Introduction to Wind Turbine Technology - Small Wind Turbines: Analysis, Design, and Application - page 7

10 -12 Watts. It is used, in combination with an equation for the propagation of the
sound, to determine the noise level at any point around the turbine or turbines, e.g.
Wagner et al. [ 7 ]. The most common ''spreading equation'' is

a

p

h 2 þ d 2

p

h 2 þ d 2 L A ¼ L P 20 log 10 8 ð 1 : 6 Þ

which is Eq. 10.4.9 of Manwell et al. [ 8 ]. It gives the noise level on the ground at
distance d from a turbine with a tower of height h. The second term is the
hemispherical spreading term which is strictly valid only when h d and the
ground is flat. The third term represents the atmospheric absorption of sound with
the coefficient a typically in the range 0.002-0.005 dB/m. For small wind turbine
purposes, this term is usually negligible. An excel spreadsheet that implements
Eqs. 1.5 and 1.6 can be downloaded from online materials for this text by starting
at: http://extras.springer.com . Useful guides to turbine noise levels and signifi-
cance can be found at the web sites listed at the end of the chapter.
Migliore et al. [ 9 ] measured the noise output from a number of commercial
small turbines. The results are too scattered to attempt to correlate in the simple
terms of ( 1.2 and 1.3 ). For example, they found the Bergey XL1 1 kW turbine
produced so little noise that it was not possible to measure it accurately; a situation
in accord to the author's experience with a 5 kW turbine similar to that shown in
Fig. 1.3 . On the other hand, the 900 W Air 403, whose blades flex to unload the
turbine in high winds, had a correspondingly high noise level during the resulting
flutter.

Fig. 1.3 Vestas 2 MW V80
wind turbines