Hansen et al, Comparison of Noise Levels Measured, Wind Farm Shutdown & Operational Conditions
Comparison of the noise levels measured in the vicinity of a wind farm for shutdown and operational conditions
Kristy HANSEN1; Branko ZAJAMŠEK1; Colin HANSEN1
1 University of Adelaide, Australia
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.
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 (1).
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 (2). Annoyance by low frequency noise often occurs in the range of an individual’s hearing threshold (3) 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 (4), 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 (4). 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 (5). 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 (5) 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.
Previous measurements recorded in the vicinity of wind farms have shown evidence of infrasound and low frequency noise, where the latter is modulated at the blade-pass frequency. Ambrose et al (6) investigated noise at a residence that was approximately 500 m from a “NOTUS” wind turbine and recorded peaks at the blade-pass frequency and harmonics as well as a 22.9 Hz tone indoors that was modulated at the blade-pass frequency of 0.7 Hz. The sound pressure level of this tone varied significantly above and below the associated outer hair cell threshold of 45 dB. Metelka (2013) conducted measurements at distances of 400 m and 500 m from the Kincardine wind farm in Ontario during the nighttime. The measured spectra showed distinct peaks at the blade-pass frequency and harmonics as well as peaks at 20.1 Hz and 40.2 Hz, which were amplitude modulated at the blade-pass frequency.
Recently, the South Australian Environmental Protection Authority (SA EPA) conducted a series of measurements in the vicinity of the Waterloo wind farm in rural South Australia. In their report (7), the EPA notes the presence of a “rumbling” noise but they asserted “low frequency content was not discernible subjectively when replaying audio records at actual levels.” The EPA report (7) also included an analysis of the noise levels when the wind farm was shutdown and operational. Noise in the 50 Hz third-octave band was found to increase by as much as 30 dB when the wind farm was operational compared to when it was shutdown. Further analysis was not carried out however, since the noise in the 50 Hz third-octave band did not meet the definition of a tone according to standards such as NZ 6808:2012 (8) and ANSI S12.9 – Part 4 which require a 15dB difference between the possible tone and adjacent third-octave components, for a frequency range from 25 to 125Hz. During wind farm operation, the noise level in adjacent third-octave bands was also up to 20 dB higher than for shutdown conditions.
The aims of the study reported on here were to quantify the wind farm contribution to the measured noise level through comparison between shutdown and operational conditions and to investigate the character of the noise signature that is associated with wind farm operation. Data were collected near the Waterloo wind farm, at three different residences, which are located at distances between 3 km and 8 km from the nearest wind turbine. The wind farm was both operational and entirely shutdown during the measurement period at each residence and the local weather conditions during shutdown and operation were matched to account for the contribution of wind-induced noise. Where possible, measurements were selected for nighttime hours between 12 am and 5 am, where the local wind speed at the microphone was negligible, to maximise the signal-to-noise ratio between wind turbine noise and ambient noise.