[Eeyore]

Richard Crowley wrote:

"Greg Locock" wrote ...
Eeyore wrote :
No. It won't work. You can only ever meaningfully cancel
sound over a tiny area (volume).

Such as the entire cabin of a passenger aircraft,

For low frequencies, perhaps. See the other posting on
first-hand experience with active noise cancellation in his
vehicle.

or in some circumstances even a car park or a harbour-
if you can superimpose the correcting source
and he original.

If you can actively counteract the noise at the source (as
an enclosure around a generator, etc.) perhaps. Only a
very limited cases of effective application, at least at this
stage of development.

You can't even do it at the source since it's impossible to precisely co-locate
the 'cancellation signal' with the noise source. never mind issues about
transmission modes etc.

It's easier with lower frequencies of course because the wavelength is longer so
it's more tolerant of the absence of precise co-location. I believe the method's
been used to reduce LF noise in ships' engine rooms. Note *reduce* not
eliminate.

Graham