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Concerns about Infrasound from Wind Turbines

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  • H G Leventhall

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VOLUME 9, ISSUE 3 JULY 2013
Environmental Noise: Aircraft and Wind Turbines
Acoustics
Today
A publication of
the Acoustical Society
of America
Acoustics of Jet Noise from
Military Aircraft
Wind Turbine Noise
Infrasound from Wind Turbines
The Waveguide Invariant in
Underwater Acoustics
And more
v9i3_cvrs_p3_ads_Layout 1 8/23/13 12:44 PM Page 2
30 Acoustics Today, July 2013
Infrasound
Infrasound has been defined as:
Acoustic oscillations whose fre-
quency is below the low frequency
limit of audible sound (about 16 Hz).
(IEC 1994)
However, sound remains audible at
frequencies well below 16 Hz. For exam-
ple, measurements of hearing threshold
have been made down to 4Hz for expo-
sure in an acoustic chamber (Watanabe
and Møller 1990) and down to 1.5 Hz for
earphone listening (Yeowart et al. 1967).
The limit of 16 Hz, or more com-
monly considered as 20 Hz, arises from the lower frequency
limit for which the standardized equal loudness hearing con-
tours have been measured, not from the lower limit of hearing.
From the subjective point of view, there is no logical reason for
terminating a continuous process of hearing at an arbitrary fre-
quency, so that from about 10Hz to 100Hz could be taken as the
low frequency range. It may also be argued that there is no log-
ical reason for terminating at 100 Hz, and the range is some-
times extended to about 200Hz and down to 5Hz. However,
objectors to wind turbine developments are now requesting
that measurements are made down to below 1Hz.
Atmospheric infrasound
This is a well-established discipline, studying frequencies
from about one cycle in 1000 seconds up to, say, 2Hz and
higher (Bedard and George 2000). Atmospheric infrasounds
are caused by weather variations, turbulence, meteorites, dis-
tant explosions, ocean waves interacting (microbaroms) or
waves breaking on the shore, practically any occurrence
which puts energy into the atmosphere over a relatively short
period of time and any process with a low repetition rate. The
attenuation with distance is small and propagation can be
complex. Monitoring of atmospheric infrasound is an essen-
tial part of ensuring the success of the Nuclear Test Ban
Treaty, since explosions in the air generate infrasound, and
there are about 60 monitoring stations around the world.
Of course, it is important to realise that our evolution has
been in the presence of naturally occurring atmospheric
infrasound, which overlaps the lower end of wind turbine
infrasound.
The Apollo Space Programme
Early work on low frequency noise and its subjective
effects was stimulated by the Apollo space programme. It was
known that large launch vehicles produce their maximum
noise energy in the infrasound region. Furthermore, as the
vehicle accelerates, the crew compartment is subjected to
boundary layer turbulence noise for a
few minutes after lift-off. Experiments
were carried out in low frequency noise
chambers on short term subjective tol-
erance to bands of noise at levels of
140dB to 150dB in the range up to
100Hz (Mohr, Cole et al. 1965). It was
concluded that subjects who were expe-
rienced in noise exposure, and who
were wearing ear protection, could tol-
erate both broad¬band and discrete fre-
quency noise in the range 1Hz to 100Hz
at sound pressure levels up to 150dB.
Later work suggests that, for 24 hour
exposure, levels of 120-130dB are tolerable below 20Hz (von
Gierke 1973, von Gierke and Nixon 1976). These high, long-
term limits were set to prevent direct physiological damage.
It was not suggested that the exposure is pleasant, or subjec-
tively acceptable, for anybody except those whose occupation
requires them to be exposed to the noise.
Work was also in progress in the UK (Yeowart, Bryan et
al. 1969, Hood and Leventhall 1971) and France (Gavreau,
Condat et al. 1966, Gavreau 1968) from the 1960s and in
Japan and Scandinavia from the 1970s (Møller 1980, Yamada
1980). Japan and Scandinavia are now the main centres for
work on infrasound and low frequency noise. A review of
studies of low frequency noise has been given by Leventhall
(Leventhall, Benton et al. 2003)
Origins of the Concerns
The early American work was published from the mid-
dle 1960s and did not attract public attention, but a few years
later infrasound entered upon its mythological” phase,
echoes of which still occur, currently in relation to wind tur-
bines. The main name associated with the early phase is that
of Gavreau from CNRS Marseille, whose work was in
progress at the same time as that of the Apollo space pro-
gramme. (Gavreau, Condat et al. 1966, Gavreau 1968).
Infrasound at 7Hz from a defective industrial fan, which may
have been operating at an unstable point on its characteristic,
led to investigations of the problem and to the design of high
intensity low frequency sound sources. The frequencies and
levels reported in Gavreau (1968) were:
Frequency Hz Reported level dB
2600 Not given
340 155dB
196 160dB
37 Not given
7 Not given
CONCERNS ABOUT INFRASOUND FROM WIND TURBINES
Geoff Leventhall
150 Craddocks Ave
Ashtead, Surrey KT21 1NL
United Kingdom
“It appears that concerns over
infrasound and low frequency
noise have found a place deep
in the national psyche of a
number of countries and lie
waiting for a trigger to bring
them to the surface.
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Concerns About Infrasound from Wind Turbines 31
Gavreau made misleading statements, which led to confusion
of harmful effects of very high levels at higher frequencies
with the effects of infrasound.
For example from the 1968 paper on “Infrasound,” which
was published in a popular science journal: (Gavreau 1968):
Infrasounds are not difficult to study but they are
potentially harmful. For example one of my col-
leagues, R Levavasseur, who designed a powerful
emitter known as the ‘Levavasseur whistle’ is now a
victim of his own inventiveness. One of his larger
whistles emitting at 2600Hz had an acoustic power
of 1kW… This proved sufficient to make him a life-
long invalid.
Of course, 2600Hz is not infrasound, but the misleading
implication is that infrasound caused injury to Levavasseur.
Gavreau’s progress
Gavreau energized his sources in a laboratory, exposing
himself and his co-workers to very high levels of noise at rel-
atively high frequencies. For example at 196Hz from a pneu-
matic “whistle” and 37Hz from a larger whistle. Exposure to
the 196Hz source at a level of 160dB led to irritation of inter-
nal organs, so that Gavreau and his colleague felt unwell fol-
lowing a five minute exposure. Again from the 1968 paper:
… after the test we became aware of a painful ‘reso-
nance’ within our bodies – everything inside us
seemed to vibrate when we spoke or moved. What
had happened was that this sound at 160 decibels…
acting directly on the body produced intense fric-
tion between internal organs, resulting in severe
irritation of the nerve endings. Presumably if the
test had lasted longer than five minutes, internal
hemorrhage would have occurred.
196 Hz is not infrasound, but the unpleasant effects are
described in a paper with the title “Infrasound.
The 37Hz whistle was run at a low level, but sufficient to
cause the lightweight walls of the laboratory to vibrate. Some
of Gavreau’s earlier work had been in the development of
pneumatic high intensity ultrasonic sources, so that he mere-
ly had to scale up the size for low frequency operation.
Gavreau also generated 7Hz with a tube of length 24m,
driven by either a loudspeaker or a motor- driven piston. He
suggested that 7Hz was particularly “dangerous” because the
frequency coincided with alpha rhythms of the brain. He also
used a tube to generate 3.5Hz, but further details were not
given.
And from the 1968 paper:
The effects of low frequency sound and infrasound
are noxious. However, we found one exception: the
intense vibration of the nasal cavities produced by
our whistle (340Hz, 155 decibels) had favorable
effects! In one case, a subject recovered a sense of
smell which he had lost some years back and was
able to breathe more easily.
Infrasound and the public
By present standards, Gavreau’s work was irresponsible,
both in the manner in which it was carried out and in the
manner in which it was described. Much of the paper with
the title “Infrasound” is not about infrasound. The work was
picked up by the media and embellished further, including
claims that 7Hz is fatal.
The misunderstanding between infrasound and low fre-
quency noise continues. A newspaper article on low frequen-
cy noise from wind turbines (Miller 24 January 2004), opens
with: “Onshore wind farms are a health hazard to people liv-
ing near them because of the low-frequency noise that they
emit, according to new medical studies.
A French translation of this article for use by objector
groups opens with: “De nouvelles études médicales indiquent
que les éoliennes terrestres représentent un risque pour la santé
des gens habitant à proximité, à cause de l’émission d’infra-
sons.
The translation of low frequency noise into infrasons con-
tinues through the article. This is not a trivial misrepresenta-
tion because, following on from Gavreau, infrasound has
been connected with many misfortunes, being blamed for
problems for which some other explanation had not yet been
found (e.g., brain tumors, sudden infant deaths, road acci-
dents). A selection of some UK press headlines from the early
years is:
The Silent Sound Menaces Drivers - Daily Mirror,
19th October 1969
Does Infrasound Make Drivers Drunk? - New
Scientist, 16th March 1972
Brain Tumours ‘caused by noise’ - The Times, 29th
September 1973
Crowd Control by Light and Sound - The Guardian,
3rd October 1973
Danger in Unheard Car Sounds - The Observer, 21st
April 1974
The Silent Killer All Around Us - Evening News, 25th
May 1974
Noise is the Invisible Danger - Care on the Road
(ROSPA) August 1974
Infrasound and low frequency noise are often associated in
the public mind with the “hum.” This is mystery noise of
unknown origin, which is heard periodically by a few people,
and causes them great distress, but is difficult to measure.
The hum has been reviewed by Demming, who gives refer-
ences to North American occurrences (Demming 2004).
Demming has established a Yahoo Group Hum Forum,
<http://tech.groups.yahoo.com/group/humforum/>, to help
hum sufferers interact and support each other.
Absurd statements were made in the book Supernature
by Lyall Watson, first published in 1973 as A Natural History
of the Supernatural and which has had a number of reprints
and large sales. This book includes an extreme instance of the
incredible nonsense which has been published about infra-
sound. It states that the technician who gave the first trial
blast of Gavreau’s whistle “fell down dead on the spot.” A post
mortem showed that all his internal organs had been
mashed into an amorphous jelly by the vibrations.” It contin-
ues that, in a controlled experiment, all the windows were
broken within a half mile of the test site and further, that two
infrasonic generators “focused on a point even five miles
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32 Acoustics Today, July 2013
away produce a resonance that can
knock a building down as effectively as
a major earthquake.
One can detect a transition from
Gavreau and his colleague feeling ill
after exposure to the high level of
196Hz to “fell down dead on the spot”
and a further transition from laborato-
ry walls vibrating to “can knock a
building down,” transitions which
resulted from repeated media exagger-
ations over a period of five or six years.
The Internet
Currently, the internet is the
favored medium for information on
infrasound and wind turbines. A web
page that features the subject is
<www.windturbinesyndrome.com>.
Main themes of this page are that wind
turbine developers are greedy and
heartless money-makers and that wind
turbine infrasound causes a range of
illnesses. The writers use colorful and
forceful language. For example, as in
<http://www.windturbinesyndrome.co
m/2013/why-wind-developers-are-
sharks-mass/>. The writers are so
focused on supposed dangers of infra-
sound, which they use as a scare tactic
on residents near proposed wind
farms, that they may be led astray by
this. For example, the long range
acoustic device (LRAD) is described as
an infrasonic weapon, whereas it is
actually based on an ultrasound carri-
er: <http://www.windturbinesyn-
drome.com/2011/the-misuse-of-infra-
sound-indust r y-military-and-now-
the-cops/>. Any evidence on the pro-
duction of even low level infrasound by
wind turbines is hailed as a victory
<http://www.windturbinesyndrome.co
m/2010/wind-turbines-produce-
major-infrasound-period-no-ques-
tion-about-it/>. Another view, not
favored by the web page, is that wind
turbines produce infrasound, but it is
of negligible impact on humans
(Leventhall 2006).
Public perceptions
We cannot blame the public for
their anxiety about infrasound and low
frequency noise when they have been
exposed to statements like those
described earlier. Public concern over
infrasound was one of the stimuli for a
growth in complaints about low fre-
quency noise during the 1970s and
1980s and has continuing effects. It
appears that concerns over infrasound
and low frequency noise have found a
place deep in the national psyche of a
number of countries and lie waiting for
a trigger to bring them to the surface.
Earlier triggers have been gas pipelines
and work at government research
establishments. A current trigger is
wind turbines.
Infrasound in the Cold War
The media follow-up of Gavreau’s
work led to interest in infrasonic
weapons, although these have not been
produced, as it is not possible to gener-
ate directional infrasound of high
enough level to be effective at a dis-
tance. However, during the cold war,
the Conference of the Committee on
Disarmament was presented with a
paper from the Hungarian Peoples’
Republic (Anon 1978) which discussed
infrasonic weapons and concluded:
… infrasound can become the
basis of one of the dangerous
types of new weapons of mass
destruction …
Fig 1. Jet engine as infrasonic weapon.
All this leads to the unequiv-
ocal conclusion that the scope
of the agreement on the pro-
hibition of the development
and manufacture of new
types of weapons of mass
destruction must also be
extended to the military use
of infrasound weapons of
mass destruction …
An example of an infrasonic weapon
was given as a jet engine attached to a
long resonance tube, shown in Fig. 1.
The physics is at fault, because the
rapid flow of the exhaust gas from the
engine will prevent the development of
resonance (Leventhall 1998). After tak-
ing advice, the Western powers con-
Fig. 2
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Concerns About Infrasound from Wind Turbines 33
cluded that infrasonic weapons were a political distraction
from the main points of the disarmament negotiations.
In relation to wind turbines, the concept that “infra-
sound is dangerous” has been absorbed into the minds of
objectors to wind turbines, who take a one dimensional view
of infrasound. That is, they consider only that it may be pres-
ent from wind turbines and ignore the very low levels. So we
have the connection shown in Fig. 2, which objectors to wind
turbines are pleased to believe and which they make use of in
planning applications. However, decibel for decibel, infra-
sound is less harmful than higher frequency noise.
The Wind Turbine Syndrome
This supposed syndrome is a collection of maladies, said
to result from exposure to infrasound from wind turbines,
including “… sleep disturbance, headache, tinnitus, ear pres-
sure, dizziness, vertigo, nausea, visual blurring, tachycardia,
irritability, problems with concentration and memory, and
panic episodes associated with sensations of internal pulsa-
tion or quivering when awake or asleep.
In her self-published, popular science book, Nina
Pierpont, who practices as a pediatrician, not an acoustics
expert, gives two hypotheses on which the wind turbine syn-
drome is based.(Pierpont 2009)
1. Wind Turbine Syndrome, I propose, is mediated by
the vestibular system — by disturbed sensory input to
eyes, inner ears, and stretch and pressure receptors in
a variety of body locations. These feed back neurolog-
ically onto a persons sense of position and motion in
space, which is in turn connected in multiple ways to
brain functions as disparate as spatial memory and
anxiety …
2. Air pressure fluctuations in the range of 4-8 Hz,
which may be harmonics of the turbine blade-passing
frequency, may resonate (amplify) in the chest and be
felt as vibrations or quivering of the diaphragm with
its attached abdominal organ mass (liver). Slower air
pressure fluctuations, which could be the blade-pass-
ing frequencies themselves or a lower harmonic (1-2
Hz), would be felt as pulsations as opposed to the
faster vibrations or quivering … The pressure fluctu-
ations in the chest could disturb visceral receptors,
such as large vessel or pulmonary baroreceptors or
mediastinal stretch receptors which function as vis-
ceral graviceptors.These aberrant signals from the vis-
ceral graviceptors, not concordant with signals from
the other parts of the motion-detecting system, have
the potential to activate the integrated neural net-
works that link motion detection with somatic and
autonomic outflow, emotional fear responses, and
aversive learning.
To summarize, Pierpont’s claim is that the low levels of infra-
sound from wind turbines have a direct pathophysiological
effect on the body, through the vestibular systems and also by
excitation of the airways and diaphragm to the viscera.
However, she has little understanding of acoustic magnitudes
and changes at will between noise and vibration. The scien-
tific backing for Hypothesis 1 is a paper on vestibular detec-
tion of vibration applied to the mastoid bone. In adopting
this she has misrepresented the original paper (Todd,
Rosengren et al. 2008) as being based on excitation by noise,
when it was actually a bone conducted vibration detection
investigation, comparing thresholds of vestibular and
cochlear detection. Following a newspaper item which con-
nected him with the wind turbine syndrome, Todd repudiat-
ed Pierpont’s use of his work (Todd 2009). The backing for
Hypothesis 2 is in body resonances resulting from whole
body vibration. However, excitation by point vibration input
at the seat or feet differs from that for long wavelength sound,
which acts over the whole body, and different resonances are
excited. Pierpont’s hypotheses are scientifically invalid.
Some people are distressed by wind turbines, and noise
is a factor in this. Those at the sharp end of environmental
noise problems know how upsetting noise can become, espe-
cially if there is an antagonism towards the source. Symptoms
which are given by Pierpont as comprising the Wind Turbine
Syndrome are paralleled in the extreme distress from any
environmental noise, which occurs with a small number of
people, especially when coupled with psychological factors.
However, Pierpont dismisses psychological effects with, “It is
important to emphasize, these symptoms are not psycholog-
ical (as if people are fabricating them), they are neurological.
This contradicts much of what is known about responses to
noise, especially low level noise. (Job 1988, Miedema and Vos
1999).
Pierpont has been the main proponent of dangers of
infrasound from wind turbines and she, along with those
who follow her, is responsible for much of the present public
attitude. A fuller critique of Pierpont’s work has been given
previously (Leventhall 2009).
Support for an adverse effect from wind turbine infra-
sound has been given by Salt and Hullar, who showed that, at
5Hz, the outer hair cell (OHC) response threshold in guinea
pigs is lower than the guinea pig hearing threshold, which
depends on inner hair cell excitation. (Salt and Hullar 2010).
This led to the definition of a rather broad brush OHC
threshold, which gradually diverges from the hearing thresh-
old below 1000Hz. Comparison of guinea pig and human
hearing thresholds was used to define an equivalent OHC
threshold for humans. The final destination in the brain of
the excitations from the OHCs, which are not transmitted
along the auditory nerve, is not known. Salt is cautious in his
scientific papers and writes The fact that some inner ear
components (such as the OHC) may respond to infrasound
at the frequencies and levels generated by wind turbines does
not necessarily mean that they will be perceived or disturb
function in any way.” (Salt and Hullar 2010). However, Salt’s
web page falls squarely in the “infrasound is hazardous”
school (<http://oto2.wustl.edu/cochlea/wind.html>).
The proposed inner and outer hair cell thresholds for
humans are compared in Fig. 3. The outer hair cell threshold
is 20dB at 100Hz, rising at 40dB/decade into lower frequen-
cies, crossing 60dB at 10Hz and 100dB at 1Hz. The next sec-
tion covers infrasound from wind turbines and it will be
noted that much of this infrasound, especially at lower infra-
sound frequencies, is below the outer hair cell threshold, so
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34 Acoustics Today, July 2013
that the relevance of the threshold to wind turbine infra-
sound is not clear. Many natural and man-made infrasonic
sources exceed the threshold in the higher infrasonic region
(Turnbull, Turner et al. 2012)
Infrasound from wind turbines
An early association of wind turbines and infrasound was
the work of NASA in the 1980s. Investigations of the MOD-1
and similar downwind turbines revealed pressure pulses from
interaction between the blades and the disturbed flow behind
the tower. Downwind turbines were largely experimental mod-
els and were completely replaced by the three bladed upwind
turbines, which make up the current operating fleet of utility
scale turbines. Analysis of pulses from the MOD-1 and similar
turbines, which typically have a repetition rate of 1Hz, leads to
a harmonic series based on 1Hz and to the linking of infra-
sound with wind turbines (Shepherd and Hubbard 1991).
However, this infrasound is of the type which might be pro-
duced by a single person hand clapping, or even a ticking clock!
The problems from the MOD-1 turbine were not from the fre-
quency, but from the peaks of the pressure pulses in the down-
wind turbine design, which caused vibration of loose building
components and were also audible. Building response is the
same over a wide range of pulse repetition rates, up to the point
where the decay time of the this response merges with the rep-
etition rate of the pulses.
There have been a number of measurements of infra-
sound from modern wind turbines (Hayes 2006, Hepburn
2006, Jung and Cheung 2008, O’Neal, Hellweg et al. 2011,
Ambrose and Rand 2011 (December), Turnbull, Turner et al.
2012, Walker, Hessler et al. 2012, Evans 2013). Current meas-
urements are often made to alleviate concerns of those objec-
tors who believe that infrasound from wind turbines is harm-
ful. Measurements show similar results. At typical nearest
residential distances, the one-third octave level at 10Hz is
around 60dB, with a negative spectrum slope of 3 to 6dB per
octave. The levels decrease with distance and may merge into
background infrasound. Evans showed that the averaged
infrasound levels at residences 1.8km and 2.7km from the
nearest turbine of a 140, 3MW turbine wind farm were simi-
lar when the turbines were on or off (Evans 2013). This con-
firms earlier work (Guldberg 2012, Howe, McCabe et al.
2012). Although the average infrasound level may not differ
between wind turbines on and off, the characteristics of the
sound may change when the turbines are operating. For
example, the inaudible infrasound close to a wind turbine
may have cyclic variations, whilst the inaudible background
infrasound has random variations in level.
There is no evidence that the low levels of infrasound
from wind turbines, as shown by these measurements, are
harmful to humans.
A narrow band analysis of noise from the Shirley wind
farm at a residence 335m from the nearest turbine is in Fig. 4
(Walker, Hessler et al. 2012). The outdoors spectrum is 38-
39dB at 10Hz, 0.05Hz band, leading to a one-third octave
level of about 55dB. The slope of the outdoors spectrum is
close to 20dB/decade (6dB/octave). The rise in the living
room spectrum in the region of 20Hz is from building reso-
nances, but is nearly 50dB below the hearing threshold at
20Hz. The residents had left their home, complaining of ill-
ness (nausea) caused by wind turbines, although all the levels
in Fig. 4 are below the Salt OHC threshold of Fig. 3.
One of the authors of the Shirley report suggested direct
action of infrasound on the vestibular otoliths as a cause of
illness, but the next section on infrasound and the ear shows
that this is an unlikely explanation.
Infrasound and the ear
The pure tone hearing threshold has been measured in a
chamber down to 4Hz (Watanabe and Møller 1990) and to
lower frequencies using earphones (Yeowart and Evans
1974). The chamber data is shown in Fig. 5, where it is com-
bined with the ISO standard threshold (ISO:226 2003). The
Watanabe and Møller threshold at 4Hz is 107dB. At 12 Hz it
is about 90dB. Yeowart and Evans give higher binaural
thresholds: 112dB at 4Hz and 121dB at 2Hz.
The mechanism of hearing down into low frequencies is
through normal excitation of the auditory cortex, as shown
by fMRI investigations (Dommes, Bauknecht et al. 2009 ).
Dommes, Bauknecht et al used functional Magnetic
Resonance Imaging (fMRI) to investigate responses of the
brain when exposed to infrasound both above and below the
hearing threshold, at the following frequencies and levels:
Freq Hz 500 48 36 12 12 12
Level dB 105 100 70 120 110 90
Audible infrasound excited the auditory cortex, which is
where hearing perception occurs. Inaudible infrasound did
not show an excitation. This is to be expected if infrasound
enters into the hearing system, and is transmitted to the brain
in a similar manner to higher frequency sounds. Dommes,
Bauknecht et al summarise the results of their work as:
In our study, no other cortical regions owed a
comparably extensive response to the high-level
stimuli as did the auditory cortex, indicating that
LFT [low frequency tones] were mainly perceived
via acoustic pathways instead of representing a
Fig. 3. Comparison of inner and outer hair cell thresholds. Above: Inner hair cell.
Below: Outer hair cell.
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Concerns About Infrasound from Wind Turbines 35
somatosensory phenomenon.
In our study, cortical activation patterns
appeared to be similar for all frequencies applied,
suggesting that LFT are processed in a similar way
as frequencies of our main hearing range (200 to
5000Hz).
We presented the 12Hz stimuli at three different
levels. Tone bursts of 120 and 110 dB resulted in
cortical activation. The 90dB stimulus did not
induce a significant response of the auditory cortex
in group analysis which, in agreement with the
findings of Møller and Pedersen (2004), indicates
that this SPL is below the estimated perception
threshold for 12 Hz. (Møller and Pedersen 2004)
This shows that low frequency tones and infrasound are per-
ceived through the normal auditory pathways, the same
pathways as for higher frequencies.
Furthermore, sounds, including infrasound, which are
below the hearing threshold, do not produce a response in
the auditory cortex, as is also the case for sub-threshold high-
er frequencies. Whilst the lowest frequency used was 12Hz,
the regular slope of the hearing threshold indicates that sim-
ilar processes are likely to apply at lower frequencies. For
example, Hensel et al showed that a biasing tone at 6Hz,
130dB was detected by the cochlea and that there was no
abrupt change in response in the transition from infrasound
to low frequency sound (Hensel, Scholz et al. 2007).
The ear is a bi-directional device
The ear operates in both forward and reverse directions.
In normal, forward operation, sound waves excite the ear
drum, which drives the ossicles to impart vibrations to the
cochlear fluid (perilymph) via the oval window. (Fig. 6)
These vibrations propagate up and down the cochlea to the
pressure release of the round window, causing waves along
the basilar membrane and exciting the inner hair cells, which
send signals via the auditory nerve to the auditory cortex,
where they are interpreted as sound The system is mechani-
cal up to the oval window and largely hydrodynamic within
the cochlea.
Reverse action of the ear was demonstrated through
otoacoustic emissions (OAE) in which ringing” of the
cochlear amplifier, which is based in the outer hair cells,
sends vibrations back through the oval window and ossicles
to excite the ear drum. Vibrations of the ear drum can then
be detected by a microphone in the ear canal (Kemp 2002).
The cochlear aqueduct and internally generated
infrasound
The brain produces a fluid (cerebrospinal fluid) which
bathes the brain and the spinal cord, providing protection,
lubrication and an egress for metabolic wastes. The cere-
brospinal fluid, which can be sampled by lumbar punctures,
carries infrasonic pressure pulses resulting from heartbeat
and breathing. A small duct, the cochlear aqueduct, connects
Fig. 4. Narrow band analysis (0.05Hz) of wind turbine noise.
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36 Acoustics Today, July 2013
the cochlea to the cerebrospinal fluid, permitting bidirec-
tional flow of fluid and allowing pressure equalisation of the
cochlea, Fig. 6. The cochlear aqueduct offers a high resistance
to high frequencies, but passes the low frequency pressure
pulses from the cerebrospinal fluid into the fluid of the inner
ear (Traboulsi and Avon 2007). This effect is strong enough
to drive the ear in reverse so that infrasound pulses generat-
ed by heartbeat and breathing, which enter the cerebrospinal
fluid and transmit to the inner ear via the cochlear aqueduct,
may be detected with a microphone in the ear canal. The
detection has similarities to detection of otoacoustic emis-
sions and the cochlea is continuously exposed to infrasound
from heartbeat and breathing at similar, and lower, frequen-
cies to wind turbine rotational infrasound.
Traboulsi and Avon detected pressure peaks in the ear at
0.2Hz from breathing and at 1Hz from heartbeat.(Traboulsi
and Avon 2007). Recent work has measured the transfer
function between pulsations occurring in the carotid artery
in the neck and the consequent pressure detected in the ear
canal, when it was occluded by a microphone (Furihata and
Yamashiti 2013). A 600 second analysis of the pressure
detected in the ear canal is shown in Fig. 7, where the heart
rate is close to 60bpm (1Hz) and two harmonics are shown .
Below 1Hz there is infrasound from breathing and other
body processes. The pressure in the small volume of the
occluded ear canal was 95 -100dB, corresponding to an aver-
age ear drum displacement of nearly 0.1µm.
The pressures produced in the inner ear fluid by exter-
nal infrasound and internally generated infrasound, can be
compared by ear drum displacements, allowing for forward
gain and reverse losses. The reverse losses in the ear are
greater than the forward gain, possibly 20-40dB greater
(Hudde and Engel 1998, Puria 2003, Cheng , Harrington et
al. 2011). By considering these, it was concluded that the
levels of infrasound in the inner ear from internal body
sources are greater than those from external infrasound
from wind turbines (Leventhall 2013). The body, and
vestibular systems, have developed to avoid disturbance
from the high levels of infrasound which are produced
internally from the heartbeat and other processes. In fact,
the hearing mechanisms and the balance mechanisms,
although in close proximity, have evolved to minimise
interaction. (Carey and Amin 2006).
Assessment
Internally generated infrasound from heartbeat and
breathing, which enters the inner ear via the cochlear aque-
duct, is greater than that received externally from wind tur-
bines at similar frequencies, perhaps by 20dB or more. Levels
of infrasound received from wind turbines at typical residen-
tial distances are well below hearing threshold and also main-
ly below the outer hair cell threshold, proposed by Salt and
Hullar as a possible onset level of adverse effects. There is no
evidence that this wind turbine infrasound is harmful, whilst
there is evidence from atmospheric infrasound that it is not.
For example, microbaroms at around 0.2Hz may be of high-
er level than wind turbine infrasound at that frequency.
Microbaroms have been measured at a power spectral densi-
ty of 120dB at 0.2Hz (Shams, Zuckerwar et al. 2013).
Certainty is never 100%, especially when biological dif-
ferences are involved, but all indications are that the Wind
Turbine Syndrome is based on fallacious reasoning and that
inaudible infrasound from wind turbines is not a problem for
residents.
However, some people are convinced that they are
harmed by infrasound from wind turbines, but this appears
to be because they have been told, repeatedly, in publicity
opposing wind turbines, that harm will occur. Frequent rep-
etition of an incorrect fact does not make it correct although,
Fig. 5. Low frequency hearing thresholds.
Fig. 6. Action of the ear. Adapted from (Maroonroge, Emanuel et al. 2009)
Fig. 7. Spectrum of infrasonic pressure in the occluded ear canal.
v9i3_pfinal_rev3_ECHOES fall 04 final 8/27/13 2:22 PM Page 36
Concerns About Infrasound from Wind Turbines 37
as with advertising and propaganda, repetition brings con-
verts. A collection from over 200 web pages and media sto-
ries, detailing supposed harmful effects from wind turbines
has been made by Simon Chapman, Professor in Public
Health at the University of Sydney, and can be viewed on:
<http://tobacco.health.usyd.edu.au/assets/pdfs/publica-
tions/DiseaseListIntro.pdf>
Deignan et al have analysed “fright factors” in Ontario
newspapers related to wind turbines and concluded that the
newspapers contained articles about wind turbines and
health that may produce fear, concern and anxiety for read-
ers.(Deignan, Harvey et al. 2013)
Similarly, Chapman considers that the Wind Turbine
Syndrome is a “communicated disease”, which is spread by
concerns of noise rather than by pathological effects
(Chapman 2012). Further, a recent study by Crichton et al
has shown, in a laboratory setting, that if participants are
concerned about the effects of infrasound upon them, they
will display symptoms whenever they believe infrasound to
be present, whether the infrasound is actually present or not.
(Crichton, Dodd et al. 2013) This emphasises the importance
of attitudes to a noise source in reactions to it. Objector
groups to wind turbine developments have fostered negative
attitudes - attitudes which can lead to distress through the
nocebo effect (opposite of placebo). (Faase and Petrie 2013,
Witthoft and Rubin 2013). The influence of complainant per-
sonality traits has been considered by Taylor et al. who have
shown that those with negative traits are more likely to be
disturbed and report non-specific symptoms.(Taylor,
Eastwick et al. 2013)
Conclusion
The reason why some may be disturbed by the low levels
of noise from wind turbines is clearly complex and requires a
multidisciplinary approach. Whilst there are instances of
genuine noise problems from wind turbines, the emphasis on
infrasound and its supposed effects on health, distracts atten-
tion from solving these. Objectors to wind turbines, who pro-
mote wind turbine infrasound as a problem, are not helping
those whom they wish to support.AT
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Geoff ’s career has been split almost equally between academic and consultancy work,
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Distinguished International Member of INCE-USA.
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... Testing of vestibular sensitivity to low-frequency and infrasound is also motivated by the common proposition that these sounds can produce dizziness, loss of balance and nystagmus. [13][14][15][16] Although such connection has generally not been backed by objective evidence, [17][18][19] in principle, such effects could indeed occur due to direct stimulation of the vestibular system. ...
... 23 Nevertheless, recent estimates from an otolith model 24 have suggested that infrasound (0.7-7 Hz) may induce sufficient otolith acceleration to produce the motion-sickness-like symptoms frequently reported by people living near wind turbines. 16,17,19 In addition, animal data suggest that infrasound can produce large endolymphatic potentials. 25 Since the inner ear is an enclosed electrical volume that comprises both, balance and hearing organs, excitatory effects of the cochlear potentials on the vestibular system cannot be ruled out and requires testing in a controlled laboratory setting. ...
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As part of pre-construction sound monitoring for four utility wind energy projects, background low frequency (12.5 to 500 Hz) sound levels were measured at four wind energy sites: two coastal areas in southern New England and two inland, rural inland areas in upper Great Lakes States. Monitoring locations were not influenced by traffic noise. The background measurements for time periods when hub height winds were at or above the turbine design wind speed were extracted, and background frequency spectra were graphed for comparison to the turbine sound level spectrum at each site. The data reveal natural low frequency sound levels near ground level are substantial when hub height winds are at the design wind speed (the speed at which maximum sound power first occurs, typically 10 m/s). Background low frequency sound at the infrasound frequency of 1/3-octave band 16 Hz exceeds utility wind turbine low frequency sound by up to 8 dB for rural-inland sites and by up to 13 dB for coastal environments. The wind turbine low frequency sound at 16 Hz is typically 30 dB below the ISO 226 hearing threshold, below which no adverse health effects have been documented.
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Infrasound and low-frequency noise below 200 Hz is known to affect the health of human beings. The main purpose of this paper is to experimentally identify the characteristics of acoustic emission of large modern upwind wind turbines with emphasis on infrasound and low-frequency noise. The sound measurement procedures of IEC 61400-11 and ISO 7196 are applied to field test and noise emission from each of 1.5-MW and 660-kW wind turbines utilizing the stall regulation and the pitch control for power regulation is evaluated. The sound spectral density showed that the blade-passing-frequency (BPF) noise is clearly dominant up to 6-7 harmonics, which generally occupy the frequency range of 1-10 Hz, i.e., infrasound. The A-weighted sound pressure levels (SPLs) of the stall control type of wind turbine were found to increase with wind speed in a more correlated way than those of the pitch control type of wind turbine while the G-weighted SPLs of low-frequency noise, including infrasound, were found to show a positive correlation with the wind speed irrespective of the method of power regulation. Potential complaints of local communities about infrasound and low-frequency noise of wind turbines are assessed by comparing the measured data with the existing hearing thresholds and criteria curves. These comparisons show that it is highly possible that low-frequency noise from the 1.5-MW and the 660-kW wind turbines in the frequency range over 30 Hz may lead to psychological complaints by ordinary adults and that infrasound in the frequency range from 5 Hz to 8 Hz could cause complaints due to rattling house fittings such as doors and windows.
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The search for ways to monitor compliance with the Comprehensive Test Ban Treaty has sparked renewed interest in sounds with frequencies too low for humans to hear. .
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An experiment was conducted to measure and characterize infrasound (and higher frequency acoustic energy) from turbines at a wind farm in Southern Alberta. Simultaneous telemetry and point measurements were acquired from three sensor types: low frequency geophones, acoustic microphones, and a precision sound analyzer. Measurements were recorded for three wind states: low, medium, and high. Down wind telemetry measurements were recorded for thirty (30) continuous 50m offsets, up to a distance of 1450 m from the wind farm. Point measurements, coincident with the telemetry measurements, were acquired with a low frequency precision sound analyzer for two offsets: 50m and 1000m from the turbines. The same measurements were recorded with the turbines on, and with the turbines off. The low frequency results of the experiment are presented in this paper.