WMOD00469

• Title: Environmental Impact of Large Windpower Farms
• Weather Modification - Doc Type: Report

to conditions generally less favorable than this one, or at least
no more favorable. Therefore one is justified in the conclusion
that no condition worse than those already found innocuous can
occur in the immediate vicinity of a wind machine. The wide
spacing of machines, combined with the natural attenuation
occurring outdoors (Kurze and Beranek, 1971), assures that no
condition will be found worse than that of a hearer near a single
machine.

When it comes to noise beyond the perimeter of the windfarm, th,e
dependence of noise propagation on the physical state of the
atmosphere becomes the major factor. Under weather conditions
that are usual when the wind is blowing hard enough for its noise
to be considerable, the decrease of temperature upward causes
sound waves to be refracted so that they curve upward away from
the ground. The consequence is that loudness diminishes with
distance from the noise source much more rapidly than it otherwise
would, and a noise even as loud as thunder becomes inaudible at a
distance of 2 or 3 kilometers if its source is at the ground.
For noise sources elevated above the ground, the same is true if
one measures the distance from where the sound ray grazes
the ground tangentially.

Two atmospheric conditions are capable of extending the range of
audibility. One is when shear is present near the ground. In
the downshear direction, the air motion tends to refract the
sound wave toward the ground and may overcome the upcurving
refraction due to temperature lapse. Under some conditions the
sound energy might then be trapped in a duct near the ground and
would be attenuated more gradually as it spread sideways. The
other condition, which might occur with light winds but practically
never with strong ones, is temperature inversion; if pronounced
enough, it also would confine the noise energy to a duct near the
ground. The most usual instance of this condition is found over
cool water on a still night when voices can often be heard at an
unusually great distance.

The lower the frequency of a sound, the less it is attenuated in
its passage through the atmosphere. Already at the boundary of
the windfarm most of the high-frequency noise of the more distant
machines - the hiss and swish of their blades - will have been
absorbed, and only the lower-frequency component will escape the
boundaries. Familiar instances of this phenomenon are the rumble
of a distant train with none of the clatter and the low-pitched
mutter of a city as heard from the top of a skyscraper.

Infrasound, air-pressure waves too low-pitched to be heard, is
generated by operation of windpowered generators with a strong
peak at a frequency which is the product of the l~otation rate

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