Sound emissions from wind turbines may have two different origins:
- Mechanical noise
- Aerodynamic noise.
Mechanical noise, i.e. metal components moving or knocking against each other may originate in the gearbox, in the drive train (the shafts) and in the generator of a wind turbine.
Machines from the early 1980s or before do emit some mechanical noise, which may be heard in the immediate surroundings of the turbine, in the worst cases even up to a distance of 200 m. However, a survey on research and development priorities of Danish wind turbine manufacturers conducted in 1995 showed that no manufacturer considered mechanical noise as a problem any longer and therefore no further research in the area was considered necessary. The reason was, that within three years noise emissions had dropped to half their previous level due to better engineering practices.
Quieting wind turbine gearboxes
Gearboxes for wind turbines are no longer standard industrial gearboxes, but they have been adapted specifically for quiet operation of wind turbines. One way of doing this is to ensure that the steel wheels of the gearbox have a semi-soft, flexible core, but a hard surface to ensure strength and long time wear.
The way this is done is basically to heat the gear wheels after their teeth have been ground and then let them cool off slowly while they are packed in a special high carbon-content powder. The carbon will then migrate into the surface of the metal. This ensures a high carbon content and high durability in the surface of the metal, while the steel alloy in the interior remains softer and more flexible.
Structural Dynamics Analysis
When going by car, plane, or train, sometimes
resonance of different components, e.g. in the dashboard of a car or a window of a train may amplify noise. An important consideration, which enters into the turbine design process today, is the fact that the rotor blades may act as membranes that may retransmit noise vibrations from the nacelle and tower.
The turbine manufacturers nowadays make computer models of their machines before building them, to ensure that the vibrations of different components do not interact to amplify noise. If you look at the chassis frame of the nacelle on some of the large wind turbines on the market today, you may discover some odd holes which were drilled into the chassis frame for no apparent reason. These holes were precisely made to ensure that the frame will not vibrate in step with the other components in the turbine.
Sound insulation plays a minor role in most wind modern turbines on the market today, although it can be useful to minimise some medium- and high-frequency noise. In general, however, it seems to be more efficient to attack noise problems at the source, in the structure of the machine itself.
When the wind hits different objects at a certain speed, it will generally start making a sound. If it hits the leaves of trees and bushes, or a water surface it will create a random mixture of high frequencies, often called white noise. The wind may also set surfaces in vibration, as sometimes happens with parts of a building, a car or even an (engineless) glider aeroplane. These surfaces in turn emit their own sound. If the wind hits a sharp edge, it may produce a pure tone, as it can be heard from musical wind instruments.
Rotor blade sound emission and the 5th power law
Rotor blades make a slight swishing sound which may be heard close to a wind turbine at relatively low wind speeds. Rotor blades must brake the wind to transfer energy to the rotor. In the process they cause some emission of white noise. If the surfaces of the rotor blades are very smooth, which indeed they must be for aerodynamic reasons, the surfaces will emit a minor part of the noise. Most of the noise will originate from the trailing (back) edge of the blades. Careful design of trailing edges and very careful handling of rotor blades while they are mounted, have become routine practice in the industry.
Other things being equal, sound pressure will increase with the fifth power of the speed of the blade relative to the surrounding air. This is why modern wind turbines with large rotor diameters have very low rotational speed.
Rotor blade tip design
The tip of the blade moves substantially faster than the root of the blade. It's therefore not surprising that the aerodynamic noise generation is very sensitive to the speed of translation at the very tip of the blade. To limit the generation of aerodynamic noise, large modern wind turbines limit the rotor rotation tip speeds under about 60 m/s.
Since the tip of the blade moves so much faster than the root of the blade, great care is taken about the design of the rotor tip. Looking closely at different rotor blades, subtle changes can be discovered in their geometry over time, as more and more research in the area is being done. The research is also done for performance reasons, since most of the torque (rotational moment) of the rotor comes from the outer part of the blades. In addition, the air flows around the tip of rotor blades is extremely complex, compared to the airflow over the rest of the rotor blade.
Research on quiter blades
Research on quieter rotor blades continues, but most of the benefits of that research will be turned into increased rotational speed and increased energy output, since noise is generally not a problem per se, given the distances to neighbouring houses etc.
For more information about noise look the pages Noise from turbines and Measuring/Calculating sound. Moreover, the page Acoustics in the Reference manual can be helpful.