As Denmark is situated between the North Sea and the Baltic Sea, the prevailing weather conditions suggest that wind power should constitute a major part of the country’s total power production. While onshore wind turbines have dominated the Danish landscape for decades, offshore wind turbine parks have been erected in Danish waters in recent years.
This development is largely a result of protests by residents against the construction of wind turbines close to their properties. The ‘not in my back yard’ principle is thus proving decisive when it comes to the construction of onshore wind turbines.
Residents have documented considerable noise problems associated with wind turbines, particularly in relation to low frequency noise.
In December 2011 the Environment Protection Agency revised the statutory order on noise from wind turbines to include mandatory limit values for low frequency noise. Denmark has thus become the first country to introduce these mandatory limit values for low frequency noise.
No evidence suggests that low frequency noise is more dangerous than other forms of noise.
Wind turbines emit a relatively quiet, but characteristic noise. This is mainly generated by the movement of the blades through the air, which produces a whirring sound, and noise from the turbine machinery (gears and generator), which is said to have a somewhat annoying resonance.
Modern turbines emit significantly less noise than older turbines, since modern turbine blades are designed to mitigate noise.
The Environmental Protection Agency opines that low frequency noise from wind turbines does not constitute a problem, as long as noise levels do not exceed the limits for ‘general noise’ from wind turbines.
However, there has been some concern as to low frequency noise in areas where the erection of wind turbines is being planned. Industry, municipalities and citizens have thus requested specific rules for this type of noise.
A detailed analysis by the agency of several specific projects based on new industry information has revealed that the new rules can be a challenge for certain new types of serial-produced wind turbine in specific situations.
Low frequency noise
‘Low frequency noise’ is defined as noise within the frequency range of 10 to 160 hertz (Hz) (between 10 and 160 cycles per second)). Noise can involve both high-pitched sounds (from high frequency sound waves) and deep sounds (from low frequency sound waves).
Noise from the movement of the blades through the air is not normally low frequency. Noise from the machinery can consist of both a high-pitched wailing (high frequency) and buzzing sounds (low frequency).
However, wind turbine noise does not feature more low frequency noise than traffic, for instance.
Wind turbines emit infrasound, which is sound at a low frequency (lower than 20Hz). In contrast to an earlier understanding of the subject, infrasound can be heard or perceived if it is strong enough, and when infrasound is discernible, it is often annoying.
Infrasound is audible to the human ear only if it is sufficiently strong. Infrasound that is inaudible to the human ear does not affect human health.
Danish wind turbines emit weak infrasound that is not audible to the human ear, even when close to the turbine. Therefore, infrasound is not a problem in regard to modern wind turbines.
Under the revised statutory order on wind turbines, which entered into force on January 1 2012, wind turbines registered after this date must observe the mandatory limit value at 20 decibels (dB) as an indoor noise level for wind speeds of six metres per second (m/s) and 8m/s. The limit value for noise from wind turbines is 44dB outdoors near residences in the open country and 39dB in residential areas, for a wind speed at 8m/s. This limit value ensures that neither the usual noise nor the low frequency noise is disturbing.
The noise limit does not mean that the noise is inaudible. The limit has been laid down to ensure that no significant disturbance is experienced.
The revised statutory order has been subject to some criticism in the media due to the fact that noise emissions are measured close to the wind turbine (not directly at residents’ houses) and the amount of noise emitted to neighbouring properties is remotely calculated.
Independent scientific studies show that the limit value has been violated in several cases.
The Environment Protection Agency has stated that the measurement method used secures the most accurate result, since measurements inside residents’ houses can be disturbed by background noise (eg, district heating stations and various machines).
It seems that the last word has yet to be said on this matter, and noise from wind turbines is expected to be one of the most prominent environmental issues in the near future.
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