Re: Ambient (external) Noise Neutralisation
From: John Jardine (john_at_jjdesigns.fsnet.co.uk)
Date: 07/16/04
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Date: Fri, 16 Jul 2004 02:20:40 +0100
Rich Grise <null@example.net> wrote in message
news:1133230.MCojpsmesD@entheos.thunderbird.ops.dsl-verizon.net...
> on Wednesday 14 July 2004 06:06 pm, John Jardine wrote:
> > Paul Burridge <pb@notthisbit.osiris1.co.uk> wrote in message
>
> >> 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)?
>
> > 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.
> ...
> > 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 :-)
>
> 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.
>
> --
> Cheers!
> Rich
>
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
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