Wind Turbine Acoustics - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 419

Blade Broadband Noise
Broadband noise arises as the rotating blades interact with the wind inflow to the rotor.
It is a significant component for all configurations of rotors, regardless of whether the low-
frequency impulsive components are present. Broadband noise components are character-
ized by a continuous distribution of sound pressure with frequency, and they dominate a
typical wind turbine acoustic spectrum at frequencies above about 100 Hz.
Example broadband-noise radiation patterns for a large-scale HAWT are shown in
Figure 7-7. Data are included for one-third-octave bands with center frequencies of 100,
200, and 400 Hz. The band levels in the upwind and downwind directions are comparable
but generally higher than those in the crosswind direction. The general shapes of these
patterns are similar to those in Figure 7-6 for the low-frequency, rotational noise
components during the daytime.
Figure 7-7. Example radiation patterns for broadband noise 200 m from a large-scale
HAWT. (one-third-octave bands, wind speed = 12.1 m/s, power = 2050 kW) [Shepherd,
Willshire, and Hubbard 1988]
The one-third-octave band spectra of Figure 7-8 were obtained for wind speeds varying
by a factor of two. At lower frequencies, dominated by the rotational harmonics, the
highest levels are shown to be associated with the highest wind speeds and the highest
power outputs. At higher frequencies, dominated by broadband components, there is no
clear trend in relation to wind speed. This result is in contrast to a scaling law given in
Sutherland, Mantey, and Brown [1987], in which A-weighted sound pressure levels increase
in proportion to the logarithm of the wind speed, and this contrast is verified by data from
a group of several small wind turbines.