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Project Help and Ideas » potential project - Active Sound Cancellation

January 20, 2011
by huzbum
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I was just wondering if anyone has put any thought and/or research into sound cancellation?

It's probably just my ignorance and tendency to oversimplify things, but wouldn't it just be a matter of determining the wave length and amplitude of sound waves and reproduce them off phase according to their frequency?

I'm sick of wind noise in my motor cycle helmet... This may be my next project (not that I have time for it anytime soon)

January 20, 2011
by Ralphxyz
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I have been thinking about Active Noise Cancellation for years now. I sure hope you come up with something.

In the commercial world some of the most successful applications of active noise canceling have been fan noise canceling in Air Conditioning systems. I believe your theory is correct I do not know why it would not work, but then again I have little education and have to keep things simple. Obviously a constant tone would be easier to work with rather than a veritable buffeting noise but it just seems like it should work. Google "Active Noise Canceling" to get some interesting reads.

Ralph

January 20, 2011
by huzbum
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It seems that most of the noise cancelling devices are based on the assumption that the microphone and speaker must be close together, so that the wave is passing the speaker and microphone at the same time... it would seem to me, that if you had a directional microphone and a set distance between the microphone and a speaker, you could use the known time it takes the sound wave to travel that known distance for processing.

It's be like a mute ray... With the flip of a switch a screaming child becomes a young Charlie Chaplin. Anyone on the mic side would hear an echo, but anyone with the speaker directly between them and the sound would be hearing it's silencing effect.

January 20, 2011
by Ralphxyz
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That is similar to my thinking. One problem (I believe) is that if you do not get phase exactly 180˚ opposite the noise you can/will add to the noise instead of decreasing it.

I have some really crazy ideas about doing this, which seem to me plausible but in reality (outside my mind) probable will not work.

How constant is the wind noise in your helmet?

Is it just dependent on speed or is it worse on a windy day?

Ralph

February 02, 2011
by huzbum
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The wind noise varies quite a bit. Between gusts from other cars, normal wind, and changes in speed, the pitch and amplitude change quite a bit.

So I've put just a tiny little bit of thought into this... In the time it takes a sound wave to travel an inch, we have about 1100 clocks. 1/2 inch about 550 clocks. This would seem to me more than ample processing time to create an inverse phase signal for a speaker.

I think we could use a variation of pulse width modulation to determine the duration of each sound wave into a mic, wait 0.000074738 seconds + duration then output a pulse for the same duration out of a speaker 1 inch from the mic.

The tricky part would be fine tuning.

I also haven't thought about how to store multiple pulses... but if my calculations are correct you're not going to need to store more than one or two pulses at a time depending on frequency and distance between mic and speaker.

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