Wind turbine acoustics - Wind Power Generation and Wind Turbine Design - page 179

Figure 20: Modifi ed source height due to curved sound path (after [4]).

6.4.3 Ray-tracing models
In order to account for complex landscapes and the meteorological effects, more
advanced methods were needed. The ray-tracing models describe the propagation of
the acoustic waves by the path along which the waves propagate (rays). For spherical
spreading (no solid objects, no temperature gradients and no wind) the rays will be
straight lines emerging from the source. When solid objects are present the rays will
be refl ected, while the presence of wind or temperature gradients will curve the rays.
The DELTA model (cited in [4]) has two steps. First, a modifi ed source height
is calculated based on the arrival angle of the ray, in this way accounting for ray
curvature (see Fig. 20):

hhd q ′ =+ cos (39)

s s

The receiver height is not corrected, because it is much smaller than the source
height and the error which is introduced by neglecting the correction is minimal.
In the second step the noise propagation is computed as if no atmospheric effects
would be present.
Recently, Prospathopoulos and Voutsinas [33] used a more advanced ray-tracing
model to compute the sound propagation even from wind turbine parks. In their
model the noise source is assumed to be located at the rotor center. The propaga-
tion of the noise is computed in three steps:

1. fi nding the eigenrays

2. calculating the energy losses along the eigenrays

3. reconstructing the sound pressure level by summing up the contribution of the

eigenrays

When computing the energy losses, both the atmospheric and ground absorp-
tion, wave refraction and diffraction and atmospheric turbulence effects have been
taken into account. Good agreements have been obtained with experimental
measurements for several testcases.