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.
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.
|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|
|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) , 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 when assessing the noise effects of windfarms, and will be used in the assessment and rating noise from wind energy developments.