"Arny Krueger" wrote in message
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"Bob Cain" wrote in message

Bob Cain wrote:



Arny Krueger wrote:


Sorry Bob, but I'm not buying.


How about this, then:

1) It is the bulk velocity, the flow of air, at the rest position
that propagates out as the velocity wave.

Frankly, over my head.

2) The bulk velocity at the rest position is the same as the surface
velocity of the piston.

Frankly, over my head.

And, finally:

3) In the frame of reference of the rest position of the
piston, no Doppler shift can be observed.

That seems wrong, because the following is right:

In the frame of reference of the piston, no Doppler shift can be
observed.

That's the listener riding the same train as the whistle.


It seems to me that the train/whistle anology is not a valid one for this
purpose, because the train's motion and the whistle's sound are not being
reproduced by the same source, as happens in a loudspeaker producing a
complex waveform.
If you make a recording of the train approaching and receeding from your
listening position and play it back, you will hear exactly the same Doppler
shift you heard when the train went by, and this can be confirmed by
measurement. Conversely, if you make a recording of the whistle from the
train and play it back on a speaker moving down the tracks at the same
velocity as the train, you will hear the same doppler shift you heard from
the passing train, even though there is no shift in the actual recording.
The above logic actually has little to do with Doppler distortion in a
speaker, but the fact remains that the train/whistle in not a valid anology!
Making the assumption that a speaker produces Doppler shift because a
whistle on a moving train does is a rationalization not based on the actual
physical facts. Even moving the whistle back and forth in approximation of a
moving speaker cone is not a valid anology, because the whistle's back and
forth motion is not generated by the sound coming from the whistle, as it
would be in a speaker reproducing a complex waveform. You're actually
dealing with two separate sources producing separate waveforms. Mounting the
whistle on the speaker cone still won't be a valid anology because the
waveforms are not being reproduced by the same mechanism, two separate
sources again.
The only valid anology for what happens with a speaker reproducing a
complex waveform would be some other single entity that is capable of
reproducing the same complex waveform. Any suggestions?