Sound from modern wind turbines contains energy spread across the audible frequency range and, like most sounds in the environment, has some inaudible energy in the infrasound range.
However, the levels of infrasound and low frequency noise from modern wind turbines are so low that they usually lie below the threshold of perception. This has been repeatedly shown by research undertaken in the UK, Denmark, Germany and the USA over the past decade and accepted by experienced noise professionals overseas and in New Zealand.
Concern about low frequency noise stems from early wind turbine designs, where the blades were down wind of the turbine tower. This design caused a strong low frequency pulse, which also had significant levels of energy in the infrasound range, when the blades passed the tower.
Modern wind turbines are designed with the blades upwind of the tower, and have an increased distance between blades and the tower to minimise any residual possibility that the blades may interact with disturbed air flow upwind of the tower. These design developments have dramatically reduced the generation of high levels of low frequency noise by wind turbines.
Low frequency noise explained
The frequency, or ‘pitch', of a sound is measured in cycles per second or ‘hertz' (Hz). Most noise in the environment contains energy at many different frequencies combined together to give it its overall character, this is often referred to as broadband noise.
For a healthy young adult the range of hearing is often quoted as extending from 20Hz to 20,000Hz although the sensitivity of the ear varies significantly with frequency and is most sensitive to sounds with frequencies between around 500Hz and 4000Hz (the range within which human speech occurs). Above and below this, the ear becomes decreasingly sensitive, especially to very low frequencies, which have to be very loud to be perceived.
‘Low frequency noise' is the term used to describe sound energy in the region below about 200Hz. The rumble of thunder and the throb of a diesel engine are both examples of sounds with most of their energy in this low frequency range.
Infrasound explained
Infrasound describes sound energy below 20Hz. Almost all noise in the environment has components in this region but they are of insignificantly low levels. Noise which has most of its energy in the ‘infrasound' range is only audible if it is at a very high level, far above normal environmental levels.
Health concerns
It is generally accepted that if infrasound and low frequency noise can not be perceived then it does not pose a concern for human health. Measured levels of infrasound and low frequency noise from wind turbines are well below levels of human perception. Expert evidence on this matter has been tested in and accepted by the Environment Court in many decisions regarding wind farm proposals.
Some people are concerned that wind turbines may cause vibroacoustic disease. Vibroacoustic disease is associated with exposure to high levels of infrasound, for example infrasound from jet engines or from using a pneumatic drill. Research has shown that wind turbines produce insignificantly low levels of infrasound.
A review conducted for the Energy Efficiency and Conservation Authority in 2004 by Bel Acoustics concluded:
"There is no information available to indicate that wind turbine generators emit infrasound anywhere near the intensity in the infrasound region for the sound to be audible even to a person with the most sensitive hearing acuity at a distance where houses are generally located relative to wind farms.
"It is important to realise that there is no reliable evidence that would indicate any effects on people when infrasound is present at a level below the hearing threshold... "[Emphasis in original]
There is no evidence to indicate that low-frequency sound or infrasound from current models of wind turbine generators should cause concern to anyone living close to a wind turbine generator or a wind farm.
A 2006 study into noise complaints at three UK wind farms concluded that low frequency noise near the turbines was measurable on a few occasions, but at low levels that were not sufficient to affect the health of nearby residents. This study made an important finding that the common cause of complaint was not associated with low frequency noise, but the occasional audible modulation of aerodynamic noise, especially at night. The study also noted that at all the measurement sites the low frequency noise from the traffic on local roads was greater than that from the wind farms.
Further reading
- Notes on Low Frequency Noise from Wind Turbines, Dr Geoff Leventhall (2004) (PDF, 939kb)
- Low frequency noise and infrasound from wind turbine generators: A literature review, Bel Acoustic Consulting (2004) (PDF, 427kb)
- Review of noise conditions, Hegley acoustic consultants (2004) (PDF, 460kb)
- The Measurement of Low Frequency Noise at Three UK Wind Farms, Hayes McKenzie (2006)