high frequency with a low frequency and applies that complex electrical
waveform to the speaker voice voil. The result is NOT a high frequency tone
riding on a low frequency tone, it's a single complex waveform containing
elements of both tones, and thus there is no Doppler distortion.

That doesn't make sense. To generate a frequency, the speaker has to
move back and forth at a certain rate. The higher the frequency, the
faster the rate at which it moves. Surely if we want to hear both
frequencies at once, the speaker has to vibrate at both speeds at once ?

If you look at a low frequency sine wave with an amplitude against time
graph, and SUM a much higher frequency much lower amplitude wave to it,
surely you'll see the original low frequency sine wave, but the line
itself instead of being a smooth sine wave, will be oscillating at the
high frequency.

This is what I imagine would happen anyway, I'm not near anything I can
test this with right now. Just looking at what the line is doing will
then surely tell you what the speaker is doing ? Surely it'll be
following the wave ?

So, the speaker would be slowly moving back and forth, following the
amplitude of the low frequency signal, but as it moves back a forth,
it'll be oscillating a small amount back and forth at its current
position in the low frequency wave because that is what the input signal
is doing.

IMHO at least ....

Here is an example;

Our keyboard player once made his keyboard output such a low frequency
signal, that you could watch the speaker slowly move back a forth quiet
far, perhaps once every second. Are you telling me that, if I "mixed" a
high frequency with that signal, the speaker would be no longer moving
back and forth slowly about once per second ?

--
Mark Simonetti.
Freelance Software Engineer.