Infrasound -- The silent and powerful sound of industrial wind turbines
NIGEL THORNLEY, 2010/02/22.
According to a recent report, residents of Waubra, Australia, are having major
trouble with a Turbine Farm near their homes.
Some mention the "pulsing nature" of the disturbance, which arises from the combined effect
of many turbines turning asynchronously -- as they variously reinforce and cancel each other's
noise, and occasionally all align to produce a powerful pulse of air pressure, because
Low Frequency Sound is primarily felt as air pressure waves, not really "heard."
Report:
www.abc.net.au/news/video/2010/02/19/2825235.htm
Low Frequency Sound -- Infrasound -- is sub sonic; you don't as much hear
it as feel it. Like radio frequencies, the propagation characteristics of
sound vary according to frequency. High frequencies are attenuated quite quickly
by ordinary structures - trees, walls, hills etc. Low Frequencies manage to punch
through much better. For instance, if you notice people who have heavy duty
speakers in their cars, you can hear the thump of the drums clearly from half
a mile away, whereas you could be quite close to the car and not hear any of the
vocals or instruments.
The car speakers' Low Frequencies are in the 20 to 200 cycles per second area,
and penetrate the steel of the car easily with little power loss. The higher
frequencies are severely attenuated by the car body and windows.
Simple phone type speech occurs from 300 cps to about 2,500 cps. Musical voice
and instruments can go from 300 to 20,000 cps. Generally the higher in frequency you
go, the more attenuation, and the shorter the distance the sound will travel.
Elephants use Infrasound to communicate - they are said to be able to transmit their
calls 4km or more. These are the Infrasound frequencies, below 20 cps, which our ears
can't hear. Blue Whales are also said to use Infrasound. In salt water, sound conducts
very well, as you will know if you have ever been swimming underwater and heard motorboats.
The speed of sound varies according to the density of the medium. In air, it's about 1,100
feet per second, thus the 5 second count per mile between lightning and thunder crack. In
salt water it's 4,400 feet per second.
Blue Whales feed in the Antarctic and Arctic and breed close to the Equator; they journey
across vast distances. How do they find each other? They communicate with sound. Claims
are made that they can hear each other at 1,500 km or more. Such is the power of these
low frequencies. The lower the frequency, the greater the physical size of the "ears"
required, so the great size of the whales make this easy, and they can use the lowest
frequencies, which go the furthest distance.
For large wind turbines, it is not the sound of the blades whishing and tearing
the air which concerns us most; those are all fairly high frequency. Rather it
is the great pulse of the big blades which produce infrasound which normal sound
meters would not be able to measure. However this infrasound would penetrate easily
into kitchens, living rooms and bedrooms, and would be fairly powerful.
In this brief overview of infrasound issues we should draw a distinction between
big industrial wind turbines which we have been discussing, and small vertical-axis
wind turbines (VAWT) which have very different characteristics.
That is a discussion
for a future article.
Please refer also to the work of Dr. Nina Pierpont whose researh deals with the
health problems related to wind turbinees, in particular to Infrasound. Her book is
titled Wind Turbine Syndrome.
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