Path: utzoo!attcan!uunet!husc6!linus!alliant!cantrell
From: cantrell@Alliant.COM (Paul Cantrell)
Newsgroups: comp.sys.next
Subject: Re: Fwd: LSC problem
Message-ID: <2636@alliant.Alliant.COM>
Date: 17 Nov 88 16:07:43 GMT
References: <6843@batcomputer.tn.cornell.edu> <77955@sun.uucp>
Reply-To: cantrell@alliant.Alliant.COM (Paul Cantrell)
Organization: Alliant Computer Systems, Littleton, MA
Lines: 33

In article <77955@sun.uucp> srw%pws1@Sun.COM (Scott Wiesner) writes:
>I'm currently reading the Rutan/Yeager book about their trip
>around the world in the Voyager airplane.  One of their problems 
>was the noise they had to deal with.  Since excess weight meant
>death to them, there was no soundproofing.  They tell of using
>a system by Bose that does exactly what is described above.  They
>said it worked great.

Yes indeed, Bose does make a set of anti-noise headphones. They are now
supposed to be standard equipment in Enstrom helicopters. There was an
article in one of the aviation rags, possibly AOPA-Pilot, where they
reviewed them. They work better at some frequency than others, but are
supposed to be quite effective. The big gain here is that they can get
a large drop in noise without having to clamp down on your head as hard
as some of the competition. Anyone who has had to fly for 6-8 hours with
a good set of noise reducing headphones knows how the clamp pressure can
induce a nice headache.

I'm sure the Bose set does no analysis of the noise - they are far too
simple for that. They have to be simply reproducing what is recorded with
a microphone with reverse phase.

I know of one system currently in use that works by analysis of the noise
waveform - MDHC Helicopters in Mesa, Arizona was doing some work called
Higher Harmonic something-or-other in which accelerometers are placed at
various places in the helicopter, and (I believe an FFT) analysis tries to
determine the basic components of the vibration. It then actually manipulates
the controls of the helicopter in such a way that the vibration is decreased
or eliminated. Since vibrations induce fatigue in both humans and helicopters,
this is fairly exciting. As far as I know, the system worked quite well.
Perhaps the NeXT box can use this to get rid of the fan noise? :)

					PC