Advice Section

Noise from Wind Turbines

Posted on: February 11th, 2009

Noise is a Minor Problem Today

It is interesting to note that the sound emission levels for all new Danish turbine designs tend to cluster around the same values. This seems to indicate that the gains due to new designs of e.g. quieter rotor blade tips are spent in slightly increasing the tip speed (the wind speed measured at the tip of the rotor blade), and thus increasing the energy output from the machines.

It thus appears that noise is not a major problem for the industry, given the distance to the closest neighbours (usually a minimum distance of about 7 rotor diameters or 300 m = 1000 ft. is observed).
The concepts of sound perception and measurement are not widely known in the public, but they are fairly easy to understand, once you get to grips with it.

Background Noise: Masking Noise Drowns out Turbine Noise
No landscape is ever completely quiet. Birds and human activities emit sound, and at winds speeds around 4-7 m/s and up the noise from the wind in leaves, shrubs, trees, masts etc. will gradually mask (drown out) any potential sound from e.g. wind turbines.
This makes it extremely difficult to measure sound from wind turbines accurately. At wind speeds around 8 m/s and above, it generally becomes a quite abstruse issue to discuss sound emissions from modern wind turbines, since background noise will generally mask any turbine noise completely.

The Influence of the Surroundings on Sound Propagation
Sound reflection or absorption from terrain and building surfaces may make the sound picture different in different locations. Generally, very little sound is heard upwind of wind turbines. The wind rose is therefore important to chart the potential dispersion of sound in different directions.

Human Perception of Sound and Noise
Most people find it pleasant listen to the sound of waves at the seashore, and quite a few of us are annoyed with the noise from the neighbour’s radio, even though the actual sound level may be far lower.  Apart from the question of your neighbour’s taste in music, there is obviously a difference in terms of information content. Sea waves emit random “white” noise, while you neighbour’s radio has some systematic content which your brain cannot avoid discerning and analysing. If you generally dislike your neighbour you will no doubt be even more annoyed with the noise. Sound experts for lack of a better definition define “noise” as “unwanted sound”.

Since the distinction between noise and sound is a highly psychological phenomenon, it is not easy to make a simple and universally satisfactory modelling of sound phenomena. In fact, a recent study done by the Danish research institute DK Teknik seems to indicate that people’s perception of noise from wind turbines is governed more by their attitude to the source of the noise, rather than the actual noise itself.

From PPs18

A76. Well designed wind farms should be located so that increases in ambient noise levels around noise-sensitive developments are kept to acceptable levels with relation to existing background noise. This will normally be achieved through good design of the turbines and through allowing sufficient distance between the turbines and any existing noise-sensitive development so that noise from the turbines will not normally be significant. Noise levels from turbines are generally low and, under most operating conditions, it is likely that turbine noise would be completely masked by wind-generated background noise. Table 1 below indicates the noise generated by wind turbines, compared with other every-day activities.

Table 1
Noise generated by wind turbines compared with other everyday activities
Source / Activity Indicative noise level dB(A)
Threshold of pain 140
Jet aircraft at 250m 105
Pneumatic drill at 7m 95
Truck at 30mph at 100m 65
Busy general office 60
Car at 40mph at 100m 55
Wind farm at 350m 35-45
Quiet bedroom 35
Rural night-time background 20-40
Threshold of hearing 0

A77. There are two quite distinct types of noise source within a wind turbine. The mechanical noise produced by the gearbox, generator and other parts of the drive train; and the aerodynamic noise produced by the passage of the blades through the air. Since the early 1990s there has been a significant reduction in the mechanical noise generated by wind turbines and it is now usually less than, or of a similar level to, the aerodynamic noise. Aerodynamic noise from wind turbines is generally unobtrusive – it is broad-band in nature and in this respect is similar to, for example, the noise of wind in trees.

A78. Wind-generated background noise increases with wind speed, and at a faster rate than the wind turbine noise increases. The difference between the noise of the wind farm and the background noise is therefore liable to be greatest at low wind speeds. Varying the speed of the turbines in such conditions can, if necessary, reduce the sound output from modern turbines.

A79. The report, ‘The Assessment and Rating of Noise from Wind Farms’ (ETSU-R-97) Opens link in a new browser window, describes a framework for the measurement of wind farm noise and gives indicative noise levels calculated to offer a reasonable degree of protection to wind farm neighbours, without placing unreasonable restrictions on wind farm development. The report presents the findings of a cross-interest Noise Working Group and makes a series of recommendations that can be regarded as relevant guidance on good practice. This methodology overcomes some of the disadvantages of BS 4142 Opens link in a new browser windowwhen assessing the noise effects of windfarms, and will be used in the assessment and rating noise from wind energy developments.

Recommended Good Practice on Controlling Noise from Wind Turbines

From ‘The Assessment and Rating of Noise from Wind Farms’ (ETSU for DTI 1997).

“The current practice on controlling wind farm noise by the application of noise limits at the nearest noise-sensitive properties is the most appropriate approach.
Noise limits should be applied to external locations and should apply only to those areas frequently used for relaxation or activities for which a quiet environment is highly desirable.
Noise limits set relative to the background noise are more appropriate in the majority of cases. Generally, the noise limits should be set relative to the existing background noise at the nearest noise-sensitive properties and the limits should reflect the variation in both turbine source noise and background noise with wind speed.
It is not necessary to use a margin above background noise levels in particularly quiet areas. This would unduly restrict developments that are recognised as having wider national and global benefits. Such low limits are, in any event, not necessary in order to offer a reasonable degree of protection to wind farm neighbours.
Separate noise limits should apply for day-time and for night-time as during the night the protection of external amenity becomes less important and the emphasis should be on preventing sleep disturbance.
Absolute noise limits and margins above background should relate to the cumulative effect of all wind turbines in the area contributing to the noise received at the properties in question. Any existing turbines should not be considered as part of the prevailing background noise.
Noise from the wind farm should be limited to 5 dB(A) above background for both day- and night-time, remembering that the background level of each period may be different.
The LA90,10min descriptor should be used for both the background noise and the wind farm noise, and when setting limits it should be borne in mind that the LA90,10min of the wind farm is likely to be about 1.5-2.5 dB(A) less than the LAeq measured over the same period. The use of the LA90,10mindescriptor for wind farm noise allows reliable measurements to be made without corruption from relatively loud, transitory noise events from other sources.
A fixed limit of 43 dB(A) is recommended for night-time. This is based on a sleep disturbance criteria of 35 dB(A) with an allowance of 10 dB(A) for attenuation through an open window (free field to internal) and 2 dB(A) subtracted to account for the use of LA90,10min rather than LAeq,10min.
Both day- and night-time lower fixed limits can be increased to 45 dB(A) to increase the permissible margin above background where the occupier of the property has some financial interest in the wind farm.
In low noise environments the day-time level of the LA90,10min of the wind farm noise should be limited to an absolute level within the range of 35-40 dB(A). The actual value chosen within this range should depend upon: the number of dwellings in the neighbourhood of the wind farm, the effect of noise limits on the number of kWh generated, and the duration of the level of exposure.
For single turbines or wind farms with very large separation distances between the turbines and the nearest properties, a simplified noise condition may be suitable. If the noise is limited to a LA90,10min of 35 dB(A) up to wind speeds of 10 m/s at 10 m height, then this condition alone would offer sufficient protection of amenity, and background noise surveys would be unnecessary.”