Wind turbine acoustics - Wind Power Generation and Wind Turbine Design - page 162

Figure 9: Diffraction behind obstacles high-frequency (left) and low-frequency
(right) waves.

rise to noise (Fig. 8f). One commonly divides the noise due to atmospheric
turbulence into a low-frequency regime (when the length-scale of the fl uctuations
is much larger than the size of the body) and a high frequency regime (the length-
scale of the fl uctuations is much smaller then the airfoil). As it will be shown in
Section 6.2, these two regimes can be treated separately in noise modeling [4].
Due to the chaotic behavior of turbulence, these models are not deterministic.
According to Lowson [9], the self-noise sources dominate at low wind speeds,
near cut-in, while at the rated power the turbulence inlet noise source dominates.
A detailed description of all these noise mechanisms can be found, e.g. in [4].

4.2 Propagation

In the previous section the noise generation mechanisms have been discussed. The
knowledge of these noise sources , however, is not enough to predict the sound
pressure level at a receiver. While the acoustic waves are traveling through the
atmosphere, several factors infl uence the propagated sound pressure levels, the
most important ones being the followings:

• The distance to the receiver. For increasing distance the acoustic energy is

spread in a larger volume which decreases the sound pressure level.

• Absorption is due to the air viscosity and converts the acoustic energy into heat.

• Refl ections due to the ground and surrounding objects.

When a wave passes around a solid object • diffraction occurs. For high frequen-

cies (wavelength much smaller than the object size) a shadow zone occurs
behind the object. The shadow zone decreases with decreasing frequency, com-
pletely disappearing for wavelengths much larger than the size of the object
(see Fig. 9).

• Refractions are caused due to temperature gradients which cause different

densities in different layers of the air, and as a consequence impose different
propagation speeds of the sound waves.
The

• wind speed and direction infl uences the directivity of the noise propagation.