Wind Turbine Acoustics - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 431

An alternative broadband-noise prediction scheme is proposed in Glegg et al .
[1987] and includes noise from unsteady lift, unsteady thickness, trailing edges, and
separated flows. Inflow turbulence at the rotor must be specified to predict unsteady
lift and thickness noises. Using the turbulence data associated with the atmospheric
boundary layer as input yielded poor agreement between calculated and measured noise
levels. Thus, the authors hypothesized that there was an additional source of turbulence:
that each blade ran into the tip vortex shed by the preceding blade. Note that Grosveld
[1985] also used atmospheric boundary layer turbulence but found that better agreement
with acoustic measurements required an empirical turbulence model. The two approaches
share the same theoretical background and therefore should give the same results.
Noise Propagation
A knowledge of the manner in which sound propagates through the atmosphere is basic
to the process of predicting the noise fields of single and multiple machines. Although
much is known about sound propagation in the atmosphere, one of the least understood
factors is the effect of the wind. Included here are brief discussions of the effects of
distance from various types of sources, the effects of such atmospheric factors as absorption
in air and refraction caused by sound speed gradients, and terrain effects.
Distance Effects
Point Sources
When there is a nondirectional point source as well as closely-grouped, multiple point
sources, spherical spreading may be assumed in the far radiation field. Circular wave
fronts propagate in all directions from a point source, and the sound pressure levels decay
at the rate of -6 dB per doubling of distance, in the absence of atmospheric effects. The
latter decay rate is illustrated by the straight line in Figure 7-18. The dashed curves in the
figure represent increased decay rates associated with atmospheric absorption at frequencies
significant for wind turbine noise.