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How Noisy is a Wind Turbine?

The question is central to whether wind turbines be placed near homes. Any answer requires an understanding of how we perceive and measure sound.

Human Hearing and the Decibel

Sound is simply vibrational energy transmitted to our ears. Evolution has protected us from damaging sound intensities by ensuring that the greater the noise level the less sensitive our ears become. If we are listening to a particular sound and wish to double the volume of what we hear we need to increase the sound power level at the source by a factor of ten. In technical terms, we perceive and measure sound volume and power levels on a "logarithmic" scale.

First some definitions:

Sound Power Level

refers to the energy per second emitted by a sound source. It is measured in watts, but may also be expressed in decibels when referenced against one decibel equaling 10^-12 watts

Sound volume or noise level

refers to the "loudness" or "change in loudness" we associate with a sound. It is measured in Bells or Decibels.

Decibel (dB)

A deciBel (dB) is one tenth of a Bell and a Bell is a doubling of sound volume. Therefore a deci-bel is a tenth of a doubling in volume. A dB is approximately the smallest change in level that can be detected by the human ear.
When sound is calibrated against a reference level of the smallest sound perceptible to the human ear (one decibel) we obtain an absolute scale of loudness ranging from our limit of hearing up to the threshold of pain at about 120 decibels (about the sound level of an affluent adolescent's car stereo).
3 dB is a 2:1 power ratio. Each 3 dB loss cuts the sound power by a factor of two, each 3 dB gain doubles the power by 2. While a 3 dB change in sound power is easily detectable it is not twice as loud. It requires a change of 10 dB to double the sound volume.

A weighted Decibels(dBA)

Hearing sensitivity varies with the frequency of the sound so adjusted Decibel scales were introduced to compensate. In the A-weighted system (dBA units), the decibel values of sounds at low frequencies are reduced, compared with unweighted decibels, in which no correction is made for audio frequency.

C weighted Decibels

Similar to A weighted decibels but compensated to account for very loud or very low frequency sounds.

These websites provide additional information:

• Sound workshop - A simple exposition of sound measurement
• UNSW Physics website. - A technical treatment of sound and its measurement.
• Engineering website - an excellent guide to common sounds in the decibel scale.

So: How much noise does a turbine generate?

Turbine noise levels are dependent on the type of turbine and the distance of the listener from the source. As a general indication, from a single 2 Megawatt turbine, we could expect noise levels of about 100 dBA¹ at a reference distance of 1 metre from the source with noise attenuating by about 6 dBA for each doubling of distance.

The quality of the noise and its intensity can however be heavily modified by atmospheric conditions and also by the presence of multiple turbines. This is discussed in temperature inversion and the Spring Range assessment at the bottom of this page

The following sites provide general information on the topic.

General Education Websites:

• American Wind Turbine Acoustic Noise Study This American study sourced from the University of Massachusetts at Amherst Renewable Energy Research Laboratory provides useful information related to wind turbine noise. Care is required interpreting the results. The site's mission is linked to introducing turbines and applies its results to American standards where tubines have been placed as close as 300 metres from homes.
• University of Massachusetts Renewable Energy Lab More information from this American source with the same qualifications as before.
• Swedish environmental survey of turbine noise annoyance.

Wind Industry Websites

• British Wind Energy Association website This sites asserts that the sound Power level of a wind turbine is about 90-100 dBA at source which reduces to a noise level of about 50-60dBA at 40 metres. By applying the common rule that doubling the distance reduces the noise level by 6 dBA in flat terrain, noise levels of 35 dBA at 500 metres can be extrapolated.
• Swedish study of turbine annoyance An MSc thesis funded from industry sources.

Interesting Links Related to Turbine Noise:

• Meridian payout for noise

References

1. German website giving rated power levels for turbines.
2. Australian OH&S workplace noise standards
3. WHO Noise and Health Standards
4. UK sound testing site with clear definition of difference between sound level, and sound power level
5. A guide to common sounds in the decibel scale
6. Engineering site with sound pressure levels and comparisons
7. Van den Burg Study: Seminal Dutch measurements of turbine noise and temperature inversion showing industry models can underestimate turbine noise.
8. The sound of high winds: the effect of atmospheric stability on wind turbine sound and microphone noise : University of Groningen dissertation on turbine noise measurement - Berg, Godefridus Petrus
9. South Australian EPA Noise Guidelines

Noise and health

Some researchers associate health issues with wind turbine noise. Dr Nina Pierpont is active in the field. She is of the view that for a radius of up to 1.5 to 3 miles wind turbines make a level of noise which the UN World Health Organisation considers impacts health.

The WHO do indeed state that 30 dB has such an impact. Refer to this: World Health Organisation website. And the noise emitted by wind turbines is confirmed by this German reference. and from Vestas , a Danish turbine manufacturer. Given the RULE OF THUMB that noise drops 6 dB with every doubling of distance, a device emitting 100 dB AT 1 METRE would be within the 30 dB level at 1.5 miles but would require specific conditions to be 30dB by 3 miles.

Spring Range Assessment:

A brief perusal of noise measurement websites shows the subject to be laced with technical pitfalls. However some clear facts emerge and it is possible to state turbine noise in a way people can relate to.

1. The reference noise power level for a two megawatt wind turbine at source is of the order of 100 dBA¹. In this regard the power of the sound source can be compared with a motor lawn mower, or traffic on a highway.
2. At sufficient distance from the turbine the noise attenuates and begins to blend into the background. Eventually it reduces to 35 dB(A), a level permitted by noise regulation - and this more innocuous number is one often quoted by turbine manufacturers.
3. Reputable scientific studies^7,8 have shown:
   • that engineering models for predicting turbine noise are subject to a degree of error.
   • that atmospheric conditions commonly encountered in Canberra (temperature inversion) can reduce background noise levels and allow turbines to be more audible than is predicted by the models used to secure their approval. The error can result in sound levels which are 15 dB(A) too high.
   • that the sound from multiple turbines can combine to produce increased noise at some locations.
   • that the "pulsing" nature of the sound noted by people living near turbines can be explained by scientific analysis.
4. A number of properties on Spring Range are located about 4 kilometres from the Barton Highway and can clearly hear the noise from traffic on it. The sound power level from a car at highway speed is about 100 dBA⁵ - about the same as the source power level for a wind turbine¹. It is reasonable to conclude that wind turbine noise on Spring Range will be audible from most of the properties near it. Some residences in close proximity to turbine sites may also experience noise levels considered excessive by WHO³.
5. Spring Range notes that the noise standards⁹ applied in development approvals permits noise levels of 35 dBA at residences and can exceed World Health Organisation recommendations. We conclude that the standards themselves are in need of review.

Most Spring Range residents moved to the area because they placed a high value on tranquility and wished to avoid the bustle of living in a suburb. The ambient noise level at Spring Range is low. (Paragliders talking to one another are audible more than a kilometre away). Wind turbine noise pollution which we may find acceptable in an industrial environment is out of place in a rural residential area.