On Mon, 08 Jun 2009 00:13:00 -0700, David Nebenzahl
wrote:
On 6/8/2009 12:05 AM Richard Crowley spake thus:
David Nebenzahl wrote:
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
Well, I don't trust Wikipedia as far as one can throw it, but I trust
you, so I'll take your word for it.
I've just been investigating the isolation performance (claimed) of
active phones vs Etymotic passive ear buds, and here is the result:
http://81.174.169.10/odds/isolation.gif
Particularly interesting is the fact that at very low frequencies the
actives actually make the noise a bit louder. Once you get beyond
1kHz, of course, the active cancellers do nothing at all, while the
passives just go on getting better.
d