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Bob Cain
 
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Default Doppler Distoriton?



Jim Carr wrote:

"Bob Cain" wrote in message
...

When you push on air, it moves and pushes on the air in
front of it but with some delay in the transfer. That's
what causes the speed of sound. The push propegates outward
from this bit of air to the bit in front of it and that's a
wave. Same when you pull on it.

Does that help?



Sorta. From what you're saying. the *origin* of each individual wave can
take place at any point within the throw of the diaphragm. Is that correct?


Not sure even how to define the origin of the wave in those
terms. Thanks for that. I just realized that the
assumptions which are being made about that are the flaw in
the intuitive description of "Doppler distortion."

Something that is ocuring dynamically is being described in
terms of a static piston in one sense and dynamically in
another. That doesn't work. The distance from the piston
to the the sensor isn't relevant to the argument if it is
riding the wave. In a way, it's effect is being included
twice if you do that. That's a no-no that will lead to
false prediction.

The flaw in the common argument for "Doppler distortion" has
proven very elusive but I think that this nails it. It
really is subtle which explains why it's been around so long.

I added rec.audio.pro to this because it's highly relevant
to the thread on this subject that is happening there.


Bob
--

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

A. Einstein
  #2   Report Post  
Jim Carr
 
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"Bob Cain" wrote in message
...

Sorta. From what you're saying. the *origin* of each individual wave can
take place at any point within the throw of the diaphragm. Is that

correct?

Not sure even how to define the origin of the wave in those
terms. Thanks for that. I just realized that the
assumptions which are being made about that are the flaw in
the intuitive description of "Doppler distortion."


I'm having a hard time envisioning just one wave being started by one thrust
of the piston. Maybe if I fully understood that rather than the quite
satisfactory "it just does and that's how a speaker works" mentality I'v
always had, I could argue intelligently one way or the other.

I added rec.audio.pro to this because it's highly relevant
to the thread on this subject that is happening there.


Great. Feed me to the wolves. Hey, RAP folks: I hold no degrees in
electronics, physics, acoustics, etc. I do not work with audio as a
profession. I just find the topic interesting and do not purport myself to
be an expert. As I noted earlier in the thread, which was not cross-posted,
if I seem condescending, it is because I am trying to explain things in
simple terms to myself. Since my logic is usually sound, my guess is that a
basic premise somewhere is wrong or incomplete, hence the detailed and
simplist explanations.

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 traveling different distances to reach my stationary ear, thus a Doppler
shift. Measurable? I dunno. Discernible to my ear? Probably not.


  #3   Report Post  
Bob Cain
 
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Jim Carr wrote:

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 traveling different distances to reach my stationary ear, thus a Doppler
shift. Measurable? I dunno. Discernible to my ear? Probably not.


To recapitulate, the problem with that intuitive view, which
is the whole basis of believing that there is "Doppler
distortion" is that it assumes that the distance from the
driver is the distance from the instantaneous position of
the piston. That's wrong. The distance from the driver,
since it is riding the wave it is creating, is the distance
from its zero or rest position, the position about which it
oscillates. That doesn't change with the nature of the
signal unless there is a DC component.

If the distance from the driver is not changing, there is no
Doppler shift. None of the proposed scenarios which have
the face of the driver oscillating about a rest position
will produce Doppler shift despite intuition.

Whew!


Bob
--

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

A. Einstein
  #4   Report Post  
Phil Allison
 
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"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 traveling 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.



.............. Phil


  #5   Report Post  
Arny Krueger
 
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"Phil Allison" wrote in message

"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 traveling 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.




  #6   Report Post  
Randy Yates
 
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Arny,

This information is probably in this thread somewhere, but it has
gotten so long and convoluted that it's much easier just to ask: Are
you asking whether FM (Doppler) modulation at the high frequency is
the ONLY effect that results when that high frequency in addition to a
low frequency (purposely left undefined since the actual values depend
on a number of factors in the physical setup) are reproduced in the
same transducer, or is there some amount of AM modulation as well?

"The Ghost" gave me an idea for determining this without requiring any
measurement of the instantaneous cone displacement. Perform an FM
discrimination of the received (microphone) signal at the high
frequency "carrier." Call the discriminated signal m(t). Regenerate a
perfect FM signal using the modulating signal m(t) and subtract that
from the original signal. The result is the residual modulation on the
signal, which could then be AM-detected to determine if AM is present.

Three practical issues which must be dealt with come to mind:

1) How to synchronize the regenerated FM carrier amplitude to the
original FM amplitude? Easy answer: emit a signal consisting of the
high frequency tone alone for a length of time adequate to measure the
amplitude.

2) What modulation index, or depth of modulation, should be used in
the regenerated FM signal? Said another way, what gain (if any) should
be applied to m(t) when regenerating the FM signal?

3) How do you synchronize the regenerated signal in time with the
original signal? There are actually two synchronization tasks to be
done: phase synchronization of the carriers, and delay in the modulating
signal, i.e., tau in A*m(t-tau). (A is the parameter in question 2).

Does this make any sense?
--
% Randy Yates % "My Shangri-la has gone away, fading like
%% Fuquay-Varina, NC % the Beatles on 'Hey Jude'"
%%% 919-577-9882 %
%%%% % 'Shangri-La', *A New World Record*, ELO
http://home.earthlink.net/~yatescr
  #7   Report Post  
Arny Krueger
 
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"Randy Yates" wrote in message


Arny,


This information is probably in this thread somewhere, but it has
gotten so long and convoluted that it's much easier just to ask: Are
you asking whether FM (Doppler) modulation at the high frequency is
the ONLY effect that results when that high frequency in addition to a
low frequency (purposely left undefined since the actual values depend
on a number of factors in the physical setup) are reproduced in the
same transducer, or is there some amount of AM modulation as well?


I'm not asking that question, because I know the answer, and I knew it
walking in the door last week.

The results of playing multiple tones through something as dirty as a
speaker produces copious amonts of both AM and FM. As a rule, the AM
dominates.

"The Ghost" gave me an idea for determining this without requiring any
measurement of the instantaneous cone displacement. Perform an FM
discrimination of the received (microphone) signal at the high
frequency "carrier." Call the discriminated signal m(t). Regenerate a
perfect FM signal using the modulating signal m(t) and subtract that
from the original signal. The result is the residual modulation on the
signal, which could then be AM-detected to determine if AM is present.


I've tried that, and a lot of other things. It has the usual problems with
nulling in the real world. You can get roughly a 2:1 to 10:1 reduction of
the unwanted distortion by that means.

Three practical issues which must be dealt with come to mind:


1) How to synchronize the regenerated FM carrier amplitude to the
original FM amplitude?


Pretty easy to do an fair job of in the digital domain.

Easy answer: emit a signal consisting of the
high frequency tone alone for a length of time adequate to measure the
amplitude.


If you've looked at the raw data page posted at
http://www.pcavtech.com/techtalk/doppler/ you'd know that finding that out
with pretty fair precision is a matter of reading numbers off a screen.

2) What modulation index, or depth of modulation, should be used in
the regenerated FM signal? Said another way, what gain (if any) should
be applied to m(t) when regenerating the FM signal?


At this point I should point out that since the AM dominates, it might make
sense to apply an AM signal to null the AM part out, leaving the FM.

3) How do you synchronize the regenerated signal in time with the
original signal? There are actually two synchronization tasks to be
done: phase synchronization of the carriers, and delay in the
modulating signal, i.e., tau in A*m(t-tau). (A is the parameter in
question 2).


Well, we know quite a bit about the signal that we are trying to clean up.

Does this make any sense?


Been there, done that. Seriously, I come back to this problem of separating
AM and FM from a real world signal every once and while, and learn a bit
more about solving it.

This time I realized that ideally, AM distortion related sidebands are
indepenendent of of the carrier frequency, but increase in amplitude with
carrier frequency for FM. Trouble is, this practical example is so heavily
dominated by the AM distortion. I hope to go back to studying jitter, and
play this card there.

I suspect that lots of people have been misidentifying AM distortion
products as jitter.


  #8   Report Post  
Jim Carr
 
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"Phil Allison" wrote in message
...

** 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*.


I disagree. The time delay or advance *is* Doppler. The speed of sound is
constant in a given medium. In the classic example of the train whistle the
velocity of the train changes the distance/time between waves, but those
waves travel just as fast as if the train were still. It seems like you are
saying Doppler has to do with adding velocities together, which is untrue.


  #9   Report Post  
Ben Bradley
 
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In alt.music.home-studio,rec.audio.tech,rec.audio.pro,
Bob Cain wrote:



Jim Carr wrote:

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 traveling different distances to reach my stationary ear, thus a Doppler
shift. Measurable? I dunno. Discernible to my ear? Probably not.


To recapitulate, the problem with that intuitive view, which
is the whole basis of believing that there is "Doppler
distortion" is that it assumes that the distance from the
driver is the distance from the instantaneous position of
the piston. That's wrong. The distance from the driver,
since it is riding the wave it is creating, is the distance
from its zero or rest position, the position about which it
oscillates. That doesn't change with the nature of the
signal unless there is a DC component.


Oh, goody. You want a DC component, I'll GIVE you a DC component!
insert emoticon here

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?

And presuming you see this as a possible cause of doppler
distortion, how is this "DC component" any different from a higher
frequency (say 20Hz or 50Hz) that also causes substantial cone
displacement?

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?

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!

Bob


-----
http://mindspring.com/~benbradley
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Phil Allison
 
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"Jim Carr"
"Phil Allison"


** 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*.


I disagree. The time delay or advance *is* Doppler.



** Doppler frequency shift is proportional to source velocity - so they
are not the same.


The speed of sound is constant in a given medium.



** If the medium is moving at some speed then that adds to, or subtracts
from, the speed of sound in still air (ie 343 m/S)


In the classic example of the train whistle the
velocity of the train changes the distance/time between waves,



** Yes, because the train is moving through the air.


but those waves travel just as fast as if the train were still.


It seems like you are
saying Doppler has to do with adding velocities together, which is untrue.



** Not at all - but the magnitude of the Doppler shift is proportional to
the velocity of the source compared to the surrounding air. A woofer cone
takes a small volume of with it for the ride.




............. Phil









  #11   Report Post  
Chris Hornbeck
 
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On Sat, 14 Aug 2004 21:04:25 -0700, "Jim Carr"
wrote:

** 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*.


I disagree. The time delay or advance *is* Doppler. The speed of sound is
constant in a given medium. In the classic example of the train whistle the
velocity of the train changes the distance/time between waves, but those
waves travel just as fast as if the train were still. It seems like you are
saying Doppler has to do with adding velocities together, which is untrue.


Doppler has to do with adding velocities together.

Chris Hornbeck
  #12   Report Post  
Jim Carr
 
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"Phil Allison" wrote in message
...
I disagree. The time delay or advance *is* Doppler.



** Doppler frequency shift is proportional to source velocity - so they
are not the same.


Maybe we just have a failure to communicate. I say the actual frequency does
not shift. Assume a constant frequency at the source. If the distance
between the source and the receiver changes, then the receiver cannot
reliably determine the frequency. This is because the distance between the
sound waves (wavelength) emitted by the source changes. The receiver
determines the frequency by measuring the wavelength, which is the distance
between a given point on a wave and the corresponding point on the next
cycle of the wave.

Now, our ears don't measure distance, they are measuring time to put it
loosely. They don't care how fast the wave is moving. They sense the time
interval (a function of distance) between waves. Since movement of either
the source, observer or both can change that distance, there is an
*apparent* shift in frequency, not a "real" shift. We know this because we
already agreed the source emitted a constant frequency. It can be expressed
like this:

fo = fs . (v - vo) / (v - vs)
fo is the apparent frequency of the observer.
fs is the frequency of the source
v is the speed of sound
vo and vs are the velocities of the source and observer.

But then again, you seemed to disagree about the speed of sound being
constant when you wrote...

** If the medium is moving at some speed then that adds to, or subtracts
from, the speed of sound in still air (ie 343 m/S)


To which I say, care to cite a reference? The apparent shift is certainly a
function of the velocity of the source and/or receiver, but the speed of
sound is constant. If it's not, there is no Doppler.

Let's just reason it out by using the whistle on the train and the changing
speed of sound which you cite. Suppose further that the listener is 3,430
meters away. The sound wave has to travel 10 seconds to arrive at the
observer, right?

Train #1 is stationary and blows the whistle. A second train is moving at
100m/S. At the exact moment is parallel with train #1 it blows its whistle.
This happens to be the exact same time train #1 blew its whistle. Based on
your statement above, the first sound of the whistle is moving at 343m/S.
The second is traveling at 443m/S.

Are you saying that the second whistle will arrive at the receiver 2.25
seconds earlier and that we would hear two separate whistles?

If they did, they would sound exactly the same anyway, which would mean no
Doppler shift.

Refer to the formula above. For Doppler to work, v (speed of sound) must
remain constant in the medium. If the velocities add together, then the
distance between each wave would therefore remain constant. Hence, no shift.


  #13   Report Post  
Phil Allison
 
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"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.


Wow.



........... Phil





  #14   Report Post  
Phil Allison
 
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"Jim Carr"
"Phil Allison"


I disagree. The time delay or advance *is* Doppler.



** Doppler frequency shift is proportional to source velocity - so

they
are not the same.



Maybe we just have a failure to communicate. I say the actual frequency

does
not shift.


Since movement of either the source, observer or both can change that

distance, there is an
*apparent* shift in frequency, not a "real" shift.



** The *observed* frequency changes - end of story.


But then again, you seemed to disagree about the speed of sound being
constant when you wrote...

** If the medium is moving at some speed then that adds to, or

subtracts
from, the speed of sound in still air (ie 343 m/S)


To which I say, care to cite a reference?



** Silly question - see any school physics text.


Let's just reason it out by using the whistle on the train and the

changing
speed of sound which you cite.
Suppose further that the listener is 3,430
meters away. The sound wave has to travel 10 seconds to arrive at the
observer, right?

Train #1 is stationary and blows the whistle. A second train is moving at
100m/S. At the exact moment is parallel with train #1 it blows its

whistle.
This happens to be the exact same time train #1 blew its whistle. Based on
your statement above, the first sound of the whistle is moving at 343m/S.
The second is traveling at 443m/S.



** That wrong conclusion simply has no connection with my statement.

Both whistle sounds travel through still air to the listener.


Are you saying that the second whistle will arrive at the receiver 2.25
seconds earlier and that we would hear two separate whistles?


** Nope.




............ Phil


  #15   Report Post  
Bob Cain
 
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Arny Krueger wrote:

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


Doesn't exist, Arny. Look he

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

Tellingly, as deep as the discussion goes, no mention is
made of "Doppler distortion" and if you read it you will see
why such nonsense wouldn't even have been considered.

It also directly supports what I have said recently that
distance from an oscilating piston, for the purposes of the
physics of piston interaction with air is the distance to
the rest position. I must say, that I found this link just
minutes ago, oddly enough looking for links to IR's for
Acoutic Modeler. I fingered it out earlier all by m'self.

Now about that data you posted...



Bob
--

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

A. Einstein


  #16   Report Post  
Bob Cain
 
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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
  #17   Report Post  
Arny Krueger
 
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"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?


  #18   Report Post  
Arny Krueger
 
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"Bob Cain" wrote in message

Arny Krueger wrote:

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


Doesn't exist, Arny. Look he

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

Tellingly, as deep as the discussion goes, no mention is
made of "Doppler distortion" and if you read it you will see
why such nonsense wouldn't even have been considered.

It also directly supports what I have said recently that
distance from an oscilating piston, for the purposes of the
physics of piston interaction with air is the distance to
the rest position. I must say, that I found this link just
minutes ago, oddly enough looking for links to IR's for
Acoutic Modeler. I fingered it out earlier all by m'self.

Now about that data you posted...


Sorry Bob, but I'm not buying.


  #19   Report Post  
Phil Allison
 
Posts: n/a
Default


"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





  #20   Report Post  
Phil Allison
 
Posts: n/a
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"Arny Krueger"
"Bob Cain"



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


Doesn't exist, Arny. Look he

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

Tellingly, as deep as the discussion goes, no mention is
made of "Doppler distortion" and if you read it you will see
why such nonsense wouldn't even have been considered.

It also directly supports what I have said recently that
distance from an oscilating piston, for the purposes of the
physics of piston interaction with air is the distance to
the rest position. I must say, that I found this link just
minutes ago, oddly enough looking for links to IR's for
Acoutic Modeler. I fingered it out earlier all by m'self.

Now about that data you posted...


Sorry Bob, but I'm not buying.




** I have a book on geography that no-where mentions that the earth is not
flat.

Will you take that as proof ???




.......... Phil






  #21   Report Post  
Arny Krueger
 
Posts: n/a
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"Phil Allison" wrote in message

"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.


Agreed. With a bullet!

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.


Again, per theory.

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.


He even bragged about all of the cone motion, when this little tyke was
driven with LF, I believe he even specifically mentioned 30 Hz.


  #22   Report Post  
Mike Rivers
 
Posts: n/a
Default


How come every time I look at a message in this thread (and I really
try to avoid it, but I'm a junkie and I can't be cured) it seems to be
cross-posted to more and more newsgroups?

Don't we have enough experts in rec.audio.pro?

--
I'm really Mike Rivers )
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me he double-m-eleven-double-zero at yahoo
  #24   Report Post  
William Sommerwerck
 
Posts: n/a
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To recapitulate, the problem with that intuitive view, which
is the whole basis of believing that there is "Doppler
distortion" is that it assumes that the distance from the
driver is the distance from the instantaneous position of
the piston. That's wrong. The distance from the driver,
since it is riding the wave it is creating, is the distance
from its zero or rest position, the position about which it
oscillates. That doesn't change with the nature of the
signal unless there is a DC component.


If the distance from the driver is not changing, there is no
Doppler shift. None of the proposed scenarios which have
the face of the driver oscillating about a rest position
will produce Doppler shift despite intuition.


Sorry, Bob, but I disagree. There is my thought experiment (which I consider
proof). And there is also the claim by an anonymous poster outlining his test
procedure and claiming he measured it.

  #25   Report Post  
Ben Bradley
 
Posts: n/a
Default

On Sun, 15 Aug 2004 00:37:44 -0700, Bob Cain
wrote:

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



I wrote:

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.


Okay, Bob, I'm going to add a DC component, but I'm not going to
tell you that this DC component is really a millihertz-frequency sine
wave. How will you know the difference? Just what IS the difference
over a time period of one second?

Bob


-----
http://mindspring.com/~benbradley


  #26   Report Post  
Bob Cain
 
Posts: n/a
Default



Ben Bradley wrote:


Okay, Bob, I'm going to add a DC component, but I'm not going to
tell you that this DC component is really a millihertz-frequency sine
wave. How will you know the difference? Just what IS the difference
over a time period of one second?


Enough. :-)


Bob
--

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

A. Einstein
  #27   Report Post  
Bob Cain
 
Posts: n/a
Default



Arny Krueger wrote:


Sorry Bob, but I'm not buying.


How about this, then:

1) It is the bulk velocity, the flow of air, at the rest
position that propegates out as the velocity wave.

2) The bulk velocity at the rest position is the same as the
surface velocity of the piston.


Bob
--

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

A. Einstein
  #28   Report Post  
Arny Krueger
 
Posts: n/a
Default

"Phil Allison" wrote in message

"Arny Krueger"
"Bob Cain"



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

Doesn't exist, Arny. Look he

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

Tellingly, as deep as the discussion goes, no mention is
made of "Doppler distortion" and if you read it you will see
why such nonsense wouldn't even have been considered.

It also directly supports what I have said recently that
distance from an oscilating piston, for the purposes of the
physics of piston interaction with air is the distance to
the rest position. I must say, that I found this link just
minutes ago, oddly enough looking for links to IR's for
Acoutic Modeler. I fingered it out earlier all by m'self.

Now about that data you posted...


Sorry Bob, but I'm not buying.


** I have a book on geography that no-where mentions that the earth
is not flat.

Will you take that as proof ???


I think you misunderstood my intent. I have no problems with Art Ludwig's
recital of the laws of physics, I have problems with Bob's interpretation of
them.


  #29   Report Post  
Phil Allison
 
Posts: n/a
Default


"Arny Krueger"
"Phil Allison"

** I have a book on geography that no-where mentions that the earth
is not flat.

Will you take that as proof ???


I think you misunderstood my intent.



** I think you missed the irony.





................. Phil


  #30   Report Post  
Bob Cain
 
Posts: n/a
Default



William Sommerwerck wrote:

To recapitulate, the problem with that intuitive view, which
is the whole basis of believing that there is "Doppler
distortion" is that it assumes that the distance from the
driver is the distance from the instantaneous position of
the piston. That's wrong. The distance from the driver,
since it is riding the wave it is creating, is the distance
from its zero or rest position, the position about which it
oscillates. That doesn't change with the nature of the
signal unless there is a DC component.



If the distance from the driver is not changing, there is no
Doppler shift. None of the proposed scenarios which have
the face of the driver oscillating about a rest position
will produce Doppler shift despite intuition.



Sorry, Bob, but I disagree.


Can you be more specific. I don't mean by offering another
scenario which I must find specific disagreement with but
doing it yourself with what I've offered. What part of it
is wrong?


Bob
--

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

A. Einstein


  #31   Report Post  
Bob Cain
 
Posts: n/a
Default



Bob Cain wrote:



Arny Krueger wrote:


Sorry Bob, but I'm not buying.



How about this, then:

1) It is the bulk velocity, the flow of air, at the rest position that
propegates out as the velocity wave.

2) The bulk velocity at the rest position is the same as the surface
velocity of the piston.


And, finally:

3) In the frame of reference of the rest position of the
piston, no Doppler shift can be observed.


Bob
--

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

A. Einstein
  #32   Report Post  
William Sommerwerck
 
Posts: n/a
Default

If the distance from the driver is not changing, there is no
Doppler shift. None of the proposed scenarios which have
the face of the driver oscillating about a rest position
will produce Doppler shift despite intuition.



Sorry, Bob, but I disagree.


Can you be more specific. I don't mean by offering another
scenario which I must find specific disagreement with but
doing it yourself with what I've offered. What part of it
is wrong?


Consider the Principle of Superposition. QED.
  #33   Report Post  
Porky
 
Posts: n/a
Default


"Ben Bradley" wrote in message
...
On Sun, 15 Aug 2004 00:37:44 -0700, Bob Cain
wrote:

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


I wrote:

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.


Okay, Bob, I'm going to add a DC component, but I'm not going to
tell you that this DC component is really a millihertz-frequency sine
wave. How will you know the difference? Just what IS the difference
over a time period of one second?

Here's a simple way out of this one, with a very slow oscillation of a
varying DC component, the cone won't be moving at a high enough velocity to
generate doppler distortion, even if it is moving two or more inches p-p.
Forget millihertz, even one hertz at 2 inches displacement is going to be a
peak velocity of less than one half foot per second. At that velocity, it
simply doesn't have time to dopple!:-)


  #34   Report Post  
Isaac Wingfield
 
Posts: n/a
Default

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
  #35   Report Post  
Phil Allison
 
Posts: n/a
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"Isaac Wingfield"
"Phil Allison"

Could this explain the phase shift you are seeing?


** No.

Static phase shift was simply adjusted out by positioning the mic at a spot
where the drive voltage and sound pressure were in phase as is evidenced by
the lissajous pattern becoming a line - only the additional shift from
the low frequency cone movement was being observed.


What happens if, instead of changing cone excursion by changing the

frequency, you keep
the frequency constant and adjust the amplitude?



** The phase shift is proportional to cone displacement - as I already
stated.

Cone velocity did not alter the peak amplitude of the oscillating
phase shift.




............ Phil






  #36   Report Post  
Bob Cain
 
Posts: n/a
Default



Porky wrote:

At that velocity, it
simply doesn't have time to dopple!:-)


That's cute. Wrong, but nonetheless cute. :-)


Bob
--

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

A. Einstein
  #37   Report Post  
Chris Hornbeck
 
Posts: n/a
Default

On Mon, 16 Aug 2004 05:22:52 GMT, Isaac Wingfield
wrote:

Now move your hand up and down fast. The weight goes up when your hand
goes down. This is well above resonance.


Actually, that *is* resonance.

Otherwise, it's an excellent intuitive teaching device about
resonance. (Nothing to do with Doppler, though.)

Chris Hornbeck
  #38   Report Post  
Porky
 
Posts: n/a
Default


"Bob Cain" wrote in message
...


Porky wrote:

At that velocity, it
simply doesn't have time to dopple!:-)


That's cute. Wrong, but nonetheless cute. :-)


True, but who's going to hear doppler shift at that level, even if it does
exist?:-)
I think in typical speaker systems, doppler distortion is among the lesser
of our distortion worries, even assuming that it does exist. I see valid
points on both sides of the discussion, but I just don't think it's that big
a priority until other forms of more severe distortion are conquered. I
don't think anyone here thinks that Doppler is the worst form of distortion
in high quality studio monitors, or even in high quality home speakers at
the current state of the art of speaker design.


  #39   Report Post  
Bob Cain
 
Posts: n/a
Default



Porky wrote:

True, but who's going to hear doppler shift at that level, even if it does
exist?:-)


To me it doesn't matter whether we can hear it or not. As
of now there is no predictive theory that yields
quantitative results we could test with experiment anyway.
I want to know whether or not it is a real phenomenon at all.

One of the reasons this issue is of importance to me is that
on the ProAudio mailing list they argue endlessly about
effects that are so small in predicted physical consequence
that they would be swamped by even a pretty darned small
"Doppler distortion" if it existed. If it really does exist
then a whole lot of what they discuss and argue about in
other areas could well be irrelevant. This is not an
unimportant issue to have settled.


Bob
--

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

A. Einstein
  #40   Report Post  
Arny Krueger
 
Posts: n/a
Default

"Bob Cain" wrote in message

Porky wrote:

True, but who's going to hear doppler shift at that level, even if
it does exist?:-)


To me it doesn't matter whether we can hear it or not. As
of now there is no predictive theory that yields
quantitative results we could test with experiment anyway.
I want to know whether or not it is a real phenomenon at all.


How are you doing with the 60KB or so of AES paper references that I posted
in RAP?

;-)


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