Wind turbine noise measurements and abatement methods - Wind Power Generation and Wind Turbine Design - page 646

Figure 4 : Broadband noise time spectrum.
Figure 5: Tonal and broadband frequency spectrum.
Generally the mechanical sound is low frequency sound although it might have
a broadband component that comes from the relative motion from each of the
above parts. The turbine's metal parts come in contact with each other, such as the
generator, the gearbox and the shafts and they emit noise and as they vibrate.
Because wind turbines can have different constructions they might have differ-
ent sound emissions because of the way in which they operate. For instance they
may have blades which are rigidly attached to the hub or may have blades that can