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The science of sound is complex and has been a subject of interest for wind farms. Environment Protection Authority Victoria (EPA) has an important role to play in providing information to Victorians about the environment, including sound and its relationship to human health.
The National Health and Medical Research Council (NHMRC), the peak national body for research in Australia, issued a statement and information paper, Evidence on wind farms and human health (NHMRC website), in February 2015.The information paper summarises the evidence on the possible health effects of wind farms in humans (with a particular focus on noise, shadow flicker and electromagnetic radiation).
This page provides basic information about sound and wind farms. For further detail, please download Wind farms, sound and health: technical information (PDF 1.3MB).
Sound – definitions
Sound is produced by vibrations that cause pressure changes in a medium, such as air. The resulting waves of pressure travel in all directions away from their source. When these sound waves fall on the human ear, you can hear them.
Audibility refers to whether or not a sound can be heard. Audible sounds are those that we can hear. Inaudible sounds are those that we cannot hear.
Noise is unwanted sound.
Environmental noise (also known as noise pollution) can be caused by air and road transport, industry, and commercial and domestic activities.
Sound level refers to the intensity of a sound. Generally, the higher the level, the louder a sound will seem.
As sound waves travel away from a source, the level decreases (the sound gets ‘quieter’).
Sound level is measured in decibels (dB). The scale of human hearing is typically 0 dB to 130 dB (which is the threshold of pain).
Loudness is how intense a sound seems when heard by the human ear. Loudness depends on the:
- sound level
- frequency, duration and character of the sound
- hearing sensitivity of the listener.
Frequency of sound (pitch)
Frequency of sound (also referred to as pitch) is the rate of reception of the sound pressure wave. It is measured in hertz (Hz) or cycles per second.
Sounds with mostly low frequencies, such as thunder, often sound like a rumble. Sounds with mostly high frequencies, such as mosquitoes, often sound like a buzz or whine.
Sounds can be grouped into categories according to frequency:
- infrasound (very low-frequency sound) – below 20 Hz
- low-frequency sound – below 200 Hz
- mid-frequency sound – 200–2000 Hz
- high-frequency sound – 2000–20,000 Hz
- ultrasound – above 20,000 Hz.
Examples of sound include the following:
- The lowest note of a double bass is 41 Hz (low-frequency).
- The highest note on a piano is 4186 Hz (high-frequency).
- Most human speech is in the range 300–3000 Hz (mid to high-frequency).
Most sounds contain a mix of many frequencies.
Sounds people can hear
The frequency range of human hearing is typically 20–20,000 Hz for young, healthy adults. However, frequencies outside this range are audible if the sound is loud enough.
The ear is most sensitive to sounds in the 300–10,000 Hz range, which is similar to the range of speech. We are less sensitive to sounds outside this range, particularly sounds below 20 Hz.
Loudness and how it is measured
Simply measuring the sound level does not tell us how loud a sound seems to the human ear, because the loudness of a sound depends not only on its level, but also on its frequency. Most sounds are a combination of many frequencies.
The sound measurement technique dBA takes account of the frequencies and levels in a sound. It is representative of how our ears respond to noise. See Table 1 for examples of different sources of noise and their corresponding sound levels.
Table 1: Typical dBA levels of environmental sounds
||Sound level (dBA)
|Rural night-time background
|Car travelling at 64 km/h at 100 m away from the source
|Busy general office
|Pneumatic drill at 15 m away from the source
|Jet aircraft at 50 m away from from the source
|Threshold of pain
The New Zealand Standard NZS 6808:2010 Acoustics – Wind farm noise recommends a limit of wind turbine sound levels outdoors at noise sensitive locations of 40 dB LA90(10min)
Low-frequency and high-frequency sounds
Sound levels decrease as sound waves travel away from their source. However, sound levels of lower frequencies decrease less quickly than those of higher frequencies.
For example, when standing next to a road, the higher-frequency sounds of the tyre against the road are most obvious. When further away from a road, the sound that remains is the rumbling low-frequency noise from the engines.
It is more difficult to insulate against low-frequency sound than against mid- and high-frequency sound. This explains why bass sounds are often the main component of music heard from the next-door neighbour’s sound system.
It also explains why low-frequency sound from wind farms can seem more prominent inside houses than higher-frequency sound.
Infrasound is very low frequency sound. It usually refers to sounds with a frequency below 20 Hz.
Many people think that infrasound is inaudible. This is partly true – we are less sensitive to sounds below 20 Hz – but the human ear can perceive sounds in this range if they are at very high levels.
Infrasound is perceived by the ear in the same way as other frequencies, so it has to be audible to be detected. However, the sensation it produces is different from that of higher frequencies. This has led to confusion about how infrasound is detected and how it affects the body.
Most infrasound is accompanied by sounds at other frequencies, so it is unusual to experience pure infrasound.
There are many sources of infrasound:
- the natural environment, such as
- household and industry, such as
- rail traffic
- power plants
- the human body, such as
- head movement.
Infrasound is produced at higher levels by the body than by many external sources, including wind farms. Humans have therefore been exposed to infrasound throughout our evolution.
Sound from wind farms
Wind farm sound production
Wind farms produce a range of sounds. The types of sound that can be heard depend on the type of turbine being used and the distance of the listener from the turbine(s).
Landscape and weather conditions also affect the character of the sound.
An intermittent ‘swish’ sound is the main sound within approximately 300 m of a wind turbine.
Wind turbine sound frequency
Sound from wind turbines contains many different frequencies. The ‘swish’ sound is in the mid to high frequencies. Low-frequency sounds may be more noticeable than the ‘swish’ at distances further away from the turbine. However, wind turbines actually produce more mid- and high-frequency sound than low-frequency sound.
Wind farms and masking the sounds
Wind turbines produce more sound when wind speeds are higher. However, increased wind speeds also make other sounds from the environment louder.
The ambient sounds may mask sounds from the wind farm, making them less noticeable. This can make the measurement of wind farm sound more complex than for other environmental sound.
Wind farms – special audible characteristics
Although sound levels from wind farms are generally low, some special audible characteristics (SACs) of wind farm sound may be present. SACs may make the sound more annoying than predicted.
SACs from wind farms that can be annoying are amplitude modulation and tonality. SACs can often be minimised by changing wind farm operating conditions.
The ‘swish’ sound described above is caused by the regular rise and fall of the sound level as the blades of the turbine rotate. This variation in sound level is known as ‘amplitude modulation’.
Under certain conditions, particularly at night, this effect may be more noticeable. The swish may become a ‘beating’ or ‘thumping’ sound. It is unclear what causes this effect, but the sound may be more prominent when it occurs from multiple turbines at the same time. There is less ambient noise at night, so the noise generated by the wind turbines may seem louder than usual.
However, this type of increased amplitude modulation causing annoyance has only been detected at a small number of wind farms.
Sometimes a distinctive sound, such as a hum or whine, can be heard. This occurs when there is a dominant frequency associated with the noise (instead of an even mix of different frequencies). This is known as tonality.
High-frequency tones can be just as annoying as low-frequency tones.
SACs may not be detected by standard measurements, so they must be specifically assessed.
Noise and health effects
Perception of, and reaction to, sound
Individuals perceive and react to sounds very differently.
For example, a dripping tap in the night may be unbearable to one person and barely noticeable to another.
This is because the way we perceive sounds depends on many factors, such as:
- the sound itself (including the sound level and any SACs)
- the individual’s response to the sound.
Inaudible sound and health effects
The evidence indicates that sound can only affect health at sound levels that are loud enough to be easily audible.
This means that, if you cannot hear a sound, there is no known way that it can affect your health. This is true for all sound frequencies.
A low level of audible noise is not a problem for many of us. However, some of us may develop a negative reaction to noise or annoyance, depending on how the noise is perceived. As described above, this is influenced by the noise itself and our response to the noise.
Our response to a noise can contribute more to annoyance and related health effects than the level or characteristics of the noise itself.
Noise and individual response
How sensitive we are to noise is one factor that influences how we respond to noise. Noise sensitivity may occur for:
- physiological reasons (such as natural hearing ability)
- psychological reasons (such as generalised anxiety or beliefs about noise generally)
- external reasons (such as other life stressors).
Sensitivity to noise – increases and decreases
Long-term exposure to an annoying noise may increase a person’s sensitivity to the noise, possibly because they become focused on the noise and its negative associations.
On the other hand, tolerance or habituation may develop over time. For example, train noise that is noticeable when you move into a new house often becomes less noticeable as you become more used to it.
Noise standards are used for environmental noise (such as wind farms and traffic noise), and also for industry and even household appliances.
Noise standards are set to protect the majority of people from annoyance. The wide individual variation in response to noise makes it unrealistic to set standards that will protect everyone from annoyance.
A small number of people may still experience annoyance even at sound levels that meet the standard. This is the case not only for wind farms but for all sources of noise.