Author: | Noise
Conventional assessment of the audibility of low-frequency wind-turbine noise has usually relied on direct graphical comparison between the hearing threshold, and narrow-band or 1/3 octave wind-turbine spectra. But the hearing threshold is defined using single, isolated pure-tone test signals, whereas the latter spectra represent a measure of broadband rms energy which inevitably varies in amplitude according to the precise measurement bandwidth.
Since these two measures are derived according to two entirely different measurement conventions, any direct, unqualified comparison between the two is completely inappropriate. Moreover, it will be shown that accurately derived, historical low-frequency sound data relating to complex sound fields that have been reported as being clearly audible, possess spectral levels that according to this simplistic criterion would be dismissed as inaudible.
A procedure will be given which enables rigorous first assessment, based on the running integration of the rms energy, weighted by the inverse frequency response of the hearing threshold. This reveals that for the particular characteristics of typical wind-turbine spectra, the 1/3 octave measurement provides a fortuitous comparison, based on integrated energy, which at first sight can be justified.
But this is based on comparing the rms energy of the acoustic sound field with the rms energy of individual pure sinusoids, and fails to take account of the larger crest-factor of actual acoustic signals. Typical low-frequency sound signals which have proven to be readily audible in practice generally have much higher crest-factors than pure sinusoids, while possessing lower overall rms levels.
Dynamic time-domain simulation of the response of the ear will be shown to provide a consistent explanation for the audibility of such signals. As a direct consequence, this approach reveals that typical wind-turbine infrasonic and low-frequency noise can be readily audible at very much lower levels than has hitherto been acknowledged.
Presented at the Fourth International Meeting on Wind Turbine Noise, Rome, Italy, 12-14 April 2011
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