"Mark Simonetti" wrote in message

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 ?

The speaker is one entity, so it has only one speed at a time. In your case
the speaker's speed is the sum of the two speeds. You later talk like you
believe this. So why not say it up front?

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.

Agreed

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 ?

Pretty much.

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.

Ironcially, the velocity due to the lower frequency may be the greater of
the two velocities that are summed together.

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 ?

Of course it will. And its instanteious velocity is added to the
instantaneous velocity due to the upper note.