ADDITIONAL ISSUES

Noise from turbines


Introduction
        Wind turbines generate noise from multiple mechanical and aerodynamic sources. As the technology has advanced, wind turbines have gotten much quieter, but noise from wind turbines is still a public concern. The problems associated with wind turbine noise have been one of the more studied environmental impact areas in wind energy engineering.
         For more information about noise you should look in the next page on Measuring/Calculating sound and also in the page Acoustics on the Reference manual.



Sources of wind turbine noise
        The sources of noise emitted from operating wind turbines can be divided into two categories:
1. Mechanical: The primary sources of mechanical noise are the gearbox and generator. Mechanical noise is transmitted along the structure of the turbine and is radiated from its surfaces. In the page Noise Issues it has been explained how turbines today are engineered to reduce sound emissions due to mechanical noise. Turbines can thus be designed or refitted to minimize mechanical noise. This can include special finishing of gear teeth, mounting components in the nacelle instead of at ground level, adding baffles and acoustic insulation to the nacelle, using vibration insulators and soft mounts for major components, and designing the turbine to prevent noises from being transmitted into the overall structure.
2. Aerodynamic: Aerodynamic noise is produced by the flow of air over the blades and is typically the largest source of wind turbine noise. Aerodynamic noise generally increases with rotor speed. In the page Noise Issues it has been explained how turbines today are engineered to reduce sound emissions due to aerodynamic noise. Efforts to reduce aerodynamic noise have included the use of lower tip speed ratios, lower blade angles of attack, variable speed operation and most recently, the use of specially modified blade trailing edges. 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.



Noise is a minor problem today
        At the present time, the noise produced by wind turbines has diminished as the technology has improved. As blade airfoils have become more efficient, more of the wind energy is converted into rotational energy and less into acoustic noise.
         Operating noise produced from wind turbines is considerably different in level and nature than the most large scale power plants. Wind turbines are often sited in rural or remote areas that have a corresponding ambient noise character. Furthermore, while noise may be a concern to the public living near wind turbines, much of the noise emitted from the turbines is masked by ambient or background noise of the wind itself. Thus, it 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 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. You can actually do the calculations yourself in a moment.



Planning wind turbine installation in regard to sound
Sound map        Fortunately, it is usually reasonably easy to predict the sound effect from wind turbines in advance. On one of the following pages you may even try for yourself, using the Sound Map Calculator, which was used to draw the picture.
        Each square measures 43 by 43 m, corresponding to one rotor diameter. The bright red areas are the areas with high sound intensity, above 55 dB(A). The dashed areas indicate areas with sound levels above 45 dB(A), which will normally not be used for housing etc (we get to the explanation of the sound level and dB(A) in a moment). It can been seen, the zone affected by sound extends only a few rotor diameters' distance from the machine.











Background noise and the influence of the surroundings on sound propagation
        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.
        Moreover, 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 modeling 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.