In article ,
"Phil Allison" wrote:
"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 ??
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.
A dynamic loudspeaker is a mechanical system operated above resonance.
That means that the instantaneous position of the cone is *not*
represented by the voltage across the voice coil at that instant -- in
other words, there is a phase shift between the driving voltage and the
driven cone.
Compared to the displacement of the cone when driven by a DC voltage of
a certain amplitude, at cone resonance, the phase shift is 90 degrees;
well above that frequency it approaches 180 degrees.
To visualize how the driving force and the cone excursion are not in
phase, experiment with a weight on the end of a rubber band, the weight
heavy enough to cause the band to be significantly stretched. Put
several bands in series to make it easier -- say 18" or so long when
stretched.
Hold the upper end of the string of bands in your hand, and move your
hand up and down very slowly. The weight follows along, in phase. This
is below resonance.
Now move your hand up and down fast. The weight goes up when your hand
goes down. This is well above resonance.
If you are careful, you can find resonance, and note that the motion of
the weight moves in quadrature to the position of your hand.
Notice also that when above resonance, the peak-to-peak displacement of
the weight goes down as the driving frequency goes up, if you hold the
excursion of your hand constant. This is exactly the way that a
loudspeaker maintains constant SPL over frequency when operated in its
"passband". It's automatic, an inevitable result of a mechanical system
being operated above resonance.
Could this explain the phase shift you are seeing? What happens if,
instead of changing cone excursion by changing the frequency, you keep
the frequency constant and adjust the amplitude?
Isaac
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