"Phil Allison" wrote in message

"Arny Krueger"
"Phil Allison"
"Jim Carr"

With that said, help me out here. I can't get myself away from the
assumption that since a speaker diaphragm has a throw of a certain
distance, then the waves started by the diaphragm may be started
from any point in that throw. As such two waves which are created a
certain time apart may end up travelling different distances to
reach my stationary ear, thus a Doppler shift.


** A time delay or advance is just that - it is not Doppler. Any
such delay or advance depends solely on the position of the cone -
not its *velocity*. If a cone is displaced by 10mm, that will
introduce a time error of 29 uS or a phase shift of 50 degrees at 5
kHz.

Any attempt to measure Doppler frequency shifts must allow for
this
- most have not.

That's because this time shift, more specifically the time rate of
change of this time shift, is the cause of Doppler.



** So this is what all the Doppler Distortion fuss is about ????

A tiny bit of phase jitter, which at 5 kHz rarely amounts to more
than a few degrees ??

It's not a lot. The most important thing is that its swamped by all teh AM
distortion.

I was looking at it on my scope yesterday:

1. A 5 inch woofer, in box, driven by an amp fed from with two sine
wave generators with outputs summed.

2. A condenser mic feeding a pre-amp and followed by a 12 dB/oct
HPF at 2 kHz thence to the scope.

3. The high frequency generator output is also linked to the scope
which operates in X-Y mode.

4. Park mic in front of woofer fed with a circa 5000 Hz sine wave
at about 10 watts. ( I used ear muffs)

5. Adjust scope and exact mic position to get a straight, diagonal
line traced on the scope screen - note that adjusting the 5000 Hz
amplitude affects the angle of the diagonal line only (ie makes it
easy to visually distinguish amplitude modulation ).

6. Turn up low frequency generator, set to say 40 Hz, and watch
the line open out to form a narrow ellipse indicating that the phase
is changing as the cone moves closer and further away from the mic.

7. Sweep low frequency generator up and down and note that cone
excursion alone controls the size of the ellipse - it never opens
out more than about 15 degrees for a linear cone excursion of 3 mm.

8. Try hard to imagine that this is the notorious, evil, Doppler
distortion before your eyes.

I never said it was notorious or evil. But net it out -we're saying pretty
much the same thing, Phil. The Doppler distortion is there but its small.

I think the guy who brought up Doppler as some kind of a serious problem did
so a few weeks ago. He used Doppler distortion as a justification for not
liking long-excursion woofers. In the end he admitted that he used 2-way
monitors with either 6.5 or 8" woofers, and no subwoofer. Ironic enough?