Greets!

     You were saying...

 DW> Here's something that mystifies me. We've had a lot of snow here of
 DW> late. Sometimes, aircraft can be heard flying above, or maybe through,
 DW> the snow clouds. When this happens, the sound of the engines gets
 DW> altered, something like the "phasing" effect that pop
musicians used to
 DW> like to use. The direction from which the sound comes also seems to be
 DW> highly variable, sometimes shifting dramatically very fast. The overall
 DW> effect suggests that sound is getting bounced around somehow.
 DW>
 DW> But how? And in what way are snow clouds able to do this but not
 DW> regular clouds?

What I believe you are hearing is essentially an acoustically generated
comb filter.  Sound waves traversing the clouds encounter several dramatic
temperature gradients.  These are randomly placed of course, due to the
ever-shifting nature of the clouds.  The nature of the engine noise is
basically pink noise.  As the wavefronts of the pink noise traverse these
thermal shifts they are delayed by a very few milliseconds (or possibly
several hundred microseconds.)  As these delayed wavefronts meet randomly
and "recombine", the result is the "phasing" (or more
likely "phlanging") effect you hear.  I suspect that the snow
clouds exhibit more drastic temperature gradients than others.

Make sense?  Or am I off in left field somewhere?

Best regards,
Marc
telnet://bbs.sursum-corda.com. >Celebrating 12 years on line!<

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