The
perception of sound in the human brain is caused by variation in pressure at
the eardrum above and below the residual pressure due to the atmosphere. The
rate of this variation gives the sound its apparent pitch with a slower
variation causing a perception of a lower pitched sound and, correspondingly, a
faster variation causing a higher pitched sound. This rate of variation is
referred to as the frequency of the sound which is measured in pressure cycles
per second or Hertz (Hz). The amount by which the sound is above a person�s
hearing threshold sets how loud the sound appears to them and is referred to as
the sensation level, measured in decibels (dB).
The
human ear is less sensitive to sounds at high and low frequencies than for
sounds in the middle of the hearing range, meaning that the low and high
frequency sounds have a lower sensation level for the same absolute noise
exposure level. The ear is most sensitive to the frequencies corresponding to
the weaker sounds of speech known as the �fricatives� which occur at
frequencies around 4,000 Hz.
If a sound at this frequency
were presented to a normally hearing human ear, and then the frequency of the
sound were gradually reduced whilst keeping the sound pressure level the same,
the sound would not only reduce in apparent pitch but appear to reduce in
volume. This reduction in volume is referred to as the �frequency response� of
the ear and occurs because the ear becomes progressively less sensitive to
sound as the frequency decreases. This frequency response changes in the case
of deafness, whether caused by noise at work, ageing or from other causes, with
different causes having their own characteristic frequency response.
As a result of the
decreasing frequency response of the ear at low and high frequencies, there
comes a point at which the sound is no longer perceptible by the brain. In the
example above, this point would depend on the level of the initial sound; what
this means is that the ear needs a higher and higher level of sound for it to
be perceptible as frequency increases or decreases.
The human ear is often said
to be only responsive to sound between 20 Hz and 20,000 Hz, with sounds above
and below these values being referred to as infrasound and ultrasound
respectively. Because of the above, this is an over-simplification and, providing
they are loud enough, such sounds can be perceived well into these ranges. It
should be noted, however, that as the frequency moves significantly into the
infrasound and ultrasound regions, the sound levels have to be very loud indeed
to be perceived in any way by the hearing mechanism, which is the most
sensitive receptor of such signals in the human body.
By way of illustration, a
sound at 4 Hz has to be around 110 dB louder than a sound at 4000 Hz for it to
be just perceptible � corresponding to an increase of 100,000,000,000 times in
terms of sound energy.
It is also important to
appreciate the tiny strength of these pressure fluctuations. Even a sound pressure level of 100 dB has a fluctuating pressure level of 2 Pascals (Pa).Atmospheric pressure reduces with height and
a decrease of 2 Pa in pressure is achieved at a person�s head by raising its
position by 20 cm. So, stepping up and
down a single stair subjects the ear to a fluctuating pressure equivalent to
100 dB. Doing this at a rate of, say,
once per second would therefore generate infrasound at 100 dB.
There is often confusion as to the difference between sound at
infrasonic and at �low� frequencies. This can be explained by the above which
shows that the distinction between the two is purely arbitrary. What happens is
that the threshold of perception; the lowest level which can be heard, decreases
as the frequency of the sound increases. At 20 Hz, which is generally referred
to as where infrasound becomes low frequency noise, the normal threshold of
perception is 78 dB. The threshold then decreases gradually such that, at the
top of what is commonly referred to as the low frequency range, at 200 Hz,
although again this arbitrary, the threshold is 14 dB. It can be seen that the
low frequency range spans a large range of hearing thresholds.
Infrasound and Wind Turbines
Wind turbines produce sound (or noise � which can be
defined as unwanted sound) at all frequencies but there has been particular
concern over noise at the frequency at which the blades pass the tower, known
as the �blade passing frequency� which invariably lies in the infrasound region
[1]. In older
designs of turbines, where the blades rotate downwind of the tower, a significant
level of noise was generated at such frequencies, resulting from the blades
moving from being exposed to the incoming wind into a region of still air behind
the tower, and then immediately back into the wind again as the turbine blades rotated
around the hub. This design flaw, in acoustic terms, has been eliminated in all
modern turbines of commercial scale which are designed with the blades rotating
upwind of the tower so that they are always exposed to the full force of the
wind.
Measurements on
upwind turbines
[2], show noise at the blade passing frequency to be around
65 dB
at around 500 metres, which can be set against a
perception threshold of around 125 dB which is 60 dB higher, or 1 million times
higher in energy terms. 70 dB of infrasound is described as equivalent to
�fluctuations
in air movement that are less than the tiniest breeze through an open window.
Perhaps just enough visibly to move a candle flame but probably not�.
These results are usefully put in the context of
infrasonic noise from a number of other sources in a German report for the
Ministry for the Environment, Climate and Energy of the Federal State of
Baden-Wuerttemberg in 2016
[3]. This shows infrasound levels inside a moving car,
with the window open, at 95 dB as compared to a range of wind turbine noise
levels (at 300 metres) between 55 and 75 dB. Noise levels from various domestic
appliances are shown to have similar levels of infrasonic noise to that from
wind turbines, at similar distances, as is the level of infrasonic noise in an
open field with a light wind blowing. Where wind turbine noise measurements
were made, there was found to be little difference in infrasonic noise levels
with or without the wind turbines operating.
A document produced for the World Health Organisation
in 1995
[4] noted that �there is no reliable evidence that
infrasound below the hearing threshold produce physiological or psychological
effects
� and although the latest WHO guidance on environmental noise[5] notes that wind turbines can generate infrasound it
states that �
few studies relating exposure to such noise from wind turbines
to health effects are available
�.
Very specific
research carried out by Crichton et al
[6] to investigate whether the expectation of
experiencing effects from infrasound, at a similar level and frequency to
typical levels from wind turbines, had any effect on their perception. This
reports that
�results indicated the number of symptoms reported and the
intensity of the symptom experienced during listening sessions were not
affected by exposure to infrasound
� but that that they �were influenced
by expectancy group allocation
�; ie. by whether they expected to experience
such effects or not.
Low Frequency Noise and Wind Turbines
No one would argue that wind turbines cannot be heard
at the sound levels which are permitted at residential locations for planning
purposes. This normally corresponds to a separation distance of between 500 and
1000 metres between such locations and the nearest turbines, depending on the
size and layout of the site and the type of turbines installed. At these
distances, the sound of wind turbines is not predominantly low frequency in
nature but contains audible components both above and below the arbitrary value
of 200 Hz which is generally used to delineate between �normal� and low
frequency sounds. Most environmental sounds of human origin, such as that from
transportation and industry, fall into this same category. The LUBW study
3
shows the noise from wind turbines, traffic and even a general urban
background, becoming perceptible at frequencies above about 30 Hz although, for
noisy road traffic, it occurred at a lower level because the overall noise
level was raised.
The most
important factor in the assessment of low frequency noise is the sensation
level (see Paragraphs 1 and 2 (above)) of the noise as it varies with frequency. If
the sensation level increases with
increasing frequency then the sound will be perceived as not having a
specifically low frequency characteristic. If, on the other hand, the sensation
level increases with decreasing frequency then the sound will be perceived as
having a specifically low frequency characteristic. This is highly unlikely to
be the case for wind turbine noise at typical separation distances. Where it
can occur, however, is if the turbines emit a very distinct low frequency tone
or tones, characterised by a sharp rise and fall in sensation level in the low
frequency region and which is normally regulated by planning condition. Alternatively, turbines may be perceived as creating
increased low frequency noise if they are a significant distance away, under
which circumstances noise levels at all frequencies are relatively low and
typically well below permitted noise limits. Specific UK guidance on low
frequency noise is provided within a report for the Government by Salford
University in 2005 and updated in 2011
[7].
Noise,
Health and Wind Turbines
Noise only has what
could be termed direct health effects where it is of such high level that it
causes hearing loss. Such high noise levels are not permitted to occur at
residential locations by planning and other regulation. Noise at lower levels can,
however, cause health effects indirectly through stress and/or sleep disturbance.
Stress in itself can cause sleep disturbance and noise can cause stress even if
it is only just audible although it is generally accepted that higher noise
levels cause higher levels of stress, all other things being equal.
Wind turbines have been cited by some as causing effects on health such
as sleep deprivation, severe chronic stress and dysfunction of the vestibular
system. In her book,
Wind Turbine Syndrome, Nina Pierpont, a
pediatrician in the USA, goes further than this and suggests that noise from
wind turbines can cause symptoms which �
include sleep disturbance, headache,
tinnitus, ear pressure, dizziness, vertigo, nausea, visual blurring, tachycardia,
irritability problems with concentration and memory, and panic episodes
associated with sensations of internal pulsing or quivering that arise while
awake or asleep
�. These assertions were based on case studies of 10
families living in proximity to wind turbines who also had health problems (38
people in total). It is suggested by Pierpont that the collection of symptoms �
resembles
syndromes caused by vestibular dysfunction
� which she attributes to
infrasound. She notes that �
statistically significant risk factors for
symptoms during exposure include pre-existing migraine disorder, motion sensitivity,
or inner-ear damage (pre-existing tinnitus, hearing loss, or industrial noise
exposure)
� meaning that those problems existed prior to any wind turbine
noise exposure. No results of any acoustic measurements at the homes of any of
the subjects of the case studies were included in the book so the extent to
which any of these subjects were exposed to levels of sound at any frequency is
unknown.
Similar claims have been made by others including a Portuguese research
team headed by Mariana Alves-Pereira who has coined the term Vibro-Acoustic Disease
to describe physiological damage caused by high levels of industrial noise and
who has made claims that the same symptoms can be caused by wind turbine noise
without any consideration of the actual levels of noise generated. Alves-Pereira has been known to graphically
demonstrate her interpretation of the impact of infrasound on the body by
punching herself in the arm or hand
[8]
to show the physical effect. However, as
we have already described the effect on the body is less than lightly blowing
on it and bears no resemblance to her self-inflicted assault.
The two examples above are symptomatic of the way the alleged effects of
wind turbine noise on �health� have been cited, and the lack of reliable
evidence as noted by the WHO and referred to in Paragraph 10 (above). The
most important issue in any study of cause and effect is to evaluate both the
possible cause, as well as the effects, in detail. In the case of wind turbine
noise this would require quantification of noise level right across the
frequency range from the blade passing frequency and upwards into the normal
audible range with particular attention given to audible sounds, which are
known to cause stress and sleep disturbance, rather than those which are orders
of magnitude (see Paragraph 8 (above)) below it. In particular, no research
alleging health effects from significantly sub-perception wind turbine infrasound
appears to have investigated the reasons why infrasound exposure from other
sources (see Paragraph 9 (above)) does
not cause similar claims of ill-health. Suggestions
that wind turbine infrasound causes ill-health fail to explain what the causal
link might be either through the hearing or vestibular mechanism and generally confuse the effects of substantial levels of
infrasound and/or whole body vibration with the tiny changes of air pressure
corresponding to the levels of infrasound generated by turbines (and many
general environmental sources.
Conclusions
The above presents only a summary of infrasound, low frequency noise and
the claimed effects on health of noise from wind turbines. It is apparent from
consideration of the evidence, however, that audible wind turbine noise has the
capacity for causing annoyance which may in turn cause stress and sleep-disturbance
even at low levels. The reason for this is not related to infrasound which
occurs at orders of magnitude below perception threshold and which is at a
similar level to many other sounds in the environment which people are exposed
to on a regular basis without any attributable effects on their health. In this
respect, wind turbines are no different to other sources of unwanted noise
except in respect of some non-acoustic factors which may affect the subjective
response to such noise.
Note by Andy McKenzie with Dick Bowdler (Dick Bowdler Noise Consultant<a href="https://www.dickbowdler.co.uk/" )
[1] The blade passing frequency is calculated from the
rate of rotation of the turbine hub in revolutions per second multiplied by the
number of blades. For a 3-bladed turbine rotating at 20 revolutions per minute
the blade passing frequency is 1 Hz
[2] Bowdler D, A Short History of the Dangers of Infrasound, Proceedings
of the Institute of Acoustics Vol.40 Page 273-281, 2018
[3] Low-frequency noise incl. infrasound from wind turbines and other
sources, Landesanstalt f�r Umwelt, Messungen und Naturschutz Baden-W�rttemberg,
2016
[4] Community Noise – Document Prepared for the World Health
Organization, Eds. Bergland B. & Lindvall T., Archives of the Centre for
Sensory Research Vol. 2(1) 1995
[5] Environmental Noise Guidelines for the European Region, World Health
Organization, 2018
[6] Crichton et al, Can Expectations Produce Symptoms from Infrasound
Associated with Wind Turbines?, Health Psychology
33(4), 360�364, 2014
[7] DEFRA Contract Report NANR45, Proposed criteria for the assessment
of low frequency noise disturbance, University of Salford, 2011
[8] For example at a presentation in Glasgow http://www.windsofjustice.org.uk/2017/09/wind-turbine-noise-seminar-review-and-up-date/ and in Slovenia https://www.youtube.com/watch?v=ZXCZ3OyklrE at 1m 10s.