[Harlan Messinger]

Don Pearce wrote:
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.

What are the units of the vertical axis?