for Low Aerodynamic Noise from Wind Turbines
Sources of Sound Emission
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 you can hear it from musical wind instruments.
Blade Sound Emission and the Fifth Power Law
Rotor blades make a slight swishing sound which you may hear
if you are 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. You will therefore notice that
modern wind turbines with large rotor diameters have very low
Blade Tip Design
Since the tip of the blade moves substantially faster than the
root of the blade, great care is taken about the design of the
rotor tip. If you look closely at different rotor blades you
will discover subtle changes 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
airflows around the tip of rotor blades is extremely complex,
compared to the airflow over the rest of the rotor blade.
on Quieter Blades
Research on quieter rotor blades continues, but as mentioned
in the section Noise is a Minor
Problem, 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.