Jim Logajan wrote:

The wavelength at ~2000 Hz is still about ~6 inches.

Closer to 7", but even so, but you need to have the microphone and the
node at which you're trying to cancel the noise VERY close together in
relation to the wavelength. Remember, 1/4 wavelength will be 90
degrees out of phase - you need something on the order of 1/16
wavelength or less to keep the phase mismatch reasonable, and ensure
that the microphone is picking up the same sounds as the ear picks up.
So even at 2K Hz, the microphone needs to be no further than 3/8"
from the ear - that's not going to happen - that's pretty much inside
your ear.

That's why the ANR rating of the Telex that's been discussed
previously is only 12 dB in the 2K Hz range, whereas the ANR rating of
the Bose/Lightspeed/etc. is in the mid to high 30 dB range at the
lower frequencies - as Thomas B. pointed out, that's a HUGE difference.

Two limiting factors would seem to be the distance between the
external microphone and the headphone, and the frequency response
of the feedback electronics.

Ummm, the external microphone (the one you speak into) is NOT the
microphone that's used for ANR sensing. There is a small microphone
INSIDE each earcup - as close to the ear as it can be placed. Using
the external mic. for sensing would be completely useless.

..... If your feedback response is slow, it
might actually pay to design it so the external microphone is
farther away from the headphone.

Again, putting the external mic. anywhere other than right near the
ear would make it impossible to do any sort of ANR.

--
Marc J. Zeitlin
http://www.cozybuilders.org/
Copyright (c) 2006