[Greg Locock]

wrote in
ups.com:

On Sep 15, 6:09 pm, 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, or in
some circumstances even a car park or a harbour-
if you can superimpose the correcting source
and he original.

'tiny' beng a somewhat loose term.

Well, even by your own somewhat extraordinary claim, "tiny"
is, in fact, a pretty well defined term: you defined it:

You state it can be done "if you can superimpose the
correcting source and he [sic] original." Fine. So let's
superimpose the original (4 engines on a 747 along
with the entire outside skin of the plane which is where
most of the in-flight noise due to turbulence comes from.

Tum te tum.


Now, using YOUR proposal, please explain to the gathered
audience how you would "superimpose the correcting
source and the original.) The original noise source is
the size of a 747. According to you, if you can superimpose
a second, out-of-phase 747-sized noise source on the
original you're done. So then only place I can put the
correcting source is NEXT to the 747, say at an average
distance of 100 feet, and place the person awaiting the
sonic remedy exactly in between the two.

And how's that working out for you?

Silly eeyore, I said that in /some circumstances/ if there was one source you
could colocate source and corrector. No thistles for you.


The problem with your rather extraordinary claim is that
in an airplane cabin, there is no single "noise source:"
it's all over the place and highly uncorrelated. I can correct
is over a very small area, to the limits of the wavelength
desired whicyh is defined by the highest frequency I
want to cancel, and that is a VERY precise definition of
"tiny."

Practically speaking we want to produce a noticeable reduction in the noise
level, across a useful volume of space. That can be done in a large arbitrary
volume, using several mics and several sound sources.

If we can go one step further and colocate source and corrector then it can be
done more simply. Alternatively if the corrector can be introduced into the
primary noise path then it is also easier.

This has been demonstrated many times.


Cheers

Greg Locock