried out at three residences located in the vicinity of a wind farm. Measurements were done for both operational and shutdown conditions and significant differences between shutdown and operational conditions were found in the infrasonic range (below 20 Hz). More specifically, distinct peaks at the blade-pass frequency, associated with operational conditions, were found to be and
generally more than 20 dB higher than the levels measured at equivalent frequencies when the wind farm is shutdown. These observations are consistent for both the outdoor and indoor results.
Despite the large differences in the infrasonic noise level measured during shutdown and operational conditions, the one-third octave sound pressure levels are well below the threshold of audibility derived by Møller and Pedersen (9) and Moorhouse et al. (8) which suggests that the infrasonic range of wind turbine noise is not perceived as sound. However, comparison of peak sound pressure levels in bandpass filtered time signals with a 1 Hz bandwidth indicated that wind farm infrasound might be detected by the ear and thus could influence how people feel in its presence. Furthermore, the infrasound was shown to have an impulsive character, which might also be an important factor.
Results presented in this paper are of a preliminary nature and thus no firm conclusions can be drawn from it. The possibility of the ear being responsive to inaudible levels of infrasound does require further research.