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Powered Sound Canceling systems, possable?

  1. Nov 1, 2004 #1
    I am wondering if I can build a powered sound canceling system, one that could be used to mask, or cancel out sounds like snoring and a generator and a/c system?

    OR even better, to take one of those noisy cars with the load boom box sound effects and capture it and cancel it right out. (Love to see one of those AH when his system suddenly seemly goes dead….)

    As I understand it noise canceling is like the old automatic volume control in old AM radios where it would take a percentage of the sound and feed it back to cancel or avenge it out, that way very load stations and very low stations would be closer to the same level of each another.

    If I understand it right the modern sound canceling system captures the sound and then plays it back at the same volume at 180 degrees out of phase and thus cancels it out.

    IS this right?

    I am wondering if I can build a powered sound canceling system, one that could be used to mask, or cancel out sounds like snorring and a gerurator adn a/c system?

    OR even better, to take one of those noisy cars with the load boom box sound effects and capture it and cancel it right out.

    As I understand it noise canceling is like the old automatic vol
     
  2. jcsd
  3. Nov 1, 2004 #2

    chroot

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    You could build a system that would cancel sound heard at a particular spot or locus of spots, but you cannot build a system that would cancel sound at all spots. In fact, the system would make things noisier at most locations.

    - Warren
     
  4. Nov 1, 2004 #3
    So it would have to be decated to certian sounds?

    Is there one I can get or build?

    Rich
     
  5. Nov 1, 2004 #4

    chroot

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    No, it would not be dedicated to certain sounds. It would only effectively cancel sounds at one specific point in space. At all other locations, it would add additional noise.

    The sort of device you're considering building cannot be built.

    - Warren
     
  6. Nov 1, 2004 #5
    Sorry but I do think I understand.

    Say I want to cancel the noise of a gas generator, and fixed and repeating sound, can I not place a system next to it that would render it silent? It would then be tuned for that sound?

    Rich
     
  7. Nov 1, 2004 #6

    Cliff_J

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    Rich think of trying to stop the wind with a fan. If you had a 5 MPH wind and a fan that could blow air at 5 MPH then you could face it into the wind. Then if you stood right in front of the fan, the two winds would cancel each other out in that one spot. Move a little and you'll find more wind as the interference adds more air movement to anywhere surrounding that one spot.

    Lets say instead of a gas generator its a big electric motor. You could encase it within a big airtight box. Now you could make the walls vibrate out of phase to the leftover noise and have it help reduce the transmission of noise through the vibration of the structure. But this is steady-state noise with a repetitive pattern, the easy problem, and its reduced and not eliminated.

    Canceling the noise of a changing source by sampling it and inverting it suffers from the problems Warren outlined. It'll work for the headphones since they can block the tranmission to one important spot(your ears), but it won't help anywhere else.

    Some speakers are designed to exactly produce one wave and an out-of-phase wave at the exact same time as a means of controlling directivity and take advantage of the cancellation effects that helps reduce the sound in certain directions. They are called a di-pole configuration and work well for surround sound speakers since the sound coming directly from them is reduced so you hear mostly reflections off the walls that in effect will surround you with sound.

    Cliff
     
  8. Nov 2, 2004 #7
    Ok so I cannot enclose a gas generator in a case, put a mike hooked up to a amp and reverse phase the sound so that out side the case the sound would be cancel, so that for all persons outside the Generator case and van would hear nothing?

    And I can not have a outside mike and a sound system inside a car that would let me cancel the boom boom of other car INSIDE my own car?

    I do understand you can not cancel a sound at it source, once canceled there would be nothing to cancel..
     
  9. Nov 2, 2004 #8

    Cliff_J

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    Rich, its possible with a high powered array (read lots of mic and speakers) and about the only things you'd get is a very empty wallet the kid with the booming stereo stopping you asking you about all your gear in envy.

    Passive noise control (sealed airtight, structurally stiff, well damped - at least 2 of the 3) is orders of magnitude cheaper.

    Cliff
     
  10. Nov 2, 2004 #9
    Roger incase it.

    I was hope for a easier way.

    Thanks.

    And to kill thoise boom boxes on wheels, that was a dream....

    Rich
     
  11. Nov 2, 2004 #10

    russ_watters

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    Active noise cancellation of the type you are describing works great the other way - at the reciever: http://reviews.cnet.com/4520-3000_7-1017728-1.html

    But cancelling it at the source would require an array of speakers and microphones (and even then would cause hot and cold spots).
     
  12. Nov 3, 2004 #11

    Gokul43201

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    Active noise reduction is most effective in low frequencies. The systems that work reasonably at medium/high frequencies will cost you over $10K !!
     
  13. Nov 3, 2004 #12

    russ_watters

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    Good point - the reason for this is that when frequency is lower, the reaction time of the device is better. When the frequency of the sound gets too high, the device may be unable to keep up (oversimplification).
     
  14. Nov 17, 2004 #13
    New hybrid Accord and its active engine-noise cancellation

    Honda's new hybrid Accord uses active noise cancelling to quiet the engine:


    • ...Bose active noise cancellation generates opposite-phase sound waves that quiet the boom of the V-6 when in 3-cylinder mode. The waves are delivered through the audio system's speakers, whether or not the system is turned on.
     
  15. Nov 19, 2004 #14
    I tested a noise cancelling headphone while at Boeing. It was good at low frequencies (<1KHz) and the high frequencies were taken care of by conventional massive dampeners.
     
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