Specifc warble sound cancellation DIY - Home Brew

So,  I've been working on a project to identify potential high frequency 22khz+ on behalf of a new dog in the house.  While chatting about it to a relative we began discussing his situation.  This field is new to me but I have experience in .Net programming.  Here's the situation:

He works in a booth in a paper mill.  The machine creates a repetitive sound ranging from about 600 hz to 1800 hz in a saw tooth pattern over 5 seconds.  I know this as he recorded it and I can look at it in Adobe Audition.  There are some "harmonics" well above this as well but those look minor in comparison.  The problem is that to cut the mind numbing psychosis inducing (that explains a lot about him BTW) noise the booth has to be sound proofed and he has to wear muffs.

 If we could create some type of noise cancelling FFT algorithm that can run on the PC in the booth or some other mini PC or device that could sync with and cancel out this pattern it would be really cool.  I was thinking of one of the small speaker type boxes or larger desk top speakers if that kind of power is needed as output.  From the reading I've been doing it looks possible and I don't think I would need the processing power to overcome ALL noise just the known frequencies.

I'm looking for any thoughts if this is possible on a DIY basis for a .Net programmer or if a device / software package already exists?  I know it is easy as pie for recorded sounds but how to transmit the cancelling sound to make it less perceptible by the guy in the booth?

Thanks!

Zip
zipnoticAsked:
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David Johnson, CD, MVPConnect With a Mentor OwnerCommented:
The answer is that there isn't a viable method of doing what the asker wants which is the elimination of all extraneous noise except for noise that is different than the normal.  
1. software is too slow and
2. Any noise cancellation will also cancel the not normal noise.
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David Johnson, CD, MVPConnect With a Mentor OwnerCommented:
This would be better done in hardware.  The problem is you have an input wave and to cancel it you have to inject a wave of the same amplitude and frequency inverting the phase to cancel out the offending item. This is normally done by using a microphone, your phase inverter and a speaker output.  You want to do this in near-real-time or you will simply be adding more noise.
http://electronics.howstuffworks.com/gadgets/audio-music/noise-canceling-headphone3.htm
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zipnoticAuthor Commented:
I'm with ya but can you suggest a hardware solution worthy of DIY use?
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Dr. KlahnConnect With a Mentor Principal Software EngineerCommented:
Mike up the booth, run the sound through a signal inverter and feed it back into the booth through an audio amplifier.  Using a dual channel scope, check the noise signal against the cancellation audio at the speaker and adjust until they are out of phase.  If necessary install a phase shifter after the signal inverter so that the signal when it hits the speakers is exactly out of phase.

Whether this will actually work in practice is problematic.  Noise cancelling headphones operate on this principle, but the enclosed space (the ear canal) is tiny and there is little phase shift across it.  Phase cancellation may not work if there is significant phase difference across the booth at the frequencies in question.

It may be less expensive and more productive of everyone's time to simply buy the workers noise cancelling headsets.
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David Johnson, CD, MVPOwnerCommented:
Mike up the booth, run the sound through a signal inverter and feed it back into the booth through an audio amplifier.  Using a dual channel scope, check the noise signal against the cancellation audio at the speaker and adjust until they are out of phase.  If necessary install a phase shifter after the signal inverter so that the signal when it hits the speakers is exactly out of phase.

Unless the noise is constant, no change in amplitude or frequency the above won't work. Reason being that the noise from the audio amplifiers will be delayed like an echo.  You also have to consider the source of the noise, the location of the speaker and the location of the persons ears.
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zipnoticAuthor Commented:
The booth is about 3 feet by 3 feet.  Just big enough for a shelf that acts as 'desk' for a PC and a stool.  I'm told it looks more like a phone booth from Dr. Who...  The trouble with noise cancelling headphones is that the warble in question is normal for the machine and can be ignored.  If the machine starts to make other sounds then that is the first indication of a problem that needs to be checked out.  This thing is a monster that runs miles of paper through it per hour.  Noise cancelling headphones like Bose would cut all noises, not just the specific frequencies of the 'normal' warble.

I would say the operator's ears are confined to a cube about 18 x 18 inches.  Not sure if this info means it may be doable or not?

THanks
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zipnoticAuthor Commented:
Thanks for thoughts
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