Noise Monitoring in the Vicinity of the Waterloo Wind Farm

Kristy Hansen, Branko Zajamsek
and Colin Hansen

School of Mechanical Engineering
University of Adelaide

May 26, 2014

1   Introduction

This report details independent noise measurements and their analysis taken in the vicinity of the Waterloo Wind Farm during the period 9/4 – 22/6, which is the same period as the study undertaken by the EPA and reported in EPA (2013). Measurements were taken outside of as well as inside a number of residences. Due to the potential for data contamination by background noise during the day, only data measured between midnight and 5am are reported here, as during those hours, the dominant noise source was generally the wind farm. The following sections of this report detail the measurement equipment, measurement procedures, data analysis and data interpretation, followed by a conclusion summarising the results detailed in the rest of the report.

The data analysis and interpretation comprises four sections:

• overall levels averaged over 10-minutes for all night-time data collected at each residence;
• unweighted third-octave spectra and overall levels for the shutdown periods;
• unweighted third-octave spectra and narrowband spectra for measurement times corresponding to noise diary entries; and
• unweighted and A-weighted third-octave spectra for measurements which exceeded 40 dB(A).

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8   Conclusions

Based on the findings in this report, the following conclusions can be drawn:

• For the 50 Hz third-octave band, the sound pressure level difference between shutdown and operational conditions can be higher than 25 dB for both outdoor and indoor measurements.
• The noise level in the 50 Hz third-octave band is often above the audibility threshold (ISO 389-7, 2005) when the wind farm is operating.
• The peak in the 50 Hz third-octave band would be classified as a tone according to some standards (NZS 6808:2010, 2010; ANSI S12.9 - Part 4, 2005).
  • The allowable limits should be reduced by 5 dB(A) to account for such tonal noise.
• The outdoor and indoor noise levels measured during the shutdown cases were consistently lower than those measured when the wind farm was operating.
• The most significant differences between shutdown and operational conditions can be observed when the residence is downwind from the nearest wind turbine and the hub height wind speed is greater than 8 m/s.
• The shutdown periods should have occurred during 12 am – 5 am when the contribution from extraneous sources would be minimised and the contribution from the wind farm more able to be quantified.
• For all shutdowns reported here, the closest wind turbine to the residence did not reach its rated speed of 15 m/s. In most cases, the wind speed at hub height was significantly lower than rated speed for the shutdown and adjacent times.
• The peak in the 50 Hz third-octave band is a consistent feature of the noise diary results and is often above the audibility threshold (ISO 389-7, 2005).
• A narrow-band analysis with frequency resolution of 0.1 Hz reveals distinct peaks at the blade-pass frequency and harmonics for many of the results corresponding to noise diary entries.
• The narrow-band analysis also shows the existence of tones, which occur at 23 Hz, 28 Hz, 46 Hz, 56 Hz and 69 Hz.
  • These tones have several sidebands which are spaced at the blade-pass frequency and allude to the occurrence of amplitude modulation.
• There is a good correlation between low frequency noise events and complaints registered in noise diaries.
• At many of the residences, there were many occasions during the hours of 12 am and 5 am where the outdoor noise level exceeded the SA EPA (EPA, 2009) criteria of 40 dB(A).
• The indoor limit for wind turbine hosts of 30 dB(A) recommended by the SA EPA (EPA, 2009) was exceeded on many occasions between 12 am and 5 am. This is also the no observed health effect limit for outdoor noise according to the WHO (2009).
• The range in the overall A-weighted levels was noticeably large indoors and could be as low as 5 dB(A) and as high as 38 dB(A). The lower value highlights that the night-time noise levels in this rural environment are sometimes so low that even low levels of wind turbine noise would be noticeable. It is plausible that the upper value is related to the presence of wind turbine noise.
• It has been shown that there can be a large variation in the results obtained by considering the LAeq as opposed to the LA90, between the hours of 12 am and 5 am.
  • Since the number of extraneous noise sources is expected to be low during these night-time hours and wind turbine noise can be highly variable with time, it does not seem justified to only consider noise levels which were exceeded 90 % of the time.
• The C-weighted level was often higher for downwind conditions and hub height wind speeds greater than 8 m/s. However, consideration of the overall level with respect to recommended limits did not prove useful in identifying any low frequency noise issues.
• The LCeq - LAeq criteria was often exceeded and there was a large scatter in the data.
• The overall G-weighted level of 85 dB(G) was never exceeded however this does not preclude the possibility that infrasound was not detectable.
• The Danish low frequency noise guidelines were exceeded on a number of occasions. In general, the exceedences occurred for downwind conditions and hub height wind speeds greater than 8 m/s.
• The DEFRA criteria were exceeded on multiple occasions, usually corresponding to downwind conditions and hub height wind speeds greater than 8 m/s.

Therefore, the results show that there is a low frequency noise problem associated with the Waterloo wind farm. Therefore, it is extremely important that further investigation is carried out at this wind farm in order to determine the source of the low frequency noise and to develop mitigation technologies. In addition, further research is necessary to establish the long-term effects of low frequency noise and infrasound on the residents at Waterloo. This research should include health monitoring and sleep studies with simultaneous noise and vibration measurements.

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