Throughout our extensive experience of undertaking wind farm noise assessments over the years, our clients often ask questions regarding wind turbine noise, including how wind turbines generate noise, how it is assessed and how can noise be minimised. We have compiled some of the most commonly asked questions in order to help you understand wind turbine noise. If you have a question that isn’t answered below, please don’t hesitate to get in contact.
Are wind turbines noisy?
Despite the fact that wind turbines generate sound, modern wind turbines are in general much quieter than people anticipate. Conversing normally at the foundation of a turbine operating at full capacity is entirely feasible without using a raised voice. The precise level of noise from a wind farm is determined by several factors such as the wind farm’s turbine type, site layout, the local topography, and the speed / direction of the wind.
What sounds do wind farms produce?
The noise produced by wind turbines is typically broadband in nature, consisting of a wide range of different frequencies. Subjectively, people describe noise from a wind turbine as a ‘swoosh’, similar to the sound of waves on the seashore. The precise frequency content of the noise at any particular location is influenced by factors such as the wind turbine model, wind speed, the listener’s distance from the turbine(s), and the surrounding terrain.
What about low-frequency noise and infrasound?
Infrasound is defined as sound below the range of human hearing (20 Hz), whereas low-frequency sound is generally taken as being frequencies below 200 Hz. As such, all infrasound is low-frequency, but not all low-frequency sound is infrasound. Concern is sometimes expressed about the possible effects of low-frequency sound from wind turbines on nearby residents. An increase in low-frequency sound was a feature of some early wind turbine designs with the blades downwind of the tower. This caused an audible ‘thump’ each time a blade passed behind the tower. However, modern wind turbines all have their blades upwind of the tower, thus reducing the level of this type of noise to below the threshold of human perception.
In February 2013, the Environmental Protection Authority of South Australia published the results of a study into infrasound levels near wind farms[1]. This study measured infrasound levels at urban locations, rural locations with wind turbines close by, and rural locations with no wind turbines in the vicinity. It found that infrasound levels near wind farms are comparable to levels away from wind farms in both urban and rural locations. Infrasound levels were also measured during organised shut-downs of the wind farms; the results showed that there was no noticeable difference in infrasound levels whether the turbines were active or inactive. The study consulted that…“the contribution of wind turbines to measured infrasound levels is insignificant in comparison with the background level of infrasound in the environment.”
Another study, Bowdler et al. (2009)[2], concludes that:
“…there is no robust evidence that low frequency noise (including ‘infrasound’) or ground-borne vibration from wind farms generally has adverse effects on wind farm neighbours”.
[1] Environment Protection authority (2013) Infrasound levels near wind farms and in other environments Available at: http://www.epa.sa.gov.au/xstd_files/Noise/Report/infrasound.pdf
[2] Bowdler et al. (2009). Prediction and Assessment of Wind Turbine Noise: Agreement about relevant factors for noise assessment from wind energy projects. Acoustic Bulletin, Vol 34 No2 March/April 2009, Institute of Acoustics.
Can wind turbine noise affect health?
Numerous studies have been undertaken to determine whether there is any link between wind turbine noise and health. Health Canada, Canada’s national health organisation, released preliminary results of a large-scale study into the effect of wind farms on human health in 2014[3]. More than 4000 hours of wind turbine noise measurements were collected and a total of 1238 households participated. No evidence was found to support a link between exposure to wind turbine noise and any of the self-reported illnesses. Additionally, the study’s results did not support a link between wind turbine noise and either stress or sleep quality (self-reported or measured).
Similarly, the Massachusetts Institute of Technology released a review of the scientific literature on wind turbine noise in December 2014[4]. The review took health effects such as stress, annoyance and sleep disturbance into consideration, as well as other effects that have been raised in association with living close to wind turbines. The study found that: “No clear or consistent association is seen between noise from wind turbines and any reported disease or other indicator of harm to human health.”
The National Health and Medical Research Council of Australia conducted a comprehensive independent assessment[5] on the scientific evidence on wind farms and human health in 2015. They stated that… “after careful consideration and deliberation, NHMRC concludes that there is currently no consistent evidence that wind farms cause adverse health effects in humans.”
[3] Health Canada 2014, Wind Turbine Noise and Health Study: Summary of Results. Available at http://www.hc-sc.gc.ca/ewh-semt/noisebruit/turbine-eoliennes/summary-resume-eng.php
[4] Journal of Occupational and Environmental Medicine (2014), Wind Turbines and Health: A Critical Review of the Scientific Literature. Available from: journals.lww.com/joem/Abstract/2014/11000/Wind_Turbines_and_Health__A_Critical_Review_of_the.9.aspx
[5] NHMRC (2015): Evidence on wind farms and human health.
How does noise influence the layout of a wind farm?
Noise limits are applied to ensure that turbines are located sufficiently far from houses or other noise-sensitive premises to protect the amenity of the location in question. This is a key factor in the design of a wind farm and where turbines can be located.
What are the noise limits for wind turbines?
In the UK, ETSU-R-97[6] provides a framework for the assessment and rating of noise from wind turbine installations, as endorsed the by the UK government. Both background noise and noise from wind turbines typically vary with wind speed. According to ETSU-R-97, wind farm noise assessments should therefore consider the site-specific relationship between wind speed and background noise, along with the particular noise emission characteristics of the proposed wind turbines.
The document recommends the application of external noise limits at the nearest noise sensitive properties, to protect outside amenity and prevent sleep disturbance inside dwellings. These limits take the form of a 5 dB margin above the prevailing background noise level, except where background noise levels are lower than certain thresholds, where fixed lower limits apply. The limits apply to the cumulative effects of all wind turbines that affect a particular location.
A ‘simplified’ criterion is also described which may be applicable where there are large separation distances between the proposed turbines and nearest noise-sensitive receptors. In such cases, a fixed limit of 35 dB, LA90,10min applies, without reference to background noise levels.
The Institute of Acoustics’ Good Practice Guide[7] was published by the IOA in May 2013 and has been endorsed by the UK Government as current industry good practice. The GPG is supported by a suite of six Supplementary Guidance Notes (SGNs), published in 2014, and presents good practice in the application of the ETSU R 97 assessment methodology at various stages of the assessment process, ensuring a robust and consistent approach to wind farm noise assessment.
How are wind turbines designed to minimise noise?
Noise generated from wind turbines is mainly of two types: mechanical and aerodynamic. Mechanical noise in wind turbine is generated by various moving components present in the nacelle such as the generator, cooling fans and other auxiliary devices. Mechanical noise can however be reduced to a large extent by applying sound absorbing materials and vibration suppression within the nacelle. This has resulted in aerodynamic noise becoming the primary noise source from wind turbines.
Aerodynamic noise is flow-induced noise caused by interaction of the air passing over the turbine blades.
Blades with serrated trailing-edge are now widely used for noise control on modern wind turbines blades, typically reducing noise levels by 2-3 dB, with little to no reduction in power generation. A number of other optimisations have been implemented or are under consideration by wind turbines manufacturers, including:
- blade trailing-edge brushes;
- porous blade surfaces;
- blade tip shaping;
- vortex generators;
- boundary layer suction applied to the blades; and
- use of blade angle of attack control systems.
The ultimate aim of this research is to maximise the energy yield of a wind turbine (thereby reducing the number of wind turbines required), whilst also minimising the levels of noise emitted.
[6] ETSU 1996, ETSU-R-97 The Assessment and Rating of Noise from Wind Turbines, ETSU for the DTI, 1996.
[7] A Good Practice Guide to the Application of ETSU-R-97 for the Assessment and Rating of Wind turbine Noise, IOA, 2013.
Our wind farm noise experts at Metrica have extensive experience in the prediction, assessment and measurement of noise from wind turbines. Please get in touch at info@metricaconsulting.co.uk to discuss how we can help you!
