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In
the news ...
Can
you actually hear 'inaudible' sound? Wind farms, Sleep
disturbances, a decline in performance, and other
negative effects
Are wind farms harmful to humans? Some believe so,
others refute this; this controversial topic makes
emotions run high. To give the debate more objectivity, an
international team of experts dealt with the fundamentals
of hearing in the lower limit range of the audible
frequency range (i.e. infrasound), but also in the
upper limit range (i.e. ultrasound). The project,
which is part of the European Metrology Research Programme
(EMRP), was coordinated by the Physikalisch-Technische
Bundesanstalt (PTB). At PTB, not only acoustics experts,
but also experts from the fields of biomagnetism (MEG) and
functional magnetic resonance imaging (fMRI) were involved
in the research activities. They have found out that
humans can hear sounds lower than had previously
been assumed. And the mechanisms of sound perception are
much more complex than previously thought. Another vast
field of research opens up here in which psychology
also has to be taken into account. And there is definitely
a need for further research.
If there is a plan to erect a wind turbine in front of
someone's property, many an eager supporter of the "energy
transition" quickly turns into a wind energy opponent.
Fear soon starts spreading: the infrasound generated by
the rotor blades and by the wind flow might make someone
ill. Many people living in the vicinity of such wind
farms do indeed experience sleep disturbances, a decline
in performance, and other negative effects.
Infrasound designates very low sounds, below the limit of
hearing, which is around 16 hertz. The wind energy sector
and the authorities often try to appease the situation,
declaring that the sounds generated are inaudible and much
too weak to be the source of health problems.
Christian Koch knows for sure, "Neither scaremongering nor
refuting everything is of any help in this situation.
Instead, we must try to find out more about how sounds in
the limit range of hearing are perceived." This expert in
acoustics from PTB is the manager of the international
project in which metrology experts from several metrology
institutes and scientists from the Max Planck Institute
for Human Development in Berlin investigated the
fundamentals of the hearing of "inaudible" sounds for 3
years. Very low sounds (i.e. infrasound, below approx. 16
hertz) or very high sounds (i.e. ultrasound, above approx.
16 000 hertz) occur in numerous situations of daily life:
infrasound is not only produced by wind turbines, but also
sometimes when a truck thunders past a house, or when a
home owner installs a power generator in his basement.
Ultrasound can, for example, originate from commercial
ultrasonic cleaning baths that are sometimes used, e.g.,
to thoroughly clean a pair of glasses. It can also be
generated by a device used as a deterrent against martens
(to keep them from gnawing on the wiring of cars). A
particular variant of such devices has been developed
to keep young people away from certain places - an internationally
controversial topic from an ethical viewpoint.
These devices, which produce very high-pitched sounds that
can only be heard by children and young people, are
sometimes used by adults who want to enjoy some peace and
quiet. "In all these areas, we have to deal with
considerable levels of loudness in some cases," Christian
Koch adds.
An audible loud sound may damage hearing - as well as
getting on your nerves. But what exactly is an "audible"
sound? And what does a human being really hear? In order
to find out more, an infrasonic source which is able to
generate sounds that are completely free from harmonics
(which is not as trivial as it may sound!) was constructed
within the scope of this project. Test persons were asked
about their subjective hearing experience, and these (also
quantitative) statements were then compared by means of
imaging procedures, namely by magnetoencephalography (MEG)
and functional magnetic resonance imaging (fMRI). The
results have shown that humans hear lower sounds - namely
from 8 hertz on - which, after all, is a whole octave than
had previously been assumed: an excitation of the primary
auditory cortex could be detected down to this frequency.
All persons concerned explicitly stated that they had
heard something - whereby this perception had not always
been tonal. In addition, the observations showed a
reaction in certain parts of the brain which play a role
in emotions. "This means that a human being has a rather
diffuse perception, saying that something is there and
that this might involve danger," Christian Koch says. "But
we're actually at the very beginning of our
investigations. Further research is urgently needed." An
application for a follow-up project has already been
filed. In this project, the investigations will be focused
on the question why some persons feel disturbed by
"inaudible" sound, whereas others are not even bothered:
many a home owner is left cold by having a wind turbine
next to their homes. And we need to take another effect
into account: namely, that some people become really ill
because they imagine risks which, in reality, might not
even exist. This is the reason why it makes sense to
involve psychologists as well.
But the researchers see a great need for further research
also in the other extreme - the ultrasound. Although the
measuring instruments used are among the most precise in
the world (PTB is the world leader, especially for MEG),
the researchers were not able to measure whether humans
can hear above the previously assumed upper threshold of
hearing, and if they can, what they then perceive. Since,
however, what applies to other ranges, also applies to
high-pitched sounds - namely that a very loud sound may
damage the hearing - here too, there is a need for further
research.
The results of the international research project might
lead to the introduction of uniform - and binding -
protection provisions for these limit ranges of hearing
within Europe, since there have been none to date.
Physikalisch-Technische
Bundesanstalt
What
about sleep disturbance and other health hazards
from electromagnetic fields, mobile telephone mast,
WiFi, etc.?
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