Notes on the rumbling noise character
A particular noise character, which diary returns described as ‘rumbling’ or ‘thumping’, was detected in amplified audio records at three sites (North East, West and South East sites), with the most prominent effects at the South East site. All of these monitoring sites are situated at similar separation distances from the wind farm (approximately 2.5 km).
It must be noted that the rumbling was only discernible when listening to amplified audio records, not in records replayed at actual levels. Analysis of shutdown and adjacent periods at the South East site indicated a direct link between operation of the wind farm and this particular noise character. It was present during periods adjacent to the shutdown and was not detectable during the shutdown by analysis of the amplified audio records.
Typically the effect was recorded under downwind conditions when the local background noise was low, notably at low local wind speeds. Spectral analysis showed that the effect could have been linked to a prominency 50Hz 1/3 octave component. The component was not that prominent at the North site and noise did not exhibit the rumbling character. As noted previously, results of the acoustics tests did not show the presence of tones in turbine emission; however the spectral content of the wind turbine generator noise contains prominent 50Hz component (not tonal) at the mid to high wind turbine generator wind speeds .
A rumbling effect is typically associated with a high degree of modulation. It was not identified at the North site, which was closer. Sometimes variations in environmental conditions can cause effect similar to the amplitude modulation , and amplification of the modulation effect due to changes of environmental parameters over a larger distance was considered plausible. The noise character was not detectable at a large distance (for example at the Township site) during the monitoring period, because of the greater attenuation of wind farm noise with increasing distance.
It is emphasised that in analysing monitoring data for these sites, the ‘rumbling’ could be heard only when the audio records were amplified.