Hi Wendell

You bring up a valid point. Active noise reduction might be a good way to go
but it would cost a lot.  And you also bring up another good point.  Air
coming in will be an acoustic leak.  The probelm with active noise
cancallation is that it is very expensive to set up in your plane and it
doesn't deaden all frequencies.  If it did, you wouldn't hear anything from
them except those frerquencies which were not matched by the amplitude of the
ANR or conversely, by being of a higher amplitude.  They also need to be
tuned for each installation.

The problem with trying to eliminate the noise only from around the head
level is that the noise comes from all over.  And it is also often coming
from the skins of the fuselage acting like a drum head. So vibration from the
engine or boundary layer can excite the skin and act like a speaker, turning
vibration into noise and transmitting it elsewhere.

Hopefully, the acoustic tests I'm hoping to start on soon can give some real
numbers to it and also help us make informed decisions as to weight
/noise/dollar trade-offs.

Dan Newland