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Q&A: Low frequency noise from wind turbines

The Environmental Protection Agency has revised the Statutory Order on Wind Turbines to include limit values for low frequency noise. The new limit values apply to turbines that are registered after January 1st 2012 where the new statutory order entered into force.

Link to an English translation of the revised Statutory Order on noise from wind turbines.

Please note that the Statutory Order is valid for wind turbines in Denmark, and that the methods for calculation of wind turbine noise are adapted to Danish conditions. This goes for the values stated for correction for ground effect, which are based on the general applicable method Nord 2000, as well as for the values stated for sound insulation of dwellings, which are based on measurements of sound insulation at low frequencies in 14 representative Danish dwellings

Read more about the regulation of noise from wind turbines (in Danish). A paper in Low Frequency Noise, Vibration and Active Control , vol. 31 no. 4, describes and explains the Danish regulation of wind turbine noise (in English)

Questions & answers

What is low frequency noise?

Noise is unwanted sound. Noise can involve both high-pitched sounds (high frequency sound) and deep sounds (low frequency sound). Low frequency noise is for example the hum or buzz from a compressor, rumble from a boiler or a combustion plant or the rumbling of an idling engine.

Wind turbine noise emanates from the rotation of the blades and from the nacelle machinery. The noise from the blades is a characteristic swishing sound, which varies in rate with their rotation. Normally this does not contain much low frequency noise. Noise from the machinery can consist of both a high-pitched wailing (high frequency) or buzzing sounds (low frequency).

Low frequency noise is technically defined as noise within the frequency range of 10 – 160 Hz (between 10 and 160 cycles per second).

Is low frequency noise a problem in relation to wind turbines?

It was earlier the opinion that low frequency noise from wind turbines does not constitute a problem as long as the noise levels do not exceed the limits for the ‘normal noise’ from wind turbines, and these limits are binding. But there was concern about low frequency noise in areas with planned wind turbines, so both municipalities, the wind industry and citizens requested specific rules for low frequency noise.

Following a detailed analysis of several specific projects based on new industry information, The Environmental Protection Agency found that the new rules can be a challenge for certain new types of serial produced wind turbines in specific situations.

No evidence suggests that low frequency noise is more dangerous than other forms of noise.

Are giant wind turbines a particular source of low frequency noise?

All turbines can emit low frequency noise, irrespective of their size either in terms of electrical power (megawatts) or height.

Current knowledge of the subject is that large wind turbines emit more noise than small ones, and should therefore be located further from properties than turbines emitting less noise. However, there is no clear correlation between the size of the wind turbine and the level of low frequency noise it emits. This depends more on construction type than on size.

In general there is no clear connection between  the size of a wind turbine and the characteristics of the emitted sound, other than large wind turbines rotate slower so the blade noise is modulated with a lower period.

Do wind turbines emit infrasound, and is this dangerous?

Wind turbines also emit infrasound, which is sound at very low frequencies. Infrasound is sensed in the same way as other sounds and is audible to the human ear if sufficiently strong. When infrasound is audible, it becomes annoying. Where infrasound is inaudible, it does not affect health.

The wind turbines we know in Denmark today emit very weak infrasound, which is below hearing threshold, even when in close proximity. Infrasound does therefore not pose a problem in regard to modern wind turbines.

The technical definition of infrasound is sound that is lower in frequency than 20 Hz (fewer than 20 cycles per second).

What is the limit value for low frequency noise?

The limit for low frequency noise is 20 decibels (dB) the low frequency noise level, calculated indoors. The limit applies to wind speeds both of 6 and 8 m/s.

The limit value for the total noise from wind turbines (the 'normal noise') is 44 dB calculated outdoors near residences in the open country and 39 dB in residential areas, for a wind speed at 8 m/s. At lower wind, 6 m/s, these limits are both 2 dB tighter.

Why are the noise levels calculated instead of being measured? 

It is not possible to measure the noise levels accurately near the surrounding dwellings. The wind causes noise from trees and vegetation, when it is sufficiently strong for the wind turbine to operate as intended. Noise from traffic, birds, and other sources also affects measurement of the low levels in question. For low frequency noise, which is assessed indoors, noise sources such as a refrigerator or a heating system disturbs the measurements.

A noise measurement where the operating conditions of the wind turbines are out of control, and where the sound propagation conditions and the background noise is not accounted for, can give any result and is invalid.

When a turbine is registered, it is not yet operating, so the noise has to be predicted. The noise emission from a turbine of same model is measured using a standardised method, and the noise is calculated in points representing the neighbours. In supervision of the noise limits after the turbines are established, the noise emission from the actual turbines are measured using the same standardised procedure, and the noise levels near the surrounding dwellings are calculated with the same method.

The calculation method is based on the contemporary and very precise model, Nord2000, which is simplified considering the high sound sources and further assumes downwind in all directions. This way a minimum of attenuation due to sound propagation is used in the calculation.

What are the consequences of the new limit value for low frequency noise?

After the new noise regulation has entered into force on January 1st 2012, wind turbines registered with municipalities will have to comply with both the limit values for the ‘normal noise’ and the limit value for low frequency noise

The municipality has an obligation to inspect wind turbines to ensure that noise disturbance is not excessive and can require wind turbine owners to have the noise generated by their turbines measured to ensure that regulations are complied with. This also applies to the limit for low frequency noise.

What about older wind turbin es?

The rules do not affect turbines that are registered with the municipalities earlier than January 1st, 2012. It only applies to turbines that are registered after the Statutory Order entered into force January 1st 2012.

When existing wind turbines are renewed, the new regulations will apply to the replacement turbines.

What should I do if I have problems with low frequency noise?

If noise has become a nuisance and the problem cannot be solved by contacting the company (or wind turbine owner) causing the noise, you can take the matter to the municipality.

Municipalities are the supervisory authority of wind turbine noise regulations. Only wind turbines registered with the municipality after January 1st 2012 are subject to the binding limit for low frequency noise.

If a wind turbine been registered with the municipalities prior to the new low frequency noise limit, will it then be exceeded?

In general, the Environmental Protection Agency does not expect problems relating to low frequency noise from wind turbines that keep limits for the ‘normal noise’. These limits are mandatory for all wind turbines.

In preparing the new regulations, the Environmental Protection Agency found that certain serial produced wind turbines may have difficulties complying with the noise limit for low frequency noise in some specific situations.