Subject: Re: [linux-audio-dev] sound cancelation with anti-sound
From: Peter Hanappe (peter_AT_hanappe.com)
Date: Thu Apr 04 2002 - 16:16:45 EEST
On Thu, 2002-04-04 at 12:46, Maarten de Boer wrote:
> i have been thinking about the following before, and now
> that i read about it on slashdot, and it got me thinking
> a bit more.
>
> the issue is sound cancelation. the article mentioned
> on slashdot,
> http://www.newscientist.com/news/news.jsp?id=ns99992094
> talks about a (very expensive) general approach of
> removing sound with anti-sound.
>
> personally, i am more interested in a very specific
> noise: the noise of the cooling fans in my pc.
>
> i am sure it would be possible to have a background
> application running that uses the soundcard (could be
> a cheap one) to do this. the sound of each fan has
> almost constant pitch, and i suppose it is harmonic.
>
> the application should record the noise, analyse it:
> determine the pitch of each fan and seperate the sounds
> of all fans, invert each signal, and play it back.
> this should work reasonably well as long as the pitch
> doesn't change. by reanalising constantly, any changes
> in the sampled signal have the be analyses and added
> to the canceling inverted signal.
>
> the problem is the sound seperation. The application
> should be low on CPU usage, so I guess time domain
> processing is the only option. but we are talking about
> let's say 3 different signals with rather simple content...
>
> any suggestions / remarks?
Noise cancelation works if the anti-noise signal is *exactly* the same
as the noise signal with a 180 degress phase inversion. This is easier
to achieve with low frequencies (your fan probably emits frequncies that
are rather high). Also, since the two audio sources are at different
locations, theoretically this works at only one point in space. So it's
easiest to put the mic and the speaker right next to the ear. Otherwise,
you'll just have two noise sources instead of one ;)
Peter
> maarten
>
>
>
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