Wind turbine acoustics - Wind Power Generation and Wind Turbine Design - page 166

Figure 12: Acoustic parabola used for noise measurements.
Figure 13: Proximity microphone used for noise measurements.
angle of incidence, it amplifi es more the noise coming from the direction where
the parabola is aimed to.
In this way noise coming from a certain part of the wind turbine can be local-
ized. If one is interested in the noise coming from a certain part of the rotating
blades (e.g. tip region), the measured signal has to be windowed to post process
only the data corresponding to the passage of the blade over the measurement spot.
The amplifi cation depends also on the wavelength of the acoustic waves, large
wavelengths compared to the size of the parabola reduces the spatial accuracy.
5.1.3 Proximity microphone
Ground boards and acoustic parabolas are used far from the wind turbine. Prox-
imity microphones (see Fig. 13) are installed close to the noise sources, either on
the blades themselves or close to the blades on fi xed devices. The disadvantage
of mounting the proximity microphones directly on the blades is that it will be
exposed to higher speeds of the air fl ow resulting in higher wind-induced noise.
If the microphone is not co-rotating with the blade a time-windowing technique
has to be used to collect data only when the noise sources are in the vicinity of the