PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

The use of synthesised or actual wind turbine noise for subjective
evaluation purposes
Steven COOPER1
1

The Acoustic Group, Australia

ABSTRACT
There are technical difficulties in producing an accurate wind turbine noise signal for subjective
testing of the noise characteristics for different operational scenarios of wind turbines. There aredifferences in the subjective response when limiting the test signals to infrasound only versus the useof full spectra. The concept of “nocebo” effect that has been presented has relied upon the use of“synthesised wind turbine infrasound” that does not reflect the signature or pressure pulsations
observed in full-spectrum field measurements. The validity of “synthesised wind farm infrasoundsignals” that have been used in such testing is examined and compared with full -spectrum signals.
Keywords: Wind Turbine, Amplitude Modulation

1. INTRODUCTION
The acoustic signature of a wind turbine in the frequency spectrum encompasses the entire audio
spectrum and extends into the infrasound region.
However, the acoustic signature is critical to identify turbines operating under different windconditions. This is reality.
In the time domain, the overall level is subject to fluctuations occurring at the blade passfrequency, that for modern day turbines is slightly less than 1 Hz.
On a linear (unweighted) spectrum analysis the maximum acoustic energy is concentrated in the
low-frequency and sub-audible ranges.
The use of the overall dB(A) level does not account for the low frequency components and tends tobe dominated by broadband mid/high frequency noise, sometimes described as a “swishing” sound.
The modulation of the overall level of noise emitted by a wind turbine (typically described as“amplitude modulation”) can vary from inaudible amplitude modulation, to audible modulation, or
excessive amplitude modulation, and is dependent upon the loading of the turbine and the angle ofattack of the turbine blades.
Typical acoustic analysis of the noise signal from wind turbines, whether by narrow band (Fast
Fourier Transform) analysis or one third octave bands identifies in the spectrum discrete components
in the infrasound region being a function of the blade pass frequency and multiple harmonics of thatfrequency, components associated with the gearbox (that are modulated by the blade pass frequency) ,and broadband noise. Measurements of the infrasound levels undertaken by such analysis reveal levels
well below the threshold of hearing, and such levels are often dismissed as irrelevant.
The high frequency components of the turbine noise are significantly reduced inside the dwelling ,due to the attenuation of building envelopes. This results in a frequency spectrum dramaticallydifferent to that obtained in the external environment immediately outside the dwelling.
Utilising external dB(A) levels for environmental assessments of wind turbines does not address
the noise levels experienced by residents inside their dwellings.
Pioneering work undertaken by Kelly [1] in 1980 in relation to the MOD-1 turbine in North
Carolina provided a schematic representation of wind turbine noise spectrum characteri stics [2] withstructural, room and human body resonances added (see Figure 1).

1912

drnoise@acoustics.com.au

FIGURE 1: Turbine noise emission components with building and human body resonances
superimposed. Ref [2]The MOD-1 turbine was of a downwind turbine (where the blades are downwind of the truss
structure). Modern turbines are upwind turbines, where the blades are upwind of the lower support(normally a round tower).
Audible characteristics of wind turbine noise can have different spectrum shapes depending upon
the relative distance of the receiver location to the turbines. On moving further away from the turbines,
the high frequency noise attenuates. Multiple turbines associated with the wind farm park can give rise
to a general low-frequency roar that in some cases is described like the sound of an aircraft that neverlands.
Field measurements have identified the presence of discrete peaks in the infrasound region at times
when residents have accurately reported sensing the operation of the turbines without visual or audible
cues. Testing in South Australia [3] indicated that for narrow band analysis levels in the 4Hz-6Hz
region when above 50dB(linear) correlated with the detection of the turbines - even though thosefrequencies were inaudible. A similar result was obtained in the Cape Bridgewater study [4].

2. AUDIBLE ASSESSMENT
A number of studies evaluating the audible noise of wind turbines have technical limitations ingenerating a full spectrum by reason of amplifiers, speakers, sampling rates of the recording devices,
frequency bandwidth of microphones and whether one is using outdoor measurements versus indoor
measurements. The significant differences in transmission loss of building elements , potential
resonance effects of buildings, or enhancement of the internal sound levels for different constructions
does not provide a uniform transmission loss performance, in terms of subjective testing cannot be
related to noise of wind turbines obtained externally to the building.
Such external noise spectrum characteristics can be subject to masking from other sources that inturn do not relate to noise levels experienced inside dwellings. As part of the Acoustic al Society ofAmerica’s Wind Turbine Working Group, a series of papers have been presented to that group in
relation to various acoustic tests and investigations to accurately reproduce signals in a test rooms
whether restricted to the infrasound region only, full-spectrum, or full-spectrum excluding infrasound.

913

3. DIFFICULTIES IN REPRODUCING WIND TURBINE “INFRASOUND”
Large areas of speakers, high-powered D class amplifiers and the use of high sampling rates forrecorded signals is necessary to be able to reproduce transient signals of wind turbine noise [5] [6].
However, there is a question as to whether infrasound is actually generated by turbines in the same
concept as audible noises associated with gearboxes or the “swish” sound from the turning of theturbines.
If one considers for a simple analysis a constant tone of 100 Hz, then under FFT analysis one
expects a peak at 100 Hz and say 20 dB lower the second harmonic, and lower still the third harmonic,
and possibly the fourth harmonic, to appear in the spectrum because there is a mechanical device (thespeaker) used to generate that sound.
If, then one considers that 100 Hz signal is created as a tone burst to run for 900 ms , off for 100 ms,
then on for 900 ms, off 100 ms etc, then there would be a periodic function of 1 Hz occurring in thetime domain.
Undertake the FFT analysis of that signal (as a pure theoretical exercise) and one would expect apeak at 1 Hz and a peak at 100 Hz because those are the two main fundamental frequencies that are
occurring in the time domain. There may be harmonics or multiple harmonics at lower levels of those
two frequencies. However, in an acoustic sense, there is no 1 Hz sound that is generated, it is purely aperiodic function that occurs in the time domain upon which in the frequency domain there appears apeak at 1 Hz. But there is definitely a 100 Hz signal that is generated.
From the results of Walker, Hessler and Hessler, Rand and Schomer [7] with respect to the Shirley
Wind Farm (see Figure 2), there is general frequency spectra shown by high-resolution analysis of theaudio signal that identifies a typical pattern in the infrasound region. The question that we need to ask
is, is that an audio signal or is it a function of the mathematical analysis of a pulse? I leave the reader
to actually look into those components and issues of concern, to ascertain whether there is an audio
signal there or not. Hint: One could start with the analysis of a periodic DC Pulse. Then examine thetime domain to determine if BT=1 is valid [8] & [9].

Figure 2: Comparison of outdoor and indoor spectra – Shirley Wind Farm (ref 7)
If an acoustician (or other disciplines) does not understand the mathematical relationship offrequency analysis or the validity of such analysis, then a question arises as to whether they areincorrectly interpreting the data?

914

3.1 Crichton
In Crichton et al “Can Expectations Produce Symptoms From Infrasound Associated with Wind
Turbines?” [10] the title of the first paper specifically refers to wind turbine infrasound. However, the“infrasound” that was used in the experiment was 40 dB at 5 Hz, as was not wind turbine infrasound.
In Crichton et al “The Power of Positive and Negative Expectations to Influence Reported
Symptoms and Mood During Exposure to Wind Farm Sound” [11] an “infrasound” signal at 50.4 dB at
9 Hz was introduced with a recording of audible wind farm noise.
If one refers to the spectra set out in Figure 2, it is clearly the case that a constant infrasound signal
at a single frequency as used in Crichton’s two studies, is not a signal generated by a wind turbine.
Neither papers refer to a fluctuating tone as the “infrasound” signal. Accordingly, how can the title ofthose two papers be correct when wind turbine infrasound noise was not actually used in theexperiment?
Furthermore, examining the specifications of the speaker system [12] that was used in the Crichton
experiments one finds there would be some difficulty in producing a clean sine wave at thosefrequencies for the level that has been indicated as it is beyond the specifications of the speaker.
The two Crichton papers have been referenced by a number of acousticians [13] in presenting
arguments supporting wind turbine applications. In light of the above one must question whether thoseacousticians have actually read and comprehended the material in the Crichton work.
3.2 Walker
In investigating the impact of “wind turbine infrasound” Walker has utilised a synthesized signal
with a low frequency speaker system and a D class amplifier to demonstrate wind turbine infrasound[14] instead of using actual wave files
Reference 14 includes the results of LAeq FFT analysis of turbines at Cape Bridgewater (Figure 55
in reference 4) to create a waveform computed as the sum of the sine waves at the six peak spectral
amplitudes. Walker considered the wave as essentially a Ch-Wave truncated at the 6 th harmonic.

Figure 3: 1600 lines, 0 – 25 Hz FFT analysis – figure 55 from ref 4
However, the signal derived by Walker does not agree with the original time signal that was used toobtain the Leq FFT. The Leq results are the result of squaring the amplitude and then the square rootof the result loses all the phase information. Deriving a time signal from an Leq result of individual
tones could only fit in with the Walker approach if the original signals were sine waves and all had zero
phase difference.
Apart from the phase issues, another possible explanation for the difference between the actual time
signal and the Walker derived signal is that the house in question is not influenced by just a single
turbine but by multiple turbines, not in sync with one another. Furthermore, the acoustic signals
obtained at Cape Bridgewater are not restricted to just infrasound but contain frequencies in thenormal audio spectrum[15].

915

FIGURE 4: Walker synthesis of Figure 3
If one is just restricting the subjective assessment of wind turbine noise to the infrasound region(where the reported levels are significantly below the hearing threshold and the sensation threshold in
the infrasound region) then one expects there to be no reported sensation when exposed to infrasoundalone.
For subjective testing of wind turbine noise Walker [16] used data from Waterloo wind farm tocreate a synthesized signal for subjective evaluation and again found synthesised infrasound produced
no reaction. Restricting the spectrum of the turbine signal to only infrasound or not the full spectrum,
places limits on the validity of the experiment whether using synthesized signals or real wave file data.
3.3 Tonin
Tonin [17] conducted a study along the lines of Crichton but used a Walker synthesised signal
applied to headphones using a pneumatic driver. Tonin acknowledges that the signal would be
deemed to be inaudible in light of the levels being below the nominal threshold of hearing in theinfrasound region.
Tonin concludes that the infrasound had no statistically significant effect on the health symptoms
reported by the volunteers and suggested the results support the hypothesis that a nocebo effec t may be
the cause of the reported symptoms. The Tonin study did not use WAVE files of actual turbine noise
restricted to the infrasound range, but used an inaudible synthesised signal with no verification that thesignal was the same as measured in the vicinity of wind turbines. Similarly, the Tonin study did notconsider inaudible or audible low frequency noise, or mid band noise generated by wind turbines.
Tonin’s conclusion [18] presents the concept that infrasound is perceived by some people to affectthe health of people and as no effect was observed supports a hypothesis of a nocebo effect. But if thesynthesised signal is inaudible, and not actual wind turbine noise, then the results of the Tonin study
cannot apply to the full spectrum of wind turbine noise.
Tonin acknowledges that his study can only apply to the application of the “infrasound” to the ears
in an enclosed space (ear muffs) and therefore does not address other potential paths (excluding theears) that may impact upon individuals. Tonin does not discuss the possibility the wind turbine
infrasound signature (obtained from narrow band FFT analysis) is the result of the modulation of theamplitude of the low/mid frequency noise, where that modulation occurs as a series of pulses repeatedat an infrasound rate.
If the purpose of the above acoustic experiments is to evaluate the perception and annoyance ofwind turbine noise, then apart from restricting the testing to signals that may not even be there , there is916

also the issue as to the relevance of the Crichton/Walker/Tonin signal with respect to Annexure D of
ANSI/ASA S12.9-2016/Part 7 [18].
Annex D to the ANSI document “advanced signal processing techniques” provide specific advice
in Section D1:
It has been observed (Bray, Swinbanks, Walker, et al) that for complex low-frequency signals (thosecomprising multiple frequencies), the temporal relationship between the components can have asignificant influence on their subjective assessment. (Indeed, al one needs do is listen to the difference
between a gun-shot and an extended Galois sequence signal to observe the two signals with the exact
same spectrum can sound dramatically dissimilar.) There is reference to figure D1.

Figure 5: Figure D1 from ANSI/ASA S12.9-2016/Part 7
Tonin used a sawtooth signal similar to above that does not agree with the actual time signal fromwave files for the Shirley wind farm. If Tonin utilised a digitised infrasound signal which is not an
actual wind turbine signal, and produced infrasound levels that are inaudible, then the likely result ofsuch a study is that there would be no impact.
3.4 Cooper
In contrast to the approached of the three researchers above, our research has used as a starting
point the full spectrum noise wave files of actual wind turbines applied as a complete sound field. I
then investigated the subjective whole-body response to this sound reproduced in a laboratory setting.
It is difficult to comprehend from just an LAeq frequency analysis of a time signal how one canreconstruct the original time signal. In any event, why would you need to use a digitised signal when it
is much easier to use the actual wave files that are available from recordings ? This digitised concept
has always perplexed me as the instrumentation for the Shirley wind farm study went down to the low
infrasound region. If Walker’s field measurements had data recording capabilities at a high sample rate
to be able to reproduce the signal, then why couldn't those signals be used? Due to space limitations thefollowing link http://acoustics.com.au/media/ICA2019SS01.pdf provides additional material withrespect to Section 3 of this paper
How does one verify the audible characteristics of an infrasound signal? If we take the originalrecorded wave file signal and put a low pass filter to only have 0 to 20 Hz from the original signal as
signal 1, and then utilise 6 generators all set to 0 phase to reconstitute an analogue signal (below 20Hz)
as per the Walker concept (signal 2) then two wave files can be obtained. When both signals are then917

sped up 100 times we find that in terms of the sped up “infrasound” signal, they do not sound the sameand therefore would be invalid under ANSI/ASA S12.9-2016/Part 7, as a test signal for subjective
testing of infrasound.
Reference 15 identifies limitations we experienced in creating full spectrum signals in ourlaboratory.
Having examined the issues of analysis of short-term burst signals and finding that frequency
analysis of these frequency analyses can produce infrasound “signals” that are not actually present
then the issues in terms of the frequency response, timing rates and bandwidths of frequency analysis
must be looked into, to ascertain the validity of the signals. We presented the concept of ghost
frequencies that are not valid and may be considered a similar concept to the issues of digitisation
requiring anti-aliasing filters.
Just as ghost frequencies associated with vibration in gearboxes lead to further developments suchas Cepstrum analysis to reveal patterns that relate to faults in the gearbox, we proposed that the FFT ofwind turbines that give rise to infrasound spectra is just simply a tool to identify that the turbines areoperating and generating pulses.
From Hansen/Walker’s observations contained in reference 16, and the analysis identified inreference 16, our research investigations into the subjective assessment of wind turbine noise ignored
infrasound [19] [20] and utilized testing with a high pass filter to eliminate frequencies below 20 Hz.
Therefore, it became a simple task of using actual wave files of wind turbine noise which could be
simply validated.
Our double blind pilot study presented to the ASA Wind Turbine Working Group in December 2017[20] and Euronise 2018 [21], used inaudible wind turbine noise from field measurements recorded ona high sample rate data recorder to which as a double blind study of 9 persons sensitised to windturbines were able to detect with 100% accuracy the signal containing the operation of the turbines.
The two additional papers presented in this session utilise wave file recordings presented to the test
subjects as an entire sound field (not headphones) that can be validated with the field measurements as
to the accuracy in both frequency and amplitude as the source data to then be used for audible
subjective testing or inaudible subjective testing.

4

CONCLUSIONSIn the past the use of traditional acoustic analysis of wind turbine noise has revealed a time varying
noise that by way of 1/3 octave bands and narrow band analysis has revealed infrasound and low
frequency signatures, commonly referred to as ILFN (Infrasound and Low frequency Noise).
The author has never claimed there to be infrasound as the concept of a noise but has identified thepresence of the narrowband signature (blade pass frequency + harmonics) in the infrasound region.
With respect to wind turbine noise much discussion has been made in relation to wind turbine“infrasound” without qualifying the basis of what is the infrasound signal – that in any event is below
the threshold of hearing.
Subjective evaluation of pseudo/sham/synthesized wind turbine infrasound has revealed there to be
no impact leaving some researcher to claim such testing supports a nocebo effect, despite the data also
identifying the power of suggestion is important.
A fundamental problem exists with such studies as to why actual wind turbine noise (in the form ofhigh quality wave files that extend down to the infrasound region) could not be used as the test signal?
Using actual wave files, we have ascertained that there is no perceptible difference with or without
infrasound in the signal when creating an entire sound field in which the test subjects are immersed.
Reference 22 presents the results of further work with wind turbine wave files set at an inaudible
level to show that limiting the spectrum to be between 20Hz and 1kHz produces less of an impact thanusing 20Hz to 16 kHz.
Having reproduced the acoustic signature of wind turbines in a laboratory and finding changes inthe EEG of the brain now permits the assessment of wind turbine noise (inaudible and audible) to beextended by the appropriate medical studies.

REFERENCES
1.

918

Kelley N.D., McKenna H.E., Hemphill R.R., Etter C. L., Garrelts R.L and Linn N.C. Acoustic NoiseAssociated with the MOD-1 Wind Turbine: Its Source, Impact, and Control, Solar Energy ResearchInstitute, February 1985
2. Kelley N.D., Hemphill R.R. and McKenna H.E. A Methodology for Assessment of Wind Turbine
Noise Generation, Transactions of the ASME 112 Vol 104 May 1982
3. Cooper S, Sensation as an impact from wind turbines, Acoustical Society of America Meeting,Pittsburgh May 2015

4 The Acoustic Group, “The Results of an Acoustic Testing Program, Cape Bridgewater Wind Farm”,
Dec2014
http://waubrafoundation.org.au/resources/cooper-s-acoustic-group-resultscape-bridgewater-acoustic-in
vestigation/2
5 Yokoyama S, Kobayashi T, Sakamoto S & Tachibana H, Subjective experiments on the auditory
impression of the amplitude modulation sound contained in wind turbine noise, International Meetingon Wind Turbine Noise, Glasgow 2015.
6. Cooper S, Subjective perception of wind turbine noise Acoustical Society of America Meeting, Boston,June 2017, ASA POMA Vol 30/10.1121/2.0000639
7. Walker B, Hessler DM, Hessler GF, Rand R & Schomer P, “A Cooperative Measurement Survey andAnalysis of Low Frequency and Infrasound at the Shirley Wind Farm in Brown County, Wisconsin”,
Report Number 122412-1, December 2012
8. Randall RB, Frequency analysis, Bruel & Kjaer, 3rd Revisions, September 1987
9. Bray W & James R (July 2011), Dynamic measurements of wind turbine acoustic signals, employing
sound quality engineering methods considering time and frequency sensitisations of human perception,
Noise-Con 2011.
10. Crichton F, Dodd G., Schmid G, Gamble G and Petrie K.J Can Expectations Produce Symptoms
From Infrasound Associated with Wind Turbines? Health Psychology, American Psychological
Association
11. Crichton F, Dodd G, Schmid G, Gamble G, Cundy T & Petrie K, The Power of Positive and Negative
Expectations to Influence Reported Symptoms and Mood During Exposure to Wind Farm Sound?,Health Psychology, American Psychological Association 201312.. Mackoid “Mackie HRS159 High Resolution Powered Subwoofer Owner’s Manual”, available at
http://www.umlib.com/brand/Mackie/category/Subwoofer/model/HRS150/document/700807

13. Berger R G, Ashtiani P, Ollson CA, Aslund MW, McCallum LC, Leventhall G and Knopper LD,
Health-based audible noise guidelines account for infrasound and low-frequency noise produced bywind turbines. Frontiers in Public Health, 3, Article 31, February 2015
14. Walker B & Celano J “Progress Report on Synthesis of Wind Turbine Noise and Infrasound” 6thInternational Meeting on Wind Turbine Noise, Glasgow 2015.
15. Cooper S, Reproducing wind farm infrasound for subjective testing - Just how accurate is the
reproduced signal?
16. Hansen K, Walker B, Zajamsek B & Hansen C, Perception and annoyance of low frequency noise
versus infrasound in the context of wind turbine noise, International Meeting on Wind Turbine Noise,
Glasgow 2015.
17. Tonin R & Brett J, Response to Simulated Wind Farm Infrasound Including Effect of Expectation,
International Meeting on Wind Turbine Noise, Glasgow 201518. American National Standard ANSI/ASA S12.9-2016/Part7, Quantities and Procedures for Description
and Measurement of Environmental Sound, Part 7: Measurement of Low-frequency Noise andInfrasound Outdoors and in the Presence of Wind and Indoors in Occupied Spaces, April 201619 . Cooper S, Subjective perception of wind turbine noise, Acoustical Society of America Meeting, Boston,June 2017, ASA POMA Vol 30/10.1121/2.0000639
20. Cooper S, Subjective perception of wind turbine noise – The stereo approach, Acoustical Society ofAmerica Meeting, New Orleans, December 2017, ASA POMA Vol 31/10.1121/2.0000653
21. Cooper S, The Inaudible Soundscape of a Wind Farm, Euronoise 2018, Crete, May 201822 Cooper S. A Comparison of inaudible wind farm noise and the natural environment noise whilst
monitoring brainwaves and heart rates.

919