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An un-recordable sound, re-visited

Subject: An un-recordable sound, re-visited
From: "Mark Fischer" aguasonic
Date: Sat Oct 6, 2007 8:30 pm ((PDT))
Dear Nature Recordists,

A while back I posted about an un-recordable sound I had heard, a
mile+ downwind of some of the largest windmills in the world... doing
the math lead to numbers that did not make much sense, as far as prop
rates, and the resultant frequencies of any generated waves... and,
yet, there was a sound, coming from these windmills...

Well, today, I was downwind, maybe a quarter mile, of a single, much
smaller windmill (not a whole field of giants, with the possibilities
of harmonics, constructive and destructive interference, and so on)
and watched the sound being made.

No kidding.

It appears to be an interaction between the blades and the tower. As
each blade passes in front of the tower, in interferes with the
windflow. On/off, On/off, what results is a wave of pulses downstream
from the tower. The blades do not make it, by themselves. The tower,
by itself, makes no sound. Put the two together, in a steady breeze...

Of course, I did not have my recorder with me. But now I know what
conditions bring this about. I bet there is some function
(relationship) of the distance between the blades and the tower, and
the wavelengths of the resulting sound.

Stay tuned...

Mark
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