[Don Pearce[_3_]]

On Thu, 04 Jun 2009 13:25:42 -0400, Harlan Messinger
wrote:

Don Pearce wrote:
On Thu, 04 Jun 2009 13:04:08 -0400, Harlan Messinger
wrote:

I've got a pair of Sony MDR-NC6 noise-canceling headphones. They do a
great job of removing the low, rumbling sounds, like the office copier
or the continuous roar on a plane. On the other hand, on a plane, they
do virtually nothing about the higher-frequency airplane sounds
(hissing, whining), and they add their own hiss.

I asked someone selling expensive Bose headphones at Denver Airport
about whether their phones cover the higher frequency ranges, but he
turned out not to be informed on such a technical topic.

Is this issue with my Sony phones normal--are only the lower frequencies
targeted by today's noise-canceling products? Or are there products that
I can rely on to cancel a wider range of frequencies and, ideally don't
introduce significant noise of their own?

It is not a case of low frequencies being targeted, but really that
dealing with high frequencies is much more technically challenging. If
you really want good HF isolation, forget noise cancelling and buy
some passive ear defender types.

Really, it's more challenging? I would have thought one approach would
be applicable across the spectrum. Do you know of any resources I can
look at that will explain the technical details? Meanwhile, thanks for
the tip.

No need, I can explain. It is all a matter of wavelength. Outside the
headshell is a microphone that picks up the environmental sound which
must be suppressed. An inverted version of that sound is played inside
the headphones along with the wanted signal. The idea is that the
inverted sound cancels out the actual sound. For low frequencies it
works nicely, because the inverted and direct sounds line up nicely.
But at higher frequencies the wavelength becomes sufficiently small
that alignment becomes impossible, For example at 10kHz the wavelength
is about one inch, so if the microphone is half an inch from the
speaker (almost inevitable), there will be a half wave error, and far
from cancelling the sound will actually reinforce. Below 10kHz, the
situation is not that bad, but bad enough that good cancellation is
impossible.

d