From: Reinhard Zwirner
Subject: Re: Sound cancellation technology?
Date: Thu, 26 Dec 2002 14:18:07 +0100
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Kevin Aylward schrieb:
[Noise cancelling system for a bedroom]
> Sure, in practice this is a very difficult thing to do, but in principle
> or theoretically, it is possible. Its called feedback. Sound
> measurements are continually made, and if any signal is non zero, the
> loop forces a correcting sound to cancel the disturbance. In effect, the
> room impedance is made to approach zero so that it S/C any sound coming
> into the room. It does not matter what the the input source spectrum is,
> as long as it is within the correction BW of the loop.
Today's (active) noise cancellation is done by acoustical addition of
the 180 degrees phase shifted noise signal to the original noise.
Therefore, you need to know the noise signal for every place where you
want to cancel it.
Have a look at those noise cancelling headphones. Though the noise just
has to be cancelled in _one_ certain place you'll get significant (about
20 dB) attenuation only for a limited frequency range (roughly 100-800
Now, what about sound cancellation for a whole bedroom? Do you only want
attenuation of the outside noise? Or would you like to cancel the
noise of the person beside you, too?
At least, you would have to know the inner and outer noise signals. Then
you need a precise model of the room's sound field and a corresponding
real-time signal processing and "anti-sound" reproducing system.
I think in practice it's almost impossible to fulfil the OP's desire
for a - by noise cancellation - quiet bedroom in a noisy environment.