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I have had similar thoughts.  One has been to put speakers facing up on the
top of the panel, reflecting off the windshield, and canceling the propeller
beat coming in through the windshield.  I subsequently learned that the most
successful ANR in a cabin was in a helicopter where the frequencies are low
and the wavelengths long.  As wavelengths shorten and bounce around,
cancellation becomes more difficult.  ANR headsets work because they cancel
right at the ear.  I spoke to a few headset guys at OH, and they said it is
much more complicated that it first appears.  So I am not hopeful, but would
like someone smarter than me to fire some neurons at the problem and see if
there is a way.  At least we should be able to get rid of the low frequency
stuff in our small cabins.  Would some acousto-techno type care to comment?  

Related: those little foam ear plugs provide 28 dB of isolation.  For
years I have used a boom mike clipped to a custom ear piece in one ear, and
foam in the other when flying in hot weather.  Works fine.  Or for the
ultimate, put foam plugs in your ears, then don your head set and turn up the
volume.  It gets nice and quiet inside the cranium.  Except for the low
frequency stuff which jiggles the bone a bit.

Fred
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