Wind turbine noise measurements and abatement methods - Wind Power Generation and Wind Turbine Design - page 644

the blade is in operation the observer receives variable acoustic signals. Also the
sound component in the direction of the observer varies with time and a sound
wave is generated. Normally, the fl ow through the blades is distorted (non-
uniform) and therefore the angle of attack of each blade varies continuously as the
turbine rotates causing the sound generation to be highly directional and more
frequent. This change in the angle of attack can be very abrupt especially when
velocity discontinuities occur in the infl ow profi le resulting in rapid changes in the
blade loading, fl ow disturbance and therefore generation of acoustic waves.
Such type of acoustic waves can be produced when the mounting tower of the
wind turbine interferes with the fl ow passing through the wind turbine blades. This
causes local velocity instabilities in the fl ow fi eld which moves through the blades
producing pulsing low frequency noise. A downwind design is shown in Fig. 2a.
It becomes self-explanatory that when the reverse order occurs, the disturbances
ease as the blades encounter only the free fi eld fl ow disturbances. An upwind
turbine is shown in Fig. 2b.
The noise generated by downwind designs has low frequency in the range of
20-100 Hz and it is caused when the turbine blade encounters localised fl ow defi -
ciencies due to the fl ow around a tower. It can also be impulsive described by short
acoustic impulses or thumping sounds that vary in amplitude with time. It is caused
by the interaction of wind turbine blades with disturbed air fl ow around the tower
of a downwind machine.

2.2 Infrasound

A special category of the tonal noise released by wind turbines is infrasound. As we
have already mentioned while low frequency ranges at the bottom of human perception
(10-200 Hz), the infrasound is below the common limit of human perception. Sound
with frequency below 20 Hz is generally considered infrasound, even though there
may be some human perception in that range. A distinctive characteristic of infrasound
is that it can travel very far because of its long wavelength that dissipates with a low
rate and therefore it makes it easier to 'survive' and be present in our everyday life.

(a) (b)

Figure 2: (a) Downwind turbine; (b) upwind turbine.