Low frequency sound can be produced by a variety of sources including machinery, refrigerators, vehicles, thunder, surf and even the wind itself.
Low frequency sound and infrasound explained
The frequency, or 'pitch', of a sound is measured in cycles per second or 'hertz' (Hz). Most sound in the environment, including that from wind turbines, contains energy at many different frequencies combined together to give it its overall character, this is often referred to as broadband sound.
A healthy young adult's hearing range is usually 20Hz to 20,000Hz. The ear becomes decreasingly sensitive to sounds above and below these frequencies, and especially to very low frequencies, which have to be very loud to be perceived.
'Low frequency sound' 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 describes sound energy below 20Hz.
Almost all sound in the environment has components in this region as infrasound is naturally occurring. Infrasound is produced continually by wind's interaction with natural topography, and by common human activities such as running, driving with a window open and swimming. Sound 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.
Sound from wind turbines
Sound from modern wind turbines contains energy spread across the audible frequency range – which is often referred to as broadband sound. Like most sounds in the environment, sound from turbines has energy in the low frequency and infrasound range. However, the levels of this sound are so low that they usually lie below the threshold of perception.
The audible whooshing or swishing sound created as the wind passes over the rotating blades is often described as infrasound or low frequency sound, but it is in fact broadband sound.
Concern about low frequency noise from wind farms stems from early wind turbine designs, where the blades were down wind of (or behind) 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 behind the tower.
Most 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 airflow upwind of the tower.
These design developments have dramatically reduced the intensity of low frequency sound created by wind turbines.
Research into low frequency noise from modern wind turbines
A 2006 study into noise complaints at three UK wind farms concluded that low frequency noise from the turbines was measurable on a few occasions. However, the levels of sound were significantly below the recognised threshold of perception.
This study also noted that at all the measurement sites the low frequency sound from the traffic on local roads was greater than that from the wind farms. This study made an important finding that the common cause of complaint regarding sound from wind farms was not associated with low frequency noise, but the occasional audible modulation of aerodynamic noise, especially at night.
A paper presented at the 159th Meeting of the Acoustical Society of America outlines the results of sound measurements from Siemens SWT-2.3 MW-93 turbines.
The paper concludes that at more than 1000 feet (305 metres) the turbine model does not pose a low frequency noise or infrasound problem as it meet standards published by the American National Standards Institute (ANSI) related to:
- indoor levels for low frequency sound for bedrooms, classrooms and hospitals
- indoor levels for moderately perceptible vibrations in light-weight walls and ceilings
- thresholds for annoyance and beginning of rattle.
- and UK Department for Environment, Food, and Rural Affairs (DEFRA) guidelines related to levels of low-frequency noise causing disturbance.
The paper suggests that some low frequency noise may become audible from 50 Hz, depending on other sounds in the environment, though noise at these frequencies is generally already present from sources such as refrigerators (indoors) and traffic, airplanes and even the wind itself (outdoors).
- The Measurement of Low Frequency Noise at Three UK Wind Farms, Hayes McKenzie (2006)
- Low Frequency Sound and Infrasound from Wind Turbines, Acoustical Society of America 159th Meeting Lay Language Paper