Environmental Engineering Reference
In-Depth Information
complaints about the irritating noise from the wind turbines has led scientists and
engineers to invent ways to assess the levels of noise in the near and in the far fi eld
and eventually close to dwellings where residents live.
Measuring noise from wind turbines is not an easy task due to the fact that back-
ground noise levels increase as the wind speed increases. So, as the wind turbines
start rotating the background noise levels are being intensifi ed. Also when we
think that the wind turbines are placed outdoors where trees, leaves and vegetation
in general, are present then it becomes apparent that the background noise is com-
parable to the noise emissions from the turbines. This makes it extremely diffi cult
to measure sound from wind turbines accurately. At wind speeds around 8 m/s and
above, it generally becomes a quite abstruse issue to discuss sound emissions from
modern wind turbines, since background noise will generally mask any turbine
noise completely.
To assess the potential levels of infrasound and low frequency noise around a
wind farm and at neighbouring locations of interest, the measurements are under-
taken using a measuring system capable of capturing frequencies from 1 Hz to
20 kHz. Measurements are performed at internal and external locations placing the
microphones at locations where noise is considered more audible when occurred.
Assuming that the appropriate equipment is being used and the calibration proce-
dures have been followed, the standard procedure to take noise measurements is
the following:
Ambient background noise levels evaluation .
Spot measurements of noise levels inside dwellings subject to access or prediction
if access is prohibited.
Exterior acoustic measurements at neighbouring facades to assess annoyance.
Spot measurements of wind speed.
The evaluation and reporting of measurements made.
It is worth mentioning that there is a variety of sophisticated model available but
they are all at developing stages. A typical time history of measured sound pressure
levels is shown in Fig. 10.
5.1 For small wind turbines
.
Our experience indicates that in practice, fi eld measuring is a challenging task
not only due to the diffi culty to estimate background levels but also due to the
complexity of the required experimental set up. One has to count for the direc-
tional noise emission depending on which side the wind is blowing and the fact
that modern small wind turbines rotate around their vertical axis making the noise
measurement techniques even more demanding.
Because of the importance of background noise in determining the acceptability
of the overall noise level, it is crucial to measure the background ambient noise
levels for all the wind conditions in which the wind turbine will be operating.
Sound propagation is a function of the source sound characteristics (direction and
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