Ben Bradley wrote:


Let's go to extremes. Say we got one of these big honking
high-power woofers (that I recall reading Arny's writings about a
while back) with X-max of one or two inches or so. Superimpose a 1kHz
tone (probably the highest frequency it will reasonably reproduce)
onto a "DC component", say a 1/2 Hz sine wave that slooowly moves the
cone in and out a total distance of two inches, all the while it's
also putting this 1kHz tone into the air. Don't think of it as a 0.5Hz
sine wave, think of it as a varying DC component (that's obviously
what it is, you can see the cone move back and forth with your eyes).
What will you say is the acoustic source of the 1kHz, the driver
frame, which does not move, or the cone, which DOES move?

Doesn't matter how slow the oscilation is, it won't produce
Doppler shift.

If the distance from the driver is not changing, there is no
Doppler shift.


The distance from which part of the driver? The frame? The cone?
Something else?

The rest position, the one it will settle to when the
driving signal is removed. If the driving signal contains a
DC component, and the piston is not restrained by a
compliance, then and only then will Doppler shift occurs.
Hard to swallow, I know but it is the truth.




None of the proposed scenarios which have
the face of the driver oscillating about a rest position
will produce Doppler shift despite intuition.
Whew!


If you think it's frustrating for you, imagine how I feel with Phil
agreeing with me!

That really must be rough. I sympathize. :-)

Look here if you aquire the math to understand it.


http://www.silcom.com/~aludwig/Physi..._of_sound.html

I just found it and everything I've said is in it if not in
the same context. It doesn't consider "Doppler distortion"
because there is no reason to.


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein