Wind turbine acoustics - Wind Power Generation and Wind Turbine Design - page 165

5 Wind turbine noise measurements
There are two main categories of measurements related to wind turbine acoustics:
on-site and wind-tunnel measurements. The fi rst group gives more realistic data
since it is applied for a whole wind turbine and gives directly the noise generated
by it. The major drawbacks of the on-site measurements are the diffi culty to install
the instrumentation and the impossibility of prescribing desired fl ow conditions.
This leads also to a limited repeatability of the measurements. Wind-tunnel mea-
surements offer more control, but they have a limited size. As a consequence,
the models have to be scaled down and sometimes only parts of the wind turbine
can be studied. The scaling of the models is not straightforward, not all the non-
dimensional parameters can be preserved. Also, the background noise from the
wind tunnel is diffi cult to subtract from the measurements. A good review of the
measurement methods applicable for wind turbine noise is presented in [4].
5.1 On-site measurements
5.1.1 Ground board
The ground board consists of a fl at, acoustically hard plate with a microphone
mounted horizontally, at the center of the plate [4]. The microphone has to be directed
towards the turbine tower and is covered by a wind screen (see Fig. 11). The major
advantage of using a ground board is that it diminishes the wind-induced noise and
that the refl ections from the ground are independent from the site. To remove the
effect of refl ections 6 dB is subtracted from the measured spectra at all frequencies.
5.1.2 Acoustic parabola
Acoustic parabolas (see Fig. 12) refl ect and focus the acoustic waves in their focal
point where a microphone registers the signal. The amplifi cation depends on the
Figure 11: Ground board used for noise measurements (after [ 4 ]).