Prediction of infrasound and low frequency noise propagation for modern wind turbines: a proposed supplement to ISO 9613-2

A number of propagation models of varying complexity have been developed for environmental noise predictions. While more complex models are correspondingly more accurate, the continued widespread use of the simpler ISO 9613-2 model suggests that its relative simplicity is valued highly. Therefore, to extend the use of ISO 9613-2, it is proposed that a correction should be added to take into account that at some distance from a point source, the propagation of low frequency noise may begin to decay at a rate of 3 dB per doubling of distance rather than 6 dB, during atmospheric conditions where the sonic velocity near the ground increases with altitude, resulting in downward refraction of sound rays and reduced attenuation with distance. Since low frequency sound is poorly attenuated by the ground and atmosphere, a distance from the source will exist beyond which multiple sound rays, which have been reflected from the ground more than once, can arrive at the receiver thereby increasing the sound pressure level at that location. There is some disagreement as to the transition distance at which the attenuation rate changes from 6 dB/doubling of distance to 3 dB, due to the influence of source height, frequency, and the strength of the wind and temperature gradients. This study aims to determine whether decay rates of 3 dB/doubling of distance are applicable to modern wind turbine noise propagation for a range of atmospheric conditions and frequencies and to determine a typical transition distance for the investigated conditions. This investigation focuses on comparison between ISO-9613-2 predictions using different transition distances and a large data set that has been collected at a number of different locations in the vicinity of a modern wind farm. A theoretical analysis is also carried out to calculate the radii of curvature of rays that leave the source at different angles. This enables prediction of the transition distance for various wind profiles and source heights.