Noise and sound are acoustic waves carried on oscillating particles in the air. The frequency of sound (Hz) is defined by the number of oscillations per second and how we perceive noise is partly dependant on what the frequency is. High frequency noise has more oscillations per second whereas low frequency noise has fewer.
Infrasound (sound that is not audible)
The frequency range of infrasound is usually defined as below 20Hz. Audible noise is usually defined as between 20-20,000 Hz. Although sound is audible below 20Hz, tones are lost between 16 and 18Hz resulting in many people not being able to hear the sound.
Dr Geoff Leventhall, expert consultant in Noise Vibration and Acoustics and author of the UK DEFRA (peer reviewed) report on infrasound, states: “I can state quite categorically that there is no significant infrasound from current designs of wind turbines. To say that there is an infrasound problem is one of the hares which objectors to wind farms like to run.”
Low frequency noise
Low frequency noise stretches across the frequency band from approximate 10 to 200 Hz, thereby being included within both the infrasonic and audible definitions.
At the beginning of 2004, a newspaper reported a claim that ‘low frequency noise’ or ‘infrasound’ could be produced by wind turbines, and that this was possibly a cause of sleep interruption and headaches.
First of all it should be understood that both human made and natural sound sources produce elements of low frequency noise or infrasound. It should also be understood that low frequency noise has been around through our evolution as human beings without hazard – there are even low frequency fluctuations within our bodies associated with blood flow known to have audible effects.
A document ‘Low frequency noise and vibrations at a modern wind farm’ (ETSU W/13/00392/REP), commissioned by the DTI in 1997, comprehensively assessed the vibrations from wind turbines. It concluded that:
• vibration levels attenuated rapidly with distance
• there was no clear increase in vibration with wind speed
• 100 metres away from the turbine (i.e. on the wind farm site itself), levels were 10 times lower than the requirements for modern laboratories.
Overall, modern wind turbines emit negligible amounts of low frequency noise.
I have heard anti-wind farm campaigners claim that the noise from a wind turbine gets louder the further away you get. This is not technically possible according to acousticians but the human ear does hear different noise frequencies for different distances. This is due to the ability of for example air, ground or barriers such as windows to reduce perceived sound being reduced at lower frequencies. Lower frequency noise is therefore often heard over a longer distance. Try an experiment by listening to the noise next to an open window and then closing it and walking a few feet away. You will probably be able to still hear the lower frequency noises such as traffic (albeit fainter) but not the higher frequency noises.
For more information about how people perceive noise from wind farms take a look at my post is the noise all in your head? – wind turbine noise, physcology and world perspective
