Wind Turbine Acoustics - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 458

Sound Power Levels of Small-Scale Wind Turbines
Sound power levels of a variety of 3-bladed small-scale wind turbines were measured
by Migliore et al. [2003]. Rated powers of these turbines ranged from 1.0 to 100 kW, with
rotor diameters of 2.1 to 19 m. A graphical summary of the results of this series of acous-
tic tests is shown in Figure 7-43. The authors refer to these power levels as “apparent”,
because they are not measured directly but inferred through the use of Equation (7-12).
The constant a in this equation was set equal to 4, and the constant b was taken equal to
-6 dB, because sound pressure measurements were made with microphones mounted on a
ground board. The atmospheric absorption rate a was assumed to be zero.
Figure 7-43. Apparent sound power levels of several small-scale, 3-bladed wind
turbines with at least 6 dB(A) separation from background noise. [Migliore et al. 2003]
It can be seen from Figure 7-43 that sound power levels from these small-scale wind
turbines were found to increase with increasing wind speed at an average rate of about 1 dB
per meter per second. Sound power also increased significantly with the rated power of the
turbines. All else being equal, one would expect a ten-fold increase in rated power to result
in an approximately 10 dB increase in sound power. The range of the rated powers in the
figure (1-100 kW) indicates an expected 20dB range in sound power, which is
approximately consistent with the measured levels.