Jim Carr wrote:

"Bob Cain" wrote in message
...

Doppler shift is an unquestionable phenomenon, easily proved
from first principles. The thing is that it depends on a
sound source that is moving with respect to, i.e. _within_
the medium (we assume for this that the listener is
stationary with respect to it.) A piston with two
frequencies driving it and stationary with respect to the
medium otherwise will supposedly show those frequencies
modulating each other. The flaw is that the piston is not
in any sense moving within the medium, it is moving the
medium which is fundamentally different.


I don't understand. Explain to me how the speaker actually transfers the
energy to the air.

By pushing and pulling on it. The velocity of the piston is
transfered to the particles (voxels) of the air in front of it.

Then I watch the speaker diaphragm move. Sometimes it moves a lot, sometimes
a little. At one point during the movement is the sound wave created?

Whenever it moves, as it moves. The air has no choice but
to go along with it and that creates a velocity and pressure
wave that are in phase and related by the characteristic
acoustic impedence of air.

When you push on air, it moves and pushes on the air in
front of it but with some delay in the transfer. That's
what causes the speed of sound. The push propegates outward
from this bit of air to the bit in front of it and that's a
wave. Same when you pull on it.

Does that help?


Bob
--

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

A. Einstein