Wind Turbine Acoustics - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 438

vary ± 45 deg in azimuth angle during normal operations. Therefore, one of the simplifying
assumptions made in the calculations that follow is that each individual machine behaves
like an omnidirectional source.
Considerations for Frequency Weighting
A-weighted sound pressure levels, expressed in dB(A), are in widespread use for eval-
uating the effects of noise on communities [Pearsons and Bennett 1974]. This particular
weighting emphasizes the higher frequencies and de-emphasizes the lower frequencies,
according to the sensitivity of the human ear. Figure 7-24 shows the results of applying
this descriptor. The solid line is the assumed single-turbine reference spectrum, at a dis-
tance of 30 m from the machine, and the upper dashed curve is the equivalent A-weighted
spectrum at the same distance. As distances increase, atmospheric absorption causes the
levels of the higher-frequency components to decay faster than those of the lower
frequencies (see Figure 7-19).
Figure 7-24. Reference and A-weighted noise spectra from a 15-m-diameter HAWT
with a rated power of 100 kW. Assumed for example calculations of noise from a wind
power station
The result of the combination of A-weighting and atmospheric absorption is that the
midrange frequencies (100 to 1000 Hz) tend to dominate the audible spectrum at long
distances. Frequencies higher than 1000 Hz will generally not be important considerations
at long distances because of the effects of atmospheric absorption. Frequency components
below about 100 Hz may not be significant in terms of audible noise, but they can be
significant in terms of such indirect effects as noise-induced building vibrations.
Representative Wind Power Station
The basic geometric arrangement of wind turbines shown in Figure 7-25 is assumed
to represent an example wind power station. The station consists of four rows, with 31
100-kW, 15 m-diameter turbines per row. The spacing between turbines is 30 m, the row
length is 900 m, and the spacing between rows is 200 m. This basic four-row configura-