"Arny Krueger"
"Phil Allison"

** 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 the AM
distortion.


** Which, unlike the puny phase jitter, is veeerrrryy audible.




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.


** I was using poetic license.


But net it out -we're saying pretty
much the same thing, Phil. The Doppler distortion is there but its small.


** I found with my test that much of it goes away if you put the woofer
cone at right angles to the mic.

Hey - I have always had my 10 inch sub mounted facing the floor.

Way to go !!!



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?



** Must just love all that IM.



.......... Phil