The weather conditions in Denmark make wind power one of the most advantageous sustainable energy sources. Since the end of the 1970s, Denmark has amassed strong technological and research expertise in wind power, and in 2011 the Danish wind industry has made for about a quarter of the global wind turbine market.
Noise from wind turbines
Wind turbines emit a relatively weak but characteristic noise. The noise is mainly generated by the movement of the blades through the air. This produces a swishing sound in rate with the rotation of the blades, as well as noise from the turbine machinery. Machine noise can have a tonal character which is particularly annoying.
Noise limits are set for both weak and strong winds
Wind turbines must observe the noise limits in accordance with the Statutory Order on wind turbines. The noise limits apply to the total noise from all wind turbines and are set for both weak winds, when noise is found to be most annoying, and stronger winds. When the noise meets the noise limits it do not mean that the noise is inaudible. The limits have been laid down to ensure that no significant disturbance is experienced.
Noise is calculated, because precise measurements are difficult to achieve
The noise from wind in trees and bushes makes it impossible to take sufficiently precise measurements of wind turbine noise at neighbouring properties under the necessary wind conditions. In addition to the wind noise, traffic noise and sound from birds and from noise sources inside or near the dwelling may disturb measurement of the low noise levels in question. Regulations governing noise experienced by neighbouring properties therefore calculate noise annoyance based on the wind turbines' noise emission.
Noise emission is measured relatively close to the wind turbine using a microphone mounted on a large plate on the ground. Here there is much less influence from the background noise. At the same time the wind speed is measured or preferable derived from the produced power, as this corresponds better to the wind speed acting on the blades. The wind turbine noise emission is determined on this basis.
Noise emissions are measured under both very windy conditions (8 m/s at 10 m height) and less windy conditions (6 m/s) to reflect the two sets of noise limits.
The calculation of the amount of noise emitted to neighbouring properties is very simple, because the noise is emitted from a significant height. The calculation presupposes downwind sound propagation. The calculated noise level is almost always higher than actual noise experienced by neighbouring properties.
Modern turbines emit significantly less noise
The latest wind turbines are considerably quieter than the first models of the 1970s and 1980s. In particular, noise from the gears and generator has been reduced. The modern wind turbine's nacelle is noise insulated and the generator and gears are mounted so that noise is dampened as much as possible. The design of the blades has been developed to mitigate noise.
Noise from a modern wind turbine is commensurate with that of a tractor. A typical 1980s turbine generating 100 kW and a 1990s turbine generating 500 kW both emit approx. 100 dB. This is only slightly less than a typical modern turbine generating 2-3 MW.
Read more about regulations on noise from wind turbines below.
Regulations on noise from wind turbines
The legislation provides requirements regarding the distance to dwellings and noise limits. Moreover, wind turbines erected in Denmark or in Danish waters must be authorised under the Energy Agency's technical certification scheme.
Approval of wind turbine plans
Municipalities must approve plans for erecting single as well as groups of wind turbines. An investigation into possible contravention of regulations takes place, for instance with regard to protection of nature and landscape.
The minimum distance to a neighbouring home is four times the turbine's total height. There are no exceptions to the distance requirement.
The construction of a turbine must be registered in advance with the municipality. Part of the necessary documentation is adherence to noise regulations. Noise limits apply to the total noise from all wind turbines in an area. Thus, an application for a new turbine may be refused due to pre-existing noise from other turbines in the area. The municipality cannot make exceptions to the noise regulations.
Following erection and operation of the wind turbine, the municipality may instruct the owner to carry out noise measurements to ensure that the provisions of the Statutory Order on wind turbines are still observed.
Noise limits for wind turbines
Wind turbines must respect noise limits in accordance with the Statutory Order. The limits are:
- For dwellings, summer cottages, etc.: 39 dB (wind speeds of 8 m/s) and 37 dB (wind speeds of 6 m/s)
- For dwellings in open country: 44 dB (wind speeds of 8 m/s) and 42 dB (wind speeds of 6 m/s)
For both categories of areas the limit for low frequency noise is 20 dB. The limit for low frequency noise applies to the calculated indoor noise level at both 6 and 8 m/s wind speed
Low frequency noise from wind turbines
Wind turbine noise can contain both low and high frequency noise. Most noise is caused by the rotation of the blades and is high frequency, but low frequencies also generated noise.
The Environment Protection Agency has revised the Statutory Order on Wind Turbines to include mandatory limit values for low frequency noise.
Low and high frequency noise
Wind turbine noise covers a broad spectrum of frequencies including deep and high pitch sounds (low and high frequencies). Most noise is caused by the rotation of the blades and resembles a periodic swish. Blade noise is strongest at high frequency, but the blades' rotation also generates low frequency noise.
The turbine's machinery components (gears and generator) also emit noise, which can contain a high pitched wailing (at high frequencies) or humming or rumbling sounds (at low frequencies). Wind turbine noise does not feature stronger low frequency sounds than for instance traffic noise.
Infrasound is sound at a very low frequency
Sound at very low frequencies is called infrasound. In contrast to earlier understanding of the subject, infrasound can be heard or perceived if it is strong enough. When infrasound is discernible, it is often annoying. Infrasound that is lower than hearing or sensory limits is not discernible and cannot harm health.
Wind turbines emit infrasound from the rotation of the blades. The wind turbines we know in Denmark have blades mounted on the wind side of the tower. They emit so faint infrasound that it is not audible, even close to the turbine.
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)