Chris Hornbeck wrote:

On Thu, 19 Aug 2004 01:23:56 -0700, Bob Cain
wrote:


Tell me if this is equivalent or not: There is Doppler type
mixing between two frequencies if and only if the pressure
in the far field due to them is a different function of the
velocity of the piston.


No, but the distance between them may vary....

Darn, you did it again. Why not? I'm looking at the case
where there is Doppler mixing but no relative motion. Think
of the compound system of a speaker emitting a tone and
swinging on a long rope. The rest position of that system
is when the speaker is off and it is sitting at bottom dead
center. When there is no motion between the rest position
of that compound Tx and a Rx , Doppler mixing will still
occur and I maintain that is just because of the low
coupling of the LF swing to the Rx.



Where the transfer function is flat
in the Fourier sense, nothing mixes.


No, that's AM.

Not talking about that here. Flat is the condition of a
piston in a tube which is the only condition (other than an
oscilating infinite plane) when Doppler mixing won't occur.
In any other configuration there will be a coupling
function that is not flat between Tx ane Rx and that is the
source of the mixing in the far field.


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein