Cooper has presented three papers at the ICA (International Congress on Acoustics, based on practical field work using real wind turbine noise, and also self-funded. Cooper is pursuing the peculiarities of wind turbine “noise,” and real time experiences of people living near wind turbine "factories". Cooper has used double-blind studies on the audibility of wind turbine noise.
Source of all the quotes below (Part 2 of a complicated article):
Part 1 of the same article::
“The description of wind turbine noise ... should be pulsations at an infrasound rate.
Modulation of the entire signal at an infrasound rate.
Amplitude modulation (AM) is present as some discrete low frequencies modulated at the blade pass (the tower) frequency (bpf).
The language should be pulsations at an infrasound rate with modulation of the entire signal at an infrasound rate (as in sensation detected by the ear).
Amplitude modulation is very important.
Because what people are calling AM ... is incorrect.
An electrical engineer will tell you that AM is the modulation (variation in the amplitude) of a carrier frequency (being a high frequency) that is modulated at a lower rate.
Turbines exhibit a tone at discrete frequencies of between 25 and 32 Hz (dependent upon the turbine model).
In the ones I have measured it relates to the speed of the shaft that drives the generator.
" ... the blade pass frequency (“bpf”) ... is in the infrasound region."
Different wind speeds and different blade angles will give different levels of variation in the amplitude of that frequency.
The pulsations should be there all the time the blades are turning.
A narrow band analysis of the frequencies around this gearbox output shaft speed will show sidebands spaced at multiples of the blade pass frequency.
That is classic AM.
In the Cape Bridgewater study when the turbines are producing power there is a peak around 31.5 Hz.
At Capital WF it is 25.5 Hz (different turbines).
If you take the swish noise in the regions of 800Hz – 2kHz there are no discrete frequencies and therefore that noise cannot be AM.
Leventhal (in the UK) provided a report in 2004 (for DEFRA) that cited work by Bradley (in 1994) on modulation of low frequency broad band noise at an infrasound rate to significantly increase the annoyance of the broad band noise.
But it seems to have been ignored.
Leventhal has stated in his evidence in Australia (a long time ago) that in relation to annoyance of wind farms, it is not infrasound but modulation of low frequency.
Being an electrical engineer first permits me to understand AM and filter theory.
A number of years ago we developed a method to show the variation in the wind turbine signal over time that showed the pulsations, amplitude modulation and frequency modulation.
It helped acousticians understand the time varying nature of the signal."