Wind turbine acoustics - Wind Power Generation and Wind Turbine Design - page 158

Figure 4: Illustration of sound propagation.
sound as acceptable or annoying. Details about these parameters are given in
Section 4.3.
4.1 Generation
There are two main categories of wind turbine noise sources: mechanical and
aerodynamic.
4.1.1 Mechanical noise
The mechanical noise is generated by the machinery in the nacelle. The main noise
source is the gearbox, but important contributions are coming from the generator
also [5]. The cooling fans and the other auxiliary devices contribute to a lower
extent to the total noise.
Depending on the transmission path, the mechanically generated noise can be
divided in two main categories (see Fig. 5). When the noise is directly radiated
in the atmosphere it is called air-borne , like the noise emitted by the gearbox
through the nacelle openings. Another part of the mechanical noise is due to
vibrations propagated through the transmission elements and fi ttings to the
nacelle casing and tower leg. This indirect noise is called structure-borne . The
structure-borne gearbox noise constitutes the main contribution to the mechanical
noise [ 5 ].
The noise and vibrations in the gearbox are due to the transmission error of the
meshing gears. The transmission error is the difference between the desired and
the actual position of the driven gear. Thus a more accurate production of the gear-
box leads to lower noise levels. There are also differences among different gear
types, helical teeth are less noisy than the spur ones. Also, epicyclic gearboxes are
usually noisier, one of the gears being directly mounted on the casing. Due to the
wear of the gears the amplitude of the generated vibrations increases with time.
A doubling of the transmission error leads to an increase of approximately 6 dB in
the noise level.