Wind turbine acoustics - Wind Power Generation and Wind Turbine Design - page 160

Figure 7: Typical frequency spectra from a wind turbine (after [5]).

Figure 7 shows typical 1/3 octave band spectra for a small 75 kW wind turbine
measured by Ohlrich (cited in [5]). One can observe that the aerodynamic noise
due to the rotor blades is dominating in almost all frequency bands, except around
1 kHz where the mechanical noise originating from the nacelle prevails. Also, the
mechanical noise spectra from the nacelle and tower is more tonal, while the rotor
noise is 'smoother,' it has a more broadband character.
As a conclusion, there are effi cient ways to reduce the mechanical noise. Since
mechanical noise is not increasing that fast with increasing turbine size like the
aerodynamic noise [6], the current research is mainly focused on the reduction of
the aerodynamic component of the wind turbine noise.

4.1.2 Aerodynamic noise
Early theoretical studies of wind turbine noise were based on analogies with acoustic
studies of semi-infi nite half-planes which were symbolizing elements of the rotor
blade [7, 8]. Later the studies were based on results from non-rotating airfoil noise.
Lowson [9] divided the wind turbine noise sources in the following categories:

• discrete frequency noise at the blade passing frequency and its harmonics

• self-induced noise sources

- trailing edge noise
- separation-stall noise
- tip vortex formation noise
- boundary layer vortex shedding noise
- trailing edge bluntness vortex shedding noise
noise due to turbulent infl ow •

The fi rst group of noise sources contains low-frequency components due to the
uneven loading of the blades due to wakes, large-scale structures or velocity
gradients in the atmospheric boundary layer.