Path: utzoo!attcan!uunet!ncrlnk!ncrcae!hubcap!gatech!bbn!jr@bbn.com
From: jr@bbn.com (John Robinson)
Newsgroups: comp.sys.next
Subject: Re: Fwd: LSC problem
Summary: sound cancelling requires holography
Message-ID: <32595@bbn.COM>
Date: 22 Nov 88 07:30:47 GMT
References: <4000001@altair>
Sender: news@bbn.COM
Reply-To: jr@bbn.com (John Robinson)
Organization: BBN Systems and Technologies Corporation, Cambridge MA
Lines: 37
In-reply-to: boyle@altair.mcs.anl.gov

In article <4000001@altair>, boyle@altair writes:
>
>Hmmm.....  Let's see.  It seems to me that sound cancellation, 
>especially for a distributed source, must be more complicated than
>this discussion suggests.  Isn't it the case that if the source of the
>cancelling cound is not coincident with the source of the sound,
>cancellation would be perfect only if a few places, not everywhere?

Can you say sound hologram?  If you are trying to gets the peaks and
valleys of air pressure to go away everywhere, you'd have to produce
the perfect inverse signal in 3-space somehow.  If you get to put your
jamming signal through the same point source as the original, this
might be possible, but once it has gotten into the room (or whatever,
as long as it isn't anechoic) the sound will be impossible to cancel.
Note that the systems that have been described (e.g., for helicopters)
use headphones, hence interpose a source pretty close to the path of
all sound getting to your ear.  Doing it in a room, though...

The only thing I am aware of that works something like this is the
conjugate mirror stuff (see a recent SciAm article) for coherent
light.  Maybe if they would tell us about how the stealth planes work
too.
 
Sound reinforcement systems for big outdoor concerts (Watkins Glen was
like this) where they put speakers a ways out from the stage have to
be careful to introduce the right amount of delay to avoid muddying
the signal.  Some musicianns use out-of-phase mike pairs to cancel
most of the ambient noise pickup (they sing into just one).  And, if
you have ever hooked up stereo speakers out of phase you probably know
that that can kill some frequencies pretty effectively.  So noise can
be canceled some, but things'll have to be a lot more fancy before:

>Guess I'd have to ask for a seat in the "No Sounding" section of that
>airplane!
--
/jr
jr@bbn.com or bbn!jr