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Designing for Low Mechanical Noise from Wind Turbines

Sound emissions from wind turbines may have two different origins: Mechanical noise which we deal with on this page, and aerodynamic noise which we deal with on the next page.

Mechanical Sources of Sound Emission
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 (600 ft.)
A survey on research and development priorities of Danish wind turbine manufacturers conducted in 1995, however, 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, you may have experienced how 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.
As explained in the tour section on Research and Development, 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
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.



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© Copyright 2000 Soren Krohn. All rights reserved.
Updated 9 September 2000