Active Noise cancellation in 3D space

Discussion in 'The Projects Forum' started by devalvyas, Jan 27, 2012.

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  1. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    Hi,

    recently i have been reading about Active Noise Cancellation. i am wondering whether it would be possible to build an ANC system that can cancel out unwanted noice in open space, like..snoring in bed room..

    Now i know that ANC works very well for a small space like in case of Headphones..but what about open space like bed room?

    i have read about ANC system in higher end cars that reduce the engine noise in the car. so it is definately possible to implement ANC in open space ( not just for headphones...)

    i have seen this ADVT from this car maker which shows ANC at work..though i dont know whether actually it will work like they show it in the ADVT.

    http://www.youtube.com/watch?v=XoB2zhgcQqY&feature=related

    can any one please guide me how possible or how difficult it can be? as of now i am very much inclined to build such a system...

    thanks
     
  2. crutschow

    Expert

    Mar 14, 2008
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    It's rather difficult to do in a space because of the different phase shifts of all the frequencies as they are reflected around the room. A speaker to cancel the sound that is coming from a different location in the room would generate a different phase shift for many of the frequencies as compared to the source, so cancellation would be poor. I believe those car sound cancellation systems use a complex computer program to control the phase shifts and multiple speakers and microphones to provide some amount of sound cancellation (but that's only my guess).

    All in all I believe it's a difficult problem which requires an advanced understanding of acoustics so beware of that if you attempt the task.
     
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  3. Audioguru

    New Member

    Dec 20, 2007
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    You can cancel very low sound frequencies because their wavelengths are long.
    You can cancel some snoring sounds if the noise-maker, your ears and the speaker are at certain distances depending on the wavelengh of the frequency. Some frequencies will cancel but other frequencies or their reflections will add and produce acoustical feedback howling which is worse than snoring sounds.
     
  4. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    thanks a lot guys...projects is my hobby...so will definately implement it.

    i have come across some basic circuits on the web for phase inversion...will try to implement in a very basic setup and see the results....will keep on comming for help and updates.....
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    Phase inversion will not work if the room is not an anechoic chamber and the distance from the snorer to the speaker and from the snorer and speaker to the listener changes slightly.

    To prove it set a microphone, amplifier and speaker in the room. Turn up the volume until there is acoustical feedback howling. Then reverse the wires to the speaker for phase inversion. Does the feedback stop? No.

    When I worked with digital echo cancellers used in tele-conference systems I thought of using it to cancel local feedback on a PA system. I never tried it but it might work to cancel snoring sounds.
    It will build a model of the room acoustics including the phase of many frequencies and all the echoes in the room. Then it cancels them from its output. But it is complicated.
    Cirrus Logic has this IC:
     
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  6. msalvati

    New Member

    Jan 28, 2012
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    In this particular application you are a lot closer to the source than you are to the walls and ceiling, and you have sound absorptive material on the other significant surface (the bed). You are hearing a lot more direct sound than reflected relative to when you are, say, listening to the TV in your living room. So I would say it's more possible than you might think. You would need very fast DACs and an extremely adaptive ANC circuit, which is kind of expensive. On top of that (if your wife snores anything like mine anyway) you would need a solid sound system with good low frequency response. You would need to place the speaker(s) somewhere in the space between your ears and her throat and tweak a lot of parameters from your sleeping position while she is sleeping.

    Ok, so not very feasible, but not out of the question either. :p
     
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  7. Audioguru

    New Member

    Dec 20, 2007
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    Since medium and high frequency phase changes with a small change in distance then you will need a spacing contraption between the snorer, your ears, the microphone and the speaker. Then a simple system will have a barely noticeable reduction of the snoring sound and a fairly complicated system might reduce the sound fairly well.
     
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