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)

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 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) 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?
It was earlier the opinion of the Environmental Protection Agency that low frequency noise from wind turbines does not constitute a problem, since the noise levels do not exceed the limits for the ‘normal 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 indicated 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 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 new limit value for low frequency noise?
The new regulation is based on a 20 decibels (dB) limit for the low frequency noise level, calculated indoors, for wind speeds both 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, wind turbines registered with municipalities will have to comply with both the current limit values for the ‘normal 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.

What about existing wind turbines?
The rules do not affect turbines that are registered with the municipalities earlier than January 1st, 2012. It will apply only 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 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. 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 has recently been registered with 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 ‘normal 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 some specific situations.