# Noise Theory



## Squintje (Oct 2, 2010)

Some weeks ago I came up with an Idea, We all have those devices that make more noise than we want them to and in that way interfere with our music/film-tracks. I figured that if I would record the sound that those devices are making (in My case I would only take the sound of my computer) and run it through my speakers exactly out of phase (at my listening position that is) i would be able to elimkinate most of the noise

P.S. I had this idea after I did something similair by adding another set of small speakers to eliminate amplifier generated noise


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## nova (Apr 30, 2006)

Interesting idea. Have you tried it yet?


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## Theresa (Aug 23, 2010)

Its done with noise cancelling headphones usually. I think this is because the distance from the ears is close to constant with headphones while with speakers the distance and therefor the phase would vary.


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## lcaillo (May 2, 2006)

This kind of phase cancellation has to be done in real time. If you record and play back noise it will not be in phase with the real time noise. You will simply add to the noise.


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## Harpmaker (Oct 28, 2007)

Some automobile companies are using this to combat road and engine noise in the passenger compartment. 
http://www.motorauthority.com/news/1023518_toyota-develops-noise-cancelling-system-for-cars

I think Cadillac has been doing this in the U.S. market for some years. Sorry, couldn't find a link right off the bat.


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## lcaillo (May 2, 2006)

There is nothing wrong with the technology, but you have to have some difference in levels to sample the noise and must feed it back with short delay.


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## Omega (Apr 21, 2010)

Well the theory is more then 50 years old, but implementation is tricky. As mentioned many automobiles and headphones already using noise cancellation. The key is to get the timing right for the out of phase signal, otherwise you will be cancelling or adding more noise than you wanted.


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## StereoClarity (Apr 22, 2008)

Not only that but unless you can vary the input it wont work. Lets say your speakers need to play the out of phase noise at 80db to have it cancel perfectly. What happens when you turn up the volume on your unit 20db to watch a movie? You'll play that noise back 20db louder and it wont really cancel. It will probably be even louder.


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## Squintje (Oct 2, 2010)

I understand that 
i wil have to playback in real-time, I knid of forgot to mention that:innocent:. However this idea will have to ge back on my list of to-do's because while fiddling with this set-up (mic distances, levels etc.) Ifound a bigger noise source:scratch:; my tft-monitor is doing some serious damage to the sound, I know it is the mon because whenever I remove it's cable from the pc the noise goes away, So i'll seperately ground that later when I get the time 
so anyway back on topic, since it will take a while before I try tis on out Imho it can /close


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## RTS100x5 (Sep 12, 2009)

If this could only cancel out the GF / WIFE / MOTHER in LAW talking in the car , you may have your next $1,000.000 idea ....


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## lcaillo (May 2, 2006)

It can. Noise cancelling headphones can do exactly that. The cannot, however, keep the W/G/M-I-L from ripping them from your head and beating you with them.


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## HNiels (Aug 20, 2011)

Definitely an interesting idea, but unfortunately I believe it would be difficult to apply to real world applications. Not impossible just difficult, and probably expensive.


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