Environmental Engineering Reference
In-Depth Information
Depending on the rotational speed of the turbine, the size and the airfl ow wind
turbines emit infrasound, low and high frequency sound waves. Large wind tur-
bines produce infrasound of 8-12 Hz range. Small turbines can achieve higher
blade tip velocities that can give low frequency noise of 20-20 kHz range.
To get a better feeling of the frequency ranges that are audible and are produced
by wind turbines, we can see Table 1.
The levels of infrasound radiated by the large wind turbines are very low in
comparison to other sources of acoustic energy in this frequency range. However,
the annoyance is often connected with the periodic nature of the emitted sounds
rather than the frequency of the acoustic energy. Because low frequencies travel
farther than the high frequencies due to their long wavelengths, they become a
cause of irritation for residents living not so close to wind farms.
Sound is a series of waves that travel through air in the form of disturbances
and reaches our eardrums. Any natural or artifi cial obstacles such as hills and
buildings play a shielding effect role, refl ecting the sound, while most of the
times absorbing some of the acoustic energy. Trees and ground vegetation also
attenuate sound and change its directivity patterns. The distance and obstacles
that are located between the source and the receiver have a signifi cant impact on
the acoustic 'line of sight'.
The ways to reduce noise from wind turbines are mostly focused on blade design
optimisation and choice of wind farm location as it still a new fi eld and new tech-
niques are under development. A major challenge that needs to be addressed before
abatement techniques are put in action; is to establish accurate measurement meth-
ods of wind turbine noise in order to defi ne the parameters of the problem and fi nd
an appropriate acoustic solution for it. Noise measurements from wind turbines is a
complex task because the background noise levels are comparable to the noise levels
from wind turbine when it starts operating at certain wind speeds. This is the reason
a commonly used approach that overcomes the above issue needs to be established.
Current acoustic treatment methods range from designing quieter wind turbine
compartments to placing wind turbines as far as possible from residential areas.
Wind turbine blades are aerodynamically shaped to avoid causing abrupt air fl ow
disturbances and the turbine is placed upwind to eliminate interference of the fl ow
between the tower and the wind turbine blades. Apart from aerodynamic solutions
other treatments include sound proofi ng of the nacelle to reduce mechanical
noise and careful selection of the wind turbine site to attenuate noise until it
reaches the receiver.
Table 1: Important frequency ranges.
Description
Frequency range
Normal hearing
20 Hz to 20 kHz
Normal speech
100 Hz to 3 kHz
Low frequency
20-200 Hz
Infra sound
<16 Hz
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