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Ambient (external) Noise Neutralisation

Discussion in 'Electronic Design' started by Paul Burridge, Jul 14, 2004.

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  1. Hi,

    I while ago I read something about a device which amplifies ambient
    environmental noise, phase reverses it and replays it in real-time to
    restore peace and quiet by largely neutralizing the sound waves from
    the offending source. Has anyone had any experience of this device -
    if it exists? How effective is it (if at all)?

    p.
     
  2. Leon Heller

    Leon Heller Guest

    Active Noise Control (ANC) works quite well for low-frequency noise. You
    have to wear headphones, of course. Tank crews use ANC systems made by
    Thales (was Racal) Acoustics.

    Leon
     
  3. Joerg

    Joerg Guest

    Hi Paul,

    It's not just tank crews, this kind of noise cancelling headsets is also
    very popular with pilots. They are in the range of a few hundred Dollars
    so you can find them also in general aviation and there are several ads
    in Flying Magazine. I was told they are especially helpful in muffling
    that constant rumble of the engine and prop on long flights.

    Regards, Joerg
     
  4. Probably a stooopid question, but why the need for phones? Could not
    the same effect be achieved by placing full-size speakers between the
    noise source and the listener?
     
  5. Ben Bradley

    Ben Bradley Guest

    I've heard advertisements for such headsets by Bose.
    This may be possible, but there are a lot more problems with a
    speaker in an "acoustic space". This could only be effective for one
    point (really only one ear), when you move your head from that point
    the noise would increase. Noise-cancelling headphones have a small mic
    on the outside of each earpiece, and for lower frequencies the outside
    and the ear are practically at the same position, so it's easy to
    cancel the low frequencies, just invert the signal and adjust its
    volume in the headphone for minimum noise. Higher frequencies are
    attentuated much more by the earpiece shell, and don't need
    cancelling.
     
  6. John Jardine

    John Jardine Guest

    20 years ago attended a lecture given by British Gas researchers wrt their
    prototype noise cancelling system. Their system was designed to reduce the
    low frequency noise generated by a large natural gas compressor terminal.
    People living within a 2 mile radius of this monster had been getting rather
    fedup with the 24/7/365 bass rumble.
    The system when switched on managed to reduce the offending noise levels by
    about 10dB. This doesn't 'sound' much but equated to many kW of offending
    noise being cancelled. Surprisingly the guys were digitising to only 8 bit
    levels.
    Noise canceling was provided by about (ISTR) 20kW of audio speakers along
    with a number of sensing mic's circularly disposed around the main noise
    emitter , which was a 100' processing stack. Organ pipe resonances were
    guessed at as being at work but the rebuild costs would have been £10's of
    millions, so the research project was quickly implemented.
    The system worked as the number of complaints dropped sharply.
    Privatisation and the subsequent bean counting saw the end of these few Bgas
    engineering research stations. A sad loss for the whole country.

    What I've never really been able to figure is where the cancelled energy
    goes to ?. I.e noise energy + noise cancelling energy apparently gives zero
    energy :)

    regards
    john
     
  7. Ken Smith

    Ken Smith Guest

    The energy goes straight up. Basically you've created a directional
    radiator.
     
  8. Rich Grise

    Rich Grise Guest

    I wondered the same thing, and when I thought it through, it goes
    to heating the voice coil of the "noise-cancelling" speaker! See, sound
    is compression and rarefaction waves. The speaker is being driven with
    a signal that causes it to move in a way such that when it's trying to
    make the compression part of the wave, it's being met by the
    rarefaction part of the wave you want to cancel. What does this do
    to the speaker? Would it affect the force of the back emf? Would it
    change the effective impedance? I think it would do something to the
    speaker impedance (conceptually similar to vswr in ham lingo), and
    the power that's sucked out of the air is dissipated in the amp's
    output stage, and the speaker coil.
     
  9. MNQ

    MNQ Guest

    What about wife (not white) noise? Will it muffle that?
     
  10. Jim Thompson

    Jim Thompson Guest

    On Thu, 15 Jul 2004 02:06:04 +0100, "John Jardine"

    [snip]
    Global warming ?:)

    ...Jim Thompson
     
  11. John Jardine

    John Jardine Guest

    Cos you guys had answers I had to actually start -thinking- myself.
    Damn:-(.
    My take on it eventually resolved to a plastic pipe with a rubber membrane
    in the middle driven at both ends by air movers. Membrane displacement by a
    position transducer (e.g the mic' and an ear). Means I could get it down to
    a mentally manageable 1/10th Hz. Had to smile when I realised all 3 answers
    fitted easily into the model.
    The membrane acts as a section of a 'plane' square-on interface between the
    incoming noise and the anti-noise
    1) There's a lot of energy damped out at the membrane interface. I.e
    energy is needed to servo the membrane to a near zero movement. Severe non
    linear distortion (large +/- pressure fluctuations) of the pressure/anti
    pressure pulses must occur within an action layer near and at the
    interface. This creates frictional energy loss and will end up as hot air to
    help with Jim's global-warming.
    2) The tube is closed at the membrane interface so all pressure fluctuations
    must in this instance occur within the tube body, but ... under free field,
    fresh air conditions the pressure fluctuations at and near the interface
    between the noise and anti-noise have the freedom to move at 90degrees
    outwards from the face-on noise/anti-noise in an effort to equalise the
    distorted pressure changes and yes, will result in Ken's directional
    radiation.
    And yes. The speakers themselves must physically, hence electrically, feel
    the effects due to the changed acoustic 'resistance' in the vincinity of the
    noise/antinoise interface.
    They can do this directly if the sources are physically very near (near
    field condition?) or at a distance via standing waves that must set up due
    the impedance mismatch that is evolving at the noise/anti-noise junction.

    A real world though presents multiple reflections, sections-of-spheres
    wavefronts, wide ranges of frequencies and large phase/frequency/attenuation
    variations. Plastic tube therefore, has decided that no matter how much DSP
    is thrown at it, general-purpose noise canceling is physically impossible.
    Tube also whispers in my ear (can't shut her up now) that I won't be easily
    predicting how the losses are shared out.
    But from knowing the single minded ways of nature, I'll warrant most of it
    ends up as heat.
    regards
    john
     
  12. Roy McCammon

    Roy McCammon Guest

    It is redirected upward. Just guessing now, but I'd guess that the main
    lobe would be around 30 degrees above horizontal.
     
  13. Roy McCammon

    Roy McCammon Guest

    The noise canceling works best when the the noise
    hits the ears as a plane wave and you can create an
    appositely phased plane wave. Of course, the sound
    entering the ear is pretty much a plane wave when it
    hits the ear drum and by wearing the headphones, the
    source of the canceling sound wave maintains a fixed
    relationship with respect to the ear drum.
     
  14. Ken Smith

    Ken Smith Guest

    Eventually, yes.

    I say the energy goes mostly straight up.
     
  15. Joerg

    Joerg Guest

    Hi Paul,
    That's not a stupid question at all. The idea of noise cancellation
    without the need for headphones has been researched in cars, to reduce
    wind, tire and engine noise. But I have not heard of any major success
    stories here. Which doesn't mean that much since to me a car is just a
    basic means of transportation so I don't follow all the trends.

    There are some technique to cancel noise and vibrations as close to the
    source as possible and there have been major breakthroughs here. One
    example was the counter-rotating shaft that Ford introduced decades ago.
    This shaft didn't drive anything, it was just there to cancel some noise
    and vibration effects. Then there is the synchronizer on multi-engine
    aircraft. It attempts to sync the props so that the frequency of most
    beat effects drops to zero or at least really close to zero.

    Regards, Joerg
     
  16. John Jardine

    John Jardine Guest

    Can't just now put my finger on it, but there's a smell-of-truth about the
    30degs aspect.
    From the symmetry I'd maybe think of a pair of opposite 30% sidelobes (+ in
    turn their own 30deg sidelobes et al). I get the sinking feeling I should
    really be looking in the aerial handbooks :)
    regards
    john
     
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