November 6, 2019
By Sherri Lange-Master Resource
“The description of wind turbine noise needs a terminological shift. 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).” [Cooper, below]
Q: You referred earlier to your second paper at the ICA that is linked to the synthesis paper. Maybe my question on ILFN ties in with the paper on Amplitude Modulation that raises questions as to terminology?
Cooper: Yes. The second paper on amplitude modulation is very important. Because what people are calling AM (by reference to the dBA signal) 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. The dBA is not a single frequency.
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 gearbox is subject to changes in loading as the rotors rotate that must be transferred to the generator. The change in loading occurs at the blade pass frequency (“bpf”) – which is in the infrasound region.
If you take the definition of AM, then for a wind turbine having a carrier frequency of say the gearbox output shaft (a low audible frequency) then that signal can have variations in time at the bpf. 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.
The loading on the gearbox will change as the rotor runs around and these pulses will get transferred through the gearbox. (Whether it is the blade passing the tower or the change in wind loading along the blade to create a specific point where there is equal pressure along the blade will still give pulses).
So the gearbox output shaft speed is the carrier frequency and the modulation is the blade pass frequency. 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). FFT (Fast Fourier Transform) for both turbines show the gearbox output frequency with sidebands. That is AM as defined.
Q: So you say that turbines do produce Amplitude Modulation (AM) but it is only related to a low frequency, but the modulation occurs at an infrasound rate?
Q: But isn’t this ILFN? (Infra and Low Frequency Noise)
Cooper: No, it is LF being modulated at an infrasound rate so it is not infrasound as a sound.
Q: But the swish noise is not low frequency, is it?
Cooper: Correct. And this leads to the next point. The modulation of the dBA value cannot be AM because it is a broad band noise. Hence this is the issue of terminology.
If you take the swish noise in the regions of 800Hz – 2kHz there are no discrete frequencies and therefore that noise cannot be AM. You can use 1/3 octave bands to see the modulation of the amplitude, modulated at the bpf. But not AM (Amplitude Modification) by definition.
The modulation of the amplitude for the remaining frequencies in the audible range of the acoustic signature (such as the swish or thump) is not AM. This modulation is defined by Zwicker and Fastl (Psychoacoustics: Facts and Models) as “fluctuation” because the modulation rate is less than 10 Hz. Zwicker and Fastl say people sense the modulation by the hearing mechanism, but may not hear it.
Q: You are saying that in Psychoacoustics: Facts and Models, Professors Zwicker and Fastl identified “fluctuation” as the modulation of a sound where the modulation occurs at an infrasound rate – but specifically below 10 Hz?
Cooper: Yes. They show that the ear has a particular sensitivity to the rate of modulation around 4 Hz and that the modulation has an excellent correlation between speech and hearing system.
Q: So acousticians have had knowledge of modulation for a while? Does this modulation cause a greater degree of annoyance?
Cooper: Correct. 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.
Q: Doesn’t this tie in with Rand and Ambrose’s work at Falmouth, where the issue was the pressure pulsations occurring at an infrasound rate?
Cooper: Yes and this ties in with Prof Salt’s work on the inner ear. My pulsation analysis concept came after Prof Salt’s work using pure tones. So can Prof Salt’s work show the mechanism where the ear can respond to the fluctuations – being modulation at an infrasound rate.
Q: In the UK there has been a discussion on excessive AM giving rise to a greater level of disturbance, but it uses the dB(A) value which as discussed above is not really AM?
Cooper: Right. What they have used is the modulation of the dB(A) level at an infrasound rate. It is simply incorrect terminology. Being an electrical engineer first permits me to understand AM and filter theory.
Q: Your AM paper discusses a modification of one format for deriving AM used in the UK and the Modulation Index.
Cooper: Yes. 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. In the synthesis paper there is a link back to our website that has examples of narrow band and 1/3 octave band “movies”.
The UK method is time consuming in determining the modulation index.
We used the underlying analysis concept for our “movies” to determine the statistical analysis in 1/3 octaves for multiple 10 minute samples of wind turbine noise and derive the modulation index of the A-weighted peaks in the spectrum by taking the L1 level minus the L90 level (to agree with parameters used in Australia). The results agree with the labour-intensive method of manually determining the Modulation Index using multiple sets of 10 second samples.
I presented the Modulation Index information for the various wind turbine noise samples (and non-wind turbine samples) in the brainwave paper. The third paper on the brainwave monitoring is just a pilot study but identified the impact on the frontal lobes.
Q: Yes: the third paper is different, and whilst only a single person test pilot study it would seem to show the automatic response of the brain to the presence of inaudible sound.
Cooper: Yes. The paper can only discuss the acoustic content of the testing. It was the next step from the work presented in ASA (American Society of Acoustics) in New Orleans and Euronoise and the brain wave results presented to the audience are not in the power point for public viewing as they are the results from the psychologist who undertook the measurement and analysis.
Q: Well, there is a lot to digest in your three papers given at the ICA (International Congress on Acoustics) and I see that we have to consider a change in terminology as more is learned about wind turbine noise. I see that your work is practical field work using real wind turbine noise and you are still undertaking this work with no funding. Could you summarise the key technical points from your latest work?
Cooper: There are other researchers working at the ‘coal face’ in getting to the bottom of the wind turbine noise issue and their work is important and blends in with my investigations.
I suggest that with respect to the description of wind turbine noise it is a matter of terminology that needs a shift as follows:
- The language should be pulsations at an infrasound rate.
- Modulation of the entire signal at an infrasound rate. (Zwicker and Fastl call this fluctuation as a sensation detected by the ear).
- AM is present as some discrete low frequencies modulated at the bpf.
- UK method of AM is determining the modulation index of the fluctuating signal (not technically AM).
- You can determine the Modulation Index of the low frequency noise that is AM.
As a result of work I conducted in 2013 as to detected by residents of the operation of the turbines when the FFT (Fast Fourier Transform) levels in the region of 4 – 6 Hz were above 50 dB that led to the re-discovery of Kelley’s work of the 1980’s, one can expand Zwicker and Fastl’s work, and Bradley’s work, to determine the annoyance adjustment for wind turbine noise. That is one of the next projects I am looking at.
We sincerely thank Mr. Steven Cooper for again explaining not only his technical work, but also for his diligence in pursuing the in-depth nature of the complexities of the peculiarities of wind turbine “noise,” and relating those to actual real time experiences of people living near wind turbine factories.
Above all, noted is the volunteerism of Mr. Cooper’s ongoing research. We emphasize that Mr. Cooper’s re-definitions and research augment the measurement of ILFN (Infra and Low Frequency Noise) and do not in any manner diminish the validity of the general discussion in relation to impacts. The shift in identifying the correct terminology and definition of amplitude modulation and fluctuation complements the work of other notable acousticians and researchers (as did Cooper’s identification of sensation and his double-blind studies on inaudible wind turbine noise).
The impacts are repeatedly proven to seriously impact persons, and animals. Cooper’s recent articles reinforce the ground breaking work of Zwicker and Fastl (that has been forgotten/ignored by the wind industry) and that of Kelley (that has also been ignored by the wind industry until Rand, Ambrose, James and Cooper “rediscovered” the Kelley work in 2014).
Credit: Sherri Lange- Master Resource
‘Sensing but not Hearing’: Latest from Steven Cooper on Wind Turbine Nuisance (Part 2)