Wind Turbine Acoustics - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 443

Directivity Considerations for a Wind Power Station
Although individual turbines have been treated as if they radiate sound equally in all
directions, an array of such sources may not have uniform directivity characteristics. Figure
7-29 compares the predicted sound pressure levels for two array configurations as received
from two different directions. Calculations are presented for a receiver located downwind
on the line of symmetry perpendicular to the rows and for a receiver located crosswind on
the line of symmetry parallel to the rows. For the case of one row of turbines, the cross-
wind sound pressure level is predicted to be about 5 dB lower than the downwind level near
the turbines, and only about 2 dB lower in the far field. For an array with eight rows, the
crosswind sound pressure level is only 3 dB lower near the turbines, and there is little
directivity once the receiver distance exceeds 300 m. Downwind levels are higher close to
the eight-row array, because the turbine spacing in the row is less than the row spacing.
Figure 7-29. Calculated noise propagation downwind and crosswind of single and
multiple rows of wind turbines in the representative wind power station. ( a = 0.54
dB/100 m)[Shepherd and Hubbard 1986]
Estimates of complete contours of sound pressure level around a wind power station
are shown in Figure 7-30. The array geometry in this case consists of five rows of 31
machines each, spaced as shown in Figure 7-25. This gives an approximately square array.
Figure 7-30 shows predicted contours for sound pressure levels of 40, 50, and 60 dB for an
atmospheric absorption rate of 0.54 dB/100 m (which corresponds to a frequency of 1000
Hz, at 20°C and 70 percent relative humidity). Assuming a hub-height wind speed of 9 m/s,
the distances to contours in the upwind direction are greatly reduced. These upwind
contours are derived from computed distances to the acoustic shadow zone and the extra
attenuation that occurs within this zone (see Figure 7-21).
An acoustic shadow zone forming upwind of the array results in greatly reduced
distances to particular noise level contours ( i.e. , greatly reduced noise propagation) for all
frequencies above about 60 Hz. The dashed curve in Figure 7-30 shows the location of the
40-dB contour in the absence of a shadow zone.