"Porky" wrote in message

"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.

It's just a matter of scale. The world is the equivalent of the chassis of
the speaker. The effective diaphragm of the whistel is the woofer cone.

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.

And, if the train travels in a really large sine wave, it's a lot like a
speaker.

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.

Agreed..

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!

Have it your way, if that's what you want. I'm not buying that there is a
substantial difference.

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.

False.

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.

Let's presume that the whistle is actually a mechanical horn with a
diaphragm which of course undergoes the identical same Doppler shift as an
air horn or steam whistle. The diaphragm of this mechanical train horn
follows a similar path, different only in scale, from a woofer cone.

You're actually dealing with two
separate sources producing separate waveforms.

Doesn't matter because the diaphragm that actually makes the sound follows a
similar path.

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.

This turns out to be an irrelevant distinction. What matters is the path of
the diaphragm, or its moral equivalent.

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?

A train, car or a boat.