Wind Turbine Acoustics - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 460

Figure 7-44 shows a downwind view of the test turbine onto which contours of down-
ward-radiated noise levels have been projected, both from the blades and from the nacelle.
The range of power levels on these contours is 12dB. The most striking phenomenon is
that practically all downward radiated blade noise (as measured by the microphone array
on the platform in the foreground) is produced during the downward movement of the
blades. Since the range of the contours shown is 12 dB, this means that the downward-
radiated noise produced during the upward movement of a blade (on the left of the tower)
is at least 12 dB less than during its downward movement on the right of the tower.
Referring to Figure 7-14, the noise emission pattern illustrated in Figure 7-44
indicates that trailing edge noise is the leading source of broadband noise from this rotor.
Furthermore, it can be seen that the majority of the blade noise is produced by the outboard
section of the blades, but not by the very tip in this blade design. The authors note that for a
different observer location the pattern of sound emission will be different. For example, an
observer located further away would hear noise being generated over the full-blade rota-
tion, not just over one side as indicated in this figure.
A second important observation is that the noise from the blades clearly dominates
the noise from the nacelle. The difference between the overall sound pressure levels from
the nacelle and those from the blades was found to increase with increasing wind speed,
from about 8 dB(A) at 6 m/s to about 11 dB(A) at 10 m/s.
Survey of Community Response to Wind Turbine Noise
Studies of the reactions of nearby residents to the sight and sound of wind power sta-
tions have been conducted as part of the planning and zoning approval processes. One of
the most detailed is a recent study in the Netherlands sponsored by the European Union
[van den Berg et al. 2008]. The study population was selected from all residents in the Neth-
erlands living within 2.5 km from a wind turbine. As the study emphasized modern wind
power stations, wind turbines were selected with a rated power of 500 kW or more and one
or more turbines within 500 m from the first. Excluded were wind turbines that were erect-
ed or replaced in the year preceding the survey. Residents lived in the countryside with or
without a busy road close to the turbine(s), or in built-up areas (villages, towns). Excluded
were residents in mixed and industrial areas. The survey was conducted by mail.
The sound level at the residents' dwellings was calculated according to the international
ISO standard for sound propagation, the almost identical Dutch legal model, and a simple
(non-spectral) calculation model. The indicative sound level used was the sound level when
the wind turbines operated at a wind speed of 8 m/s in the daytime, at high but not maximum
power. Respondents were exposed to levels of wind turbine sound between 24 and 54 dB(A)
and wind turbines at distances from 17 m to 2.1 km. The angular elevations of the largest
wind turbines ranged from 2 degrees to 79 degrees, with an average value of 10 degrees.
Wind turbines occupied about 2 percent of the space above the horizon, on average.
Summary of the Main Conclusions of the Survey
With Respect to Hearing Wind Turbines
- Not having wind turbines visible from the dwelling and high levels of background
(road traffic) sound decreased the probability of hearing wind turbine sound, though
the influence of background sound was found to be small.