"Bob Cain" wrote in message
...


Porky wrote:


I would go so far as to say that anything generating sound and in
motion
relative to the listener will generate Doppler shift, but in the special
case of a speaker reproducing a complex waveform consisting of more than
one
pure tone, the velocity of the source relative to the listener is
effectively zero, and therefore no Doppler shift will be generated,
because:

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. Where the transfer function is flat
in the Fourier sense, nothing mixes. That is what it all
boils down to in the end. That is not at all the same as
the standard argument because it won't happen in a tube and
the standard argument says it will.

Correct, for the simple reason that a speaker reproducing a complex
waveform does so coherently, however, if you add another force vector such
as moving the speaker, Doppler shift will occur in the entire reproduced
sound, not just the HF component, this is how a Leslie speaker system works.

Also, a single tone cannot produce Doppler distortion so I
am definitely wrong about that. Yikes! I see why. The
definition I've been bandying about for a linear system is
actually the definiton of a linear, time invariant system.
The system we are considering must be time variant in terms
of the impedences involved. This is getting wierd.

Can this yield what I've been asking for, a general
expression for far field pressure as a function of piston
velocity that includes the Doppler distortion? I'm not sure
yet, but I'll be thinking about it. It is straightforward
for any two frequencies, however, and is left as an exercise
for the student. :-)

It's just as straightforward for a complex musical waveform, you don't hear
any Doppler shift unless you start moving the speaker.
Here's another way to look at it, there is no Doppler shift in a speaker
because the speaker is not the source of the sound, the air it compresses
and rarifies is the actual sound source, and since moving air can't cause
Doppler distortion (try having a friend whistle a steady tone in a variable
gusty wind if you doubt it), there is no Doppler distortion introduced by a
speaker.