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.
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 (12) 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 (12), although the variability in the noise results in the peak levels being much higher than the rms audibility
Outdoor infrasonic noise levels associated with wind farm operation vary depending on the local wind speed at the microphone. During periods of negligible wind at the microphone, distinct peaks corresponding to blade-pass harmonic frequencies are clearly distinguishable. The outdoor results presented for House 3, where the wind speed at the microphone was zero, showed the most distinct peaks in the infrasonic frequency range out of the three residences investigated. For Houses 1 and 2, these peaks in the outdoor spectra were evidently masked by wind-induced noise and this is further confirmed by their presence in the indoor spectra for measurements at these locations, as shown in Section 3.1.1 and 3.1.2. The wind-induced noise is caused by pressure fluctuations and vortex shedding, which are sensed by the microphone but bear no relation to acoustic
disturbances. Therefore, to adequately portray the levels of infrasound outdoors, it is imperative that there is negligible wind in the vicinity of the microphone. The shutdown times selected by the wind farm operator gave few opportunities to record such conditions. Hence, it is suggested that in future studies, times between 12 am and 5 am with negligible wind at the measurement locations are selected for shutdown/operational comparisons.
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.