🚀 go-pugleaf

In article <5845@videovax.tv.tek.com>, bart@videovax.tv.tek.com (Bart Massey) writes:
> A pilot friend of mine tells me that active noise-suppressing headsets are
> quite common in small-plane cockpits now.  As you can imagine, this is a

    I had the privilege of listening to a demonstration of an active noise
cancellation system at Wright-Patterson AFB a couple of years ago.  You 
could select helicopter, transport, or fighter jet noise.  The cancelling
could be activated independently so that you could hear the underlying
intended message.  It was fairly impressive.

    Cancellation of low frequencies is pretty straightforward where the
wavelengths are long.  It's when wavelengths get under two feet or so that
stability problems crop up.  At that lambda 180 degree phase shift is only
one foot.  The corresponding frequency is 1100 fps divided by 2 feet or
about 550 Hz.  Most cancellers will be effective only below 500 Hz or 
especially below 1 kHz.

C. David Covington (WA5TGF)  cdc@uafhcx.uark.edu     (501) 575-6583
Asst Prof, Elec Eng          Univ of Arkansas        Fayetteville, AR 72701