Infrasound and low-frequency noise at Waterloo wind farm
Comparison of the noise levels measured in the vicinity of a wind farm for shutdown and operational conditions.
By Kristy HANSEN, Branko ZAJAMŠEK and Colin HANSEN - University of Adelaide
Presented at inter.noise 2014 - Melbourne, Australia, November 16-19, 2014
Abstract
Outdoor and indoor microphone measurements have been taken in the vicinity of the Waterloo wind farm at a number of locations during periods when the nearby wind farm was operational as well as when it was shutdown. The majority of the shutdowns were of short duration and deliberate on the part of the wind farm operator, as they were associated with the recent EPA noise impact study. However, one of the shutdowns lasted for several days as it was related to a cable fault. Comparisons are made between both the third-octave spectra and narrowband spectra measured during the shutdown and operational periods. Operational times immediately adjacent to the shutdown times, as well as at other times when the wind conditions at hub height and at the residence matched the conditions recorded during a shutdown time, are considered in the analysis. It is shown that there are consistent and significant differences in noise spectra at the residence for the shutdown and operational cases, particularly for frequencies below 100 Hz. These differences can be observed at distances up to 8.7 km from the wind farm.
Extracts
Rural areas in South Australia are characterised by low ambient noise levels, particularly during nighttime hours. The operation of a large industrial wind farm can present a significant contrast to these ambient conditions, particularly at a downwind location when there is high wind shear. Under such conditions, the low frequency and infrasonic components of wind farm noise can travel large distances due to a combination of refraction, which causes sound waves to bend towards the ground, small atmospheric absorption and insignificant losses on reflection from the ground. This phenomenon has been investigated for a single wind turbine under downwind conditions, where it was found that the attenuation rate of noise in the frequency range of 2 Hz to 20 Hz is 3 dB/doubling of distance, rather than the 6 dB/doubling of distance which occurs due to spherical spreading.
Several residents who live in close proximity to wind farms report annoyance even when the measured noise levels are relatively low. The noise is often described as “thumping” or “rumbling” in character, which suggests that the amplitude of the noise varies with time and that it is low-frequency in nature. A periodic variation of the amplitude with time is referred to as amplitude modulation and listening tests have shown that for a given noise level, the presence of amplitude modulation significantly contributes to perceived annoyance . Annoyance by low frequency noise often occurs in the range of an individual’s hearing threshold and therefore if a low frequency noise is audible to the individual and is amplitude modulated, it is likely to be annoying.
Some residents have reported annoyance when the wind farm is inaudible to them. They describe such symptoms as dizziness and nausea as well as unfamiliar sensations in their ears. According to Salt et al., these symptoms may be related to infrasound, which stimulates the outer hair cells of the human ear at levels below the audibility threshold. This results in information transfer via pathways that do not involve conscious hearing, which may lead to sensations of fullness, pressure or tinnitus, or have no sensation. The pressure fluctuations or cyclic variations in local barometric pressure caused by wind turbine noise have also been compared to similar fluctuations which are experienced by an individual on a ship in high seas. The pressure fluctuations experienced by the individual on the ship occur due to changes in the elevation which cause the barometric pressure to vary relative to the individual. Dooley proposed that this cyclic pressure variation may be the cause of motion sickness on ships as well as nausea in the vicinity of wind farms.
There is a significant difference in the unweighted third-octave spectra when the Waterloo wind farm is shut down compared to when it is operational for each of the three residences investigated in this study. The most prominent difference occurs in the 50 Hz third-octave band and it has been shown that operational levels can be as much as 30 dB higher than shutdown levels. The peak in this third-octave band is also higher than the audibility threshold defined in ISO 389-7 by as much as 10 dB for the outdoor measurements. This peak was also measured indoors when the wind farm was operational but the magnitude is slightly lower and the rms level averaged over 10 minutes is at the same level as the audibility threshold defined in ISO 389-7, although the variability in the noise results in the peak levels being much higher than the rms audibility threshold.
The narrow-band spectra associated with wind farm operation show a consistent occurrence of peaks at specific frequencies in the infrasonic and low frequency ranges. The frequencies of these peaks are the same at each residence and they are not present when the wind farm is shut down, which indicates that they are the result of wind farm noise. The low frequency peaks at 23.3 Hz, 28 Hz, 46.6 Hz and 56 Hz are surrounded by side-bands spaced at the blade-pass frequency of 0.8 Hz. Results obtained by increasing the frequency resolution indicate that it is quite feasible that the low frequency peaks are harmonics of the blade-pass frequency. Thus their presence can either be attributed to selected amplification of blade-pass frequency harmonics or amplitude modulation of a turbine associated noise source at the blade-pass frequency. Further investigation into the source of the noise is currently being undertaken.
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