Notice the swirls (the black spots) due to localized aerodynamics dumped off the blades, like what you get of the end of a paddle worked in water. Those swirls by themselves aren't the infrasound acoustic waves (pressure pulsations) traveling away from the turbine at the speed of sound. The swirls are slow-moving in comparison.
Pressure pulsations are emitted every time a blade passes in front of the tower. The blade passing frequency and its harmonics are in the infrasound range.
Let's take an example: at the maximum rotational speed of 15 revolutions per minute, the blade passing frequency is:
15 rpm × 3 blades = 45 passings per minute
per second, it is: 45 / 60 = 0.75 passing per second
The frequency in Hertz is equal to the number of passings per second: f = 0.75 Hz.
Harmonics are integer multiples of the fundamental f:
|0.75 Hz||1.5 Hz||2.25 Hz||3 Hz||3.75 Hz||4.5 Hz||5.25 Hz||6 Hz||6.75 Hz||7.5 Hz||etc.|
⇒ All these frequencies are indeed in the infrasound range (frequencies < 20 Hz).
For years, people have been told that infrasound you cannot hear cannot affect you. This is completely WRONG.
As the inner ear DOES respond to infrasound at levels that are not heard, people living near wind turbines are being put at risk by infrasound effects on the body that no-one presently understands.
Until a scientific understanding of this issue is established we should not be dismissing these effects, but need to be erring on the side of caution.”
Alec N. Salt, Ph.D. — Department of Otolaryngology - Washington University - School of Medicine - St. Louis, MO 63110