> From: "Pastor Doug Coats" <dcoats at heritagemail.org>

> I asked a an engineer a similar question when I was working on setting up a
> sound system for a gym.  Our concern was canceling out the echoes.  His
> response was that theoretically you can cancel out sound with the reverse
> wave length of the same sound.
> In practice you would need one of three things to happen. 
> 
> 1.  You would need to mic the sound at it's source and place the speakers at
> the same location. 
> 
> 2.  You would need mic the location of the ears and place the speakers at
> the same location. 
> 
> or 
> 
> 3.  You would have to have a limitless number of mics to pick up all of the
> sound waves bouncing off of all of the surfaces for the speakers to conceal
> out the sounds. 
> 
> The only one that is somewhat practical is number one.  Say if you had an
> extremely load air conditioner or something and wanted to cancel out it's
> noise.  Number two would require some sort of headphone contraption
> connected to a battery source and a portable/wearable PC. 
> 
> As far a Linux being able to accomplish the first idea I don't know.

Active noise cancellation is a bit more involved than what you suggest.  As 
an engineer, yes, the principle of creating null points is the idea, but in 
practice you are talking about a lot of predictive filtering.  Ford Motor 
had a research group that did quite a bit of work on it, maybe 5-7 years ago 
 -- at least they published a fair amount of work in the Journal of the Audio 
Engineering Society around that time. 

As far as Linux being able to do it, it doesn't make a whole ton of sense.  
A standalone / embedded DSP is much more appropriate, but if you wanted to 
do it for grins, you would want to look at low latency kernels, like the 
real-time linux project, since it would play havoc with your cancellation 
(i.e., make the room noisier, since the adjusting signal would basically be 
noise if *IT* wasn't cancelled out) if the cancelling algorithm's process 
got niced by some hoggy process, like Java or something. 

P 

 --
"To misattribute a quote is unforgivable." -- Anonymous 

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