Wind Turbine Acoustics - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 436

Figure 7-23 illustrates the special case of propagation of low-frequency rotational-
harmonics when the atmospheric absorption and extra attenuation in the shadow zone are
very small. Measured sound pressure levels are shown as a function of distance for both
the upwind and downwind directions. For comparisons, the curves representing decay rates
of -6 dB and -3 dB per doubling of distances are also included. Note that in the upwind
case the sound pressure levels tend to follow a decay rate of -6 dB per doubling of distance,
which is equal to the rate of spherical spreading. No extra attenuation from a shadow zone
was measured.
In the downwind direction, the sound pressure levels tend to follow a decay rate of -3
dB per doubling of distance, similar to that for cylindrical spreading. This reduced decay
rate in the downwind direction at very low frequencies is believed to result from atmo-
spheric refraction which introduces a channeling sound path in the lower portions of the
earth's boundary layer [Willshire and Zorumski 1987, Thomson 1982, Hawkins 1987].
Figure 7-23. Measured effect of wind on the propagation of low-frequency rotational
harmonic noise from a large-scale HAWT. (Harmonics with frequencies from 8 to 16 Hz,
rotor diameter = 78.2 m) [Willshire and Zorumski 1987]
Terrain Effects
Terrain effects include ground absorption, reflection, and diffraction. Furthermore,
terrain features may cause complex wind gradients, which can dominate noise propagation
to large distances [Kelley et al. 1985, Thompson 1982]. Wind turbines are generally
located in areas devoid of trees and other large vegetation. Instead, ground cover usually
consists of grass, sagebrush, plants, and low shrubs, which are minor impediments to noise
propagation except at very high frequencies. At frequencies below about 1,000 Hz, the
ground attenuation is essentially zero.