"Ben Bradley" wrote in message
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On Mon, 16 Aug 2004 21:57:16 -0500, "Porky" wrote:


"PenguiN" wrote in message
. com...

If you still don't believe that this scenario validates the
train-whistle analogy, why not make the bass waveform move at the
speed of, oh say a train, and have the high pitched signal on top of
it be, oh say the sound of a whistle.

Doesn't apply, the train and whistle are supplied by two separate sources
of
energy.

Have two amplifiers, one for the low frequency connected to the
speaker through an inductor, another amp outputting the high frequency
connected to the speaker through a capacitor. Thus the low and the
high are supplied by two separate sources of energy.

Actually, isn't the train whistle powered by steam from the same
boiler that powers the wheels, moving the train? What's up with that?

But seriously, for Bob and other anti-doppler-distortion folks, how
does cone-and-frame movement cause doppler while cone movement only
does not? What is it about the frame that causes doppler?

If you put the speaker on a shaker table, run 1kHz and 50 Hz
through the speaker, move the shaker table at 50 Hz opposite the phase
to the speaker so the cone only moves at the 1kHz rate with respect to
the air, will it generate doppler distorion? According to my
understanding of Bob's position, it should.

The whole thing hinges on the single complex waveform vs multile simple
waveform idea. If the two sound signals are mixed into a single complex
signal and that signal is what drives the cone, there are not actually two
discrete signals being converted to sound, there is a single complex signal
that our ears interpret as two different sounds. This isn't an exact anology
either, but it's better than the train/whistle thing.
If you play a CD of an orchestral performance, there is not one piece of
analytic equipment man has ever designed or built that can completely
separate and isolate the individual instruments, but any human with decent
hearing and a bit of training can. This would seem to support the argument
that the speaker is producing a complex waveform as a whole, not as a bunch
of individual tones. If this is true then there can be no Doppler distortion
in a speaker that is reproducing the signal in a linear manner. Doppler
shift can only exist when one vibration is riding on another vibration or
source of motion, it cannot exist when all vibrations are being produced as
a single complex waveform.
That is as simply as I can explain it. I'll go either way with the
Doppler argument if someone can prove that the speaker isn't generating a
complex waveform, but is generating a series of simple tones.
The fact that I drove my big speakers with a 20Hz tone combined with a
500Hz tone at a level that the online proponents of Doppler distortion said
would produce audible Doppler shift, and it didn't, tends to make me think
that Doppler shift doesn't occur under those conditions.