Wind Turbine Acoustics - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 446

Table 7-1. Estimated Community Response to Noise (ISO 1971)

Estimated community response

Amount by which
received noise exceeds
threshold level
(dB) Category Description

0 None No observed reaction

5 Little Sporadic complaints

10 Medium Widespread complaints

15 Strong Threats of community action

20 Very Strong Vigorous community action

Perception of Noise Outside Buildings

Evaluating people's responses to wind turbine noise outside buildings involves the
physical characteristics of the noise of the machines, the pertinent atmospheric phenome-
na, and the ambient or outdoor background noise at the receiver's location. Both broadband
and narrow-band noise components must be considered if they are present in the noise
spectrum.
In Figure 7-33, a one-third-octave band spectrum of broadband wind turbine noise is
compared with a one-third-octave band spectrum of the typical background noise in a resi-
dential neighborhood. In this case, the background noise is a combination of noises from
numerous distant sources, with no dominant specific source. Wind effects are also absent.
Note that the turbine noise levels are generally lower than the background noise levels,
except at 1000 Hz, where they are about equal. In the laboratory, human subjects exposed
to the spectra of Figure 7-33 can just perceive the wind turbine noise. High-frequency wind
turbine noise is generally not perceived in laboratory tests when the turbine's one-third-
octave band levels are below the corresponding levels of background noise (which, in this
case, had small temporal fluctuations).
The same general findings apply to the perception of low-frequency impulsive noise.
A series of laboratory tests [Stephens et al. 1982; Shepherd 1985] were conducted to
determine the detection thresholds of impulsive wind turbine noises in the presence
of ambient noise with a spectral shape similar to that in Figure 7-33. In contrast to the
relatively simple detection model for higher-frequency noises, understanding the perception
of low-frequency impulsive noise requires that a full account be taken of the blade passage
frequency of the wind turbine, the ambient noise spectrum, and the absolute hearing thres-
hold. The latter is important because the human ear is relatively insensitive to the low freq-
uencies that characterize impulsive wind turbine noise.
In addition to laboratory tests with sample spectra, field tests can be used to determine
thresholds of perception around wind turbines, including directivity effects. For example,
aural (hearing) detectability contours were determined for two large-scale HAWTs surroun-
ded by flat terrain. The results are shown in Figure 7-34.