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Q&A: Low frequency noise from wind turbines  

Credit:  Danish Ministry of the Environment, Environmental Protection Agency, www.mst.dk 5 January 2012 ~~

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

Link to the new statutory order on noise from wind turbines (in Danish). 

Read more about the regulation of noise from wind turbines (in Danish)

Read more about wind turbines on the Wind Turbine Secretariat website (Danish Nature Agency) (in Danish)

 

Questions & answers:

What is low frequency noise?
Noise is unwanted sound. Noise can involve both high-pitched sounds (from high frequency sound waves) and deep sounds (from low frequency sound waves). Low frequency noise can for example be 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 nacelle machinery. The noise from the blades is a characteristic whirring, 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) and 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?
Earlier, the opinion of the Environmental Protection Agency has been that low frequency noise from wind turbines does not constitute a problem, as long as the noise levels do not exceed the limits for “general noise” from wind turbines.

But there has been concern about low frequency noise in areas where wind turbines are being planned. Industry, municipalities and citizens have thus requested specific rules for this type of noise.

A detailed analysis, by the Environmental Protection Agency, of several specific projects based on new industry information has 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 not be located as close to properties. 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 rather than size.



Do wind turbines emit infrasound, and is this dangerous?
Wind turbines also emit infrasound, which is sound at very low frequencies. Infrasound is detected 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 new limit value for low frequency noise?
The new regulation is based on a 20 decibels (dB) limit indoors for wind speeds of 6 and 8 m/s.

The limit value for noise from wind turbines is 44 dB outdoors near residences in the open country and 39 dB in residential areas, for a wind speed at 8 m/s.



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, applications for wind turbines submitted to municipalities will have to comply with both the current limit values for “general noise” and the new 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 new limit for low frequency noise.



Will the low frequency noise regulations also apply to privately owned wind turbines?
The new limit values will apply to all turbines, irrespective of ownership.



What about existing wind turbin
es?
The rules do not affect existing turbines or already approved applications. The new limit values will apply only to turbines for which applications are made 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 am having 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 monitoring.



If a wind turbine has recently received approval from the municipalities, will it exceed the new low frequency noise limit value?
In general, the Environmental Protection Agency does not expect problems relating to low frequency noise from wind turbines that keep current limits for “the general noise”.

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

Source:  Danish Ministry of the Environment, Environmental Protection Agency, www.mst.dk 5 January 2012

This article is the work of the source indicated. Any opinions expressed in it are not necessarily those of National Wind Watch.


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