[Arny Krueger]

"David Nebenzahl" wrote in message
.com...
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?


You're missing the fact that it becomes progressively more difficult to
control the phase of the cancellation signal as the frequency increases.

Elsewhere in this thread I think I saw a discussion of the fact that
wavelengths get shorter as frequency increases. As the wavelength gets
shorter, the harder it gets to control the phase of the wave at any point.
Think of picking up a needle with a small tweezers. Think of picking up a
needle with a large bolt cutter.

For example, at 100 feet, a sound wave is about 10' long. If the microphone
that picks up the noise that is to be cancelled out is a fraction of an inch
from the point where the cancellation is to take place, that fractional inch
distance causes very little error.

At 10 KHz, a sound wave is about an inch. Now, you have to somehow put the
microphone far, far less than one inch from the point where the cancellation
it to take place for equal effectiveness as you easily obtained at 100 Hz.

If you don't accurately control the timing of the cancellation signal, the
system oscillates.

So, the difficulties involved in canceling high frequencies are due to the
laws of physics. Part is due to wavelength effects, and part is due to the
requirements for a system that is based on negative feedback to be stable.