Wind turbine acoustics - Wind Power Generation and Wind Turbine Design - page 164

turbine noise is evaluated. When wind turbines are considered as ugly struc-
tures being in contrast with the surroundings, the probability of annoyance by
the noise increases, regardless of the measured sound pressure levels. Even if
the visual impact is not considered, the annoyance of the noise with the same
equivalent sound pressure level can be rated differently. Waye and Öhrström
[12] exposed several persons to noise registered from different wind turbine
types, scaled to 40 dB L eq ( A ). The differences in annoyance response could not
be explained by the psycho-acoustic parameters considered (sharpness, loud-
ness, roughness, fl uctuation strength and modulation). In regions with high
background noise levels the wind turbine noise is considered less disturbing
[6, 13]. Thus the acceptance level of wind turbines is higher in regions with
large traffi c, industrial areas or where a lot of noise is generated by vegeta-
tion or waves, while it is signifi cantly lower in recreational and rural areas. For
the same reason, wind turbine noise is considered more disturbing during the
night than during the day. As it is emphasized by Grauthoff [14], not only the
audible levels are important, high infrasound (below 16 Hz) levels might also be
percepted as annoying.
4.4 Wind turbine noise regulations
A recent survey of noise regulations in several countries is presented by Pedersen
[6]. Since it is considered unneeded to present all the details for the specifi c coun-
tries, here we summarize the major strategies adopted in the legislations. There are
three different kinds of noise limitation strategies:
1. Fixed values . As an example, in Sweden the highest recommended sound
pressure level originating from wind turbines is set to 40 dB, with a penalty of
5 dB for pure tones. Although this strategy is straightforward to apply it is the
least fl exible. In quiet areas even noise with sound pressure level correspond-
ing to the 40 dB limit might lead to the annoyance of the people. Contrarily,
in regions with high background noise levels even higher wind turbine noise
levels would be accepted and a fi xed value of the limit leads to suboptimal
power production.
2. Relative values . Wind turbine noise limits in Great Britain recommend a maxi-
mum noise emission of 5 dB above the background noise levels. This approach
is much more fl exible, but diffi cult to implement practically.
3. Variable values . In the Netherlands, the maximum limits of the emitted noise
vary as function of the wind speed. This method is more fl exible than the fi rst
one and easier to implement than the second one. Nevertheless, there are fur-
ther issues to be solved. Depending on the atmospheric stability conditions,
the wind speed measured at a fi xed height above the ground might not be an
accurate indicator of the wind speed met by the rotor blades. Van den Berg
[15] presents an example where the practically emitted sound pressure levels
exceeded the ones predicted with 'standard' methods because of the higher
velocities at rotor height than predicted.