David Nebenzahl wrote:
On 6/7/2009 10:04 PM Richard Crowley spake thus:
David Nebenzahl wrote:
Mr.T spake thus:
and reinject a perfectly matched inverted copy in exactly the same
spot,
Trivially easy, no? A simple phase inverter oughta do the trick.
No. Because it will almost instantly turn into a closed-loop
feedback oscillator and likely deafen the wearer. If it were
as easy as simple signal inversion, we would have had noise
cancelling headphones decades sooner.
Hmm; that seems counterintuitive. Not disputing you, but I thought
that only positive (i.e., same-phase) signals would cause that kind of
feedback. An inverted signal should (nearly) cancel the original
signal, n'est-ce pas? What am I missing here?
The space inside the headphone forms a resonant cavity
and a broadband microphone - amplifier-speaker system
would seek the most resonant frequency within milliseconds.
Anyone who has ever operated a sound reinforcement
(PA) system knows the effect.
Noise cancellation systems work by sampling the waveform
and independently synthesizing an inverted copy of the noise
waveform.
http://en.wikipedia.org/wiki/Active_noise_control