Audible thumping from wind farms can travel up to 3.5 km

Technically, wind energy is a form of solar energy. Consequently, it is renewable and can be used to generate electricity; this knowledge has been exploited greatly, particularly in the past decade where many wind farms have been installed in different regions worldwide. Wind farms are comprised of numerous large wind turbines that rotate, and the generated motion is transferred to a generator that in turn converts the kinetic energy to electric energy. So far, the process seems flawless; unfortunately, reports from residents of areas surrounding the wind farm have raised complaints regarding noise pollution generated by the turbines.  More specifically, the motion of the large blades and mechanical parts of the turbines generate noise that has been reported to cause annoyance, sleep disturbance and consequently adverse health effects. Therefore, to ensure that residents living near wind farms are not subjected to excessive noise-related disturbance, it is important to identify potentially disturbing wind farm noise components. Moreover, suitable methods for quantifying these components are required. Acceptable threshold levels also need to be defined to determine the prevalence of potential noise disturbance.

Literature has it that the presence of amplitude modulation (AM) in wind farm noise is the result of annoyance. Presently, an extensive review of existing methods for AM detection and quantification can be found in various studies. Of particular interest is the IOA ‘reference method’ that incorporates various concepts, can be automated thus allowing analysis over long time periods, and is robust to background noise contamination, thus reducing the instances of false positives.

Overall, it is important to determine how often AM is present at residential locations near a wind farm. In this view, Australian researchers from the Flinders University: Dr. Kristy Hansen, Phuc Nguyen, Dr. Branko Zajamšek, Prof. Peter Catcheside, in collaboration with Prof. Colin Hansen at The University of Adelaide studied the prevalence and characteristics of wind farm AM of a certain windfarm in Australia. Their goal was to determine how often AM occurred at various distances from the wind farm and to assess the suitability of the IOA ‘reference method’ for detecting low-frequency AM of a tone that is generated by wind turbines. Their research work is currently published in Journal of Sound and Vibration.

Their approach involved outdoor measurements for a total of 64 days at 9 different residences located between 1 and 9 km from the nearest wind turbine of a South Australian wind farm, which at the time of measurements was made up of 37 operational turbines, each with a rated power of 3 MW. The motivation for their analysis was to investigate the prevalence of a low-frequency ‘thumping’ or ‘rumbling’ noise that had been mentioned in complaints from residents.

The authors reported that an audible indoor low-frequency tone was amplitude modulated at the blade-pass frequency for 20% of the time up to a distance of 2.4 km. In addition, they found that the audible AM occurred for a similar percentage of time between wind farm percentage power capacities of 40 and 85%, indicating that it was important that the AM analysis should not restricted to high power output conditions only.

In summary, the study investigated the prevalence and characteristics of wind farm AM at 9 different residences located near a South Australian wind farm. Their work showed that, despite the number of AM events being recorded to reduce with distance, audible indoor AM still occurred for 16% of the time at a distance of 3.5 km. At night-time, audible AM occurred indoors at residences located as far as 3.5 km from the wind farm for up to 22% of the time. In a statement to Advances in Engineering, Dr. Kristy Hansen pointed out that the adopted approach was successful, although more research was needed to quantify the annoyance and sleep disturbance potential of the recorded type of tonal AM.