Doppler Distortion - Fact or Fiction

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I think there are two simple refutations, one theoretical, the other practical.

Theoretical -- If the output of a driver accurately follows its input waveform,
how can there be Doppler distortion?

Practical -- It should be trivial to compute the sidebands produced by combining
(say) a 60Hz signal and 6000 Hz signal. By looking at the phase of the
sidebands, it should be possible to determine what part of them is IM distortion
(AM components) and what part is Doppler (FM components).

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As a non-mathematical type I understood Doppler distortion to be caused
when a high frequency was generated by a driver already in motion with a
low frequency.

The example was a woofer moving full excursion on a very low tone while
generating a higher tone. Let's do an extreme case of a 10 Hz excursion
and a 1000 Hz tone. Every 20th of a second (change in direction at 10 Hz)
the pitch of the 1000 Hz tone would change as its vibrating medium (the
woofer cone) changed from moving toward the listener to moving away.

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Arny Krueger <arnyk@hotpop.com> wrote:
>"Bob Cain" <arcane@arcanemethods.com> wrote in message
>
>> I've got an argument that so far has withstood some scrutiny
>> which shows that Doppler distortion in a myth.
>
>> What would refute it and point out any flaw in the reasoning
>> would be the dynamical expression for the time varying
>> function of the pressure wave in an infinite tube with an
>> ideal piston as a function of an arbitrary, time varying
>> function of the force applied to that piston.
>
>Fool that I am, I'm kinda stuck down here in the real world. Forget the
>math, forget the long-winded discussions, the question that interests me
>most is whether or not there's Doppler distortion where it really matters -
>in the sound field in front of the speaker.

Well, surprisingly enough, Phil actually made the good point that the woofer
position does not directly follow the input signal, but that the excursion
at lower frequencies is exaggerated. This is indeed the reason that we get
Doppler distortion. But, how do we compensate for this? And can we, even?

Of course, reducing the bandwidth to each driver and reducing the driver
excursion as much as possible are crude ways around the problem.

A more exaggerated example of the distortion, though, is found in coaxial
speakers where the moving woofer cone is used as the horn for the tweeter.
Here, though, I am not sure the math model is quite so easy, and it would
be interesting to see if anyone can model the boundary effects near the
moving woofer cone.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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Arny Krueger <arnyk@hotpop.com> wrote:
>"Scott Dorsey" <kludge@panix.com> wrote in message
>
>> Well, surprisingly enough, Phil actually made the good point that the
>> woofer position does not directly follow the input signal, but that
>> the excursion at lower frequencies is exaggerated. This is indeed
>> the reason that we get Doppler distortion.
>
>I would say that the exaggerated excursion at low frequencies is a
>contributing cause for Doppler distortion, but not the only cause.

On a typical full-range speaker _not_ breaking up, what other good
causes are there? With coaxials and with speakers in breakup, there
are all kinds of wacky things going on.

The only other cause I can think of has to do with compressibility of
air and it would seem to be a comparatively small issue.

>> But, how do we compensate for this? And can we, even?
>
>> Of course, reducing the bandwidth to each driver and reducing the
>> driver excursion as much as possible are crude ways around the problem.
>
>Crude but effective! ;-)
>
>Doppler is exactly proportional to the upper frequency being modulated. Drop
>the upper crossover frequency on that woofer by a factor of two, and you
>drop the Doppler distortion by 2.

That's reducing the bandwidth.

>Double the diaphragm area, and you get the
>same benefit.

That's reducing the driver excursion.

>Subwoofers make even more sense!

Yes, but they bring another whole set of issues along with them.

>> A more exaggerated example of the distortion, though, is found in
>> coaxial speakers where the moving woofer cone is used as the horn for
>> the tweeter.
>
>Yes, and I even have a KEF Q-15 to test that with.

A better one would be one of the Radian drivers, which are really bad about
it.

It would be interesting to see if the Urei horn assemblies on the Altec
coaxial drivers really do minimize doppler modulation compared with the
original Altec horn assemblies. That was one of the arguments the Urei
guys used for the extended horns they employed.

>However, our preliminary results show that even with a reasonable worst case
>(small woofer, relatively high upper frequency) the Doppler tends to get
>lost in the AM distortion. Claiming it isn't there is wrong, but getting
>worked up about it seems a little foolish.

Yes, but how audible is it? You can treat the doppler modulation sort of
like spurious sidebands, BUT they are sidebands that are modulated by
the signal. Does it make it mode or less audible than a fixed sideband?

>>Here, though, I am not sure the math model is quite so
>> easy, and it would be interesting to see if anyone can model the
>> boundary effects near the moving woofer cone.
>
>It's tough enough to work with the case we're working with, which seems to
>be far simpler.

Right, because you have pretty much one dominant distortion source, and it
is an easy one to model. You should be able to plug and chug and get a
simple value for doppler distortion due to increased excursion at low
frequencies, knowing little more than the tone frequencies and the driver
excursion for a given cabinet. How does that compare with the measured
doppler modulation on that cabinet? That will tell you if there are any
other hidden effects to worry about.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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"Scott Dorsey" <kludge@panix.com> wrote in message
news:cfdaf6$41b$1@panix2.panix.com

> Arny Krueger <arnyk@hotpop.com> wrote:
>> "Scott Dorsey" <kludge@panix.com> wrote in message

>>> Well, surprisingly enough, Phil actually made the good point that
>>> the woofer position does not directly follow the input signal, but
>>> that the excursion at lower frequencies is exaggerated. This is
>>> indeed the reason that we get Doppler distortion.

>> I would say that the exaggerated excursion at low frequencies is a
>> contributing cause for Doppler distortion, but not the only cause.

> On a typical full-range speaker _not_ breaking up, what other good
> causes are there?

My point is that you don't need exaggerated excursion, just any substantial
excursion at all.

> With coaxials and with speakers in breakup, there
> are all kinds of wacky things going on.

Agreed.

> The only other cause I can think of has to do with compressibility of
> air and it would seem to be a comparatively small issue.

>>> But, how do we compensate for this? And can we, even?

>>> Of course, reducing the bandwidth to each driver and reducing the
>>> driver excursion as much as possible are crude ways around the
>>> problem.

>> Crude but effective! ;-)

>> Doppler is exactly proportional to the upper frequency being
>> modulated. Drop the upper crossover frequency on that woofer by a
>> factor of two, and you drop the Doppler distortion by 2.

> That's reducing the bandwidth.

Agreed. I just added some quantification.

>> Double the diaphragm area, and you get the
>> same benefit.

> That's reducing the driver excursion.

Agreed. I just added some quantification.

>> Subwoofers make even more sense!

> Yes, but they bring another whole set of issues along with them.

Again, agreed.

>>> A more exaggerated example of the distortion, though, is found in
>>> coaxial speakers where the moving woofer cone is used as the horn
>>> for the tweeter.

>> Yes, and I even have a KEF Q-15 to test that with.

> A better one would be one of the Radian drivers, which are really bad
> about it.

I really don't see a lot of difference. Compression driver tweeter versus
small direct radiator tweeter. At least the Radians have relatively big
woofers. OTOH, they are probably more likely to be driven very hard.

> It would be interesting to see if the Urei horn assemblies on the
> Altec coaxial drivers really do minimize Doppler modulation compared
> with the original Altec horn assemblies. That was one of the
> arguments the Urei guys used for the extended horns they employed.

Underlying this all is how much Doppler is there, and how audible is it?
Right now we've got a fairly extreme case, and we're scratching pretty hard
to find the Doppler FM in and among all of the AM distortion.

>> However, our preliminary results show that even with a reasonable
>> worst case (small woofer, relatively high upper frequency) the
>> Doppler tends to get lost in the AM distortion. Claiming it isn't
>> there is wrong, but getting worked up about it seems a little
>> foolish.

> Yes, but how audible is it? You can treat the Doppler modulation
> sort of like spurious sidebands, BUT they are sidebands that are
> modulated by the signal. Does it make it mode or less audible than a
> fixed sideband?

For small modulation indices, the sidebands related to AM and FM follow
similar rules. One difference is the fact that the modulation index for FM
rises with carrier frequency. AM sidebands maintain a constant relative
amplitude as the carrier frequency rises, all other things being equal.

>>> Here, though, I am not sure the math model is quite so
>>> easy, and it would be interesting to see if anyone can model the
>>> boundary effects near the moving woofer cone.

>> It's tough enough to work with the case we're working with, which
>> seems to be far simpler.

> Right, because you have pretty much one dominant distortion source,
> and it is an easy one to model.

But, that dominant modulation source seems to want to be AM, not FM.

>You should be able to plug and chug
> and get a simple value for Doppler distortion due to increased
> excursion at low frequencies, knowing little more than the tone
> frequencies and the driver excursion for a given cabinet.

There's quite a bit about that in the technical literature. I cited some of
the JAES papers about a week ago in another forum.

> How does
> that compare with the measured Doppler modulation on that cabinet?
> That will tell you if there are any other hidden effects to worry
> about.

Theory and practice seem to correspond pretty well.

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"William Sommerwerck" <williams@nwlink.com> wrote in message
news:10hjsuu23dlsqfe@corp.supernews.com...
> I think there are two simple refutations, one theoretical, the other
practical.
>
> Theoretical -- If the output of a driver accurately follows its input
waveform,
> how can there be Doppler distortion?

If the driver converted voltage to air pressure, there could be no Doppler
distortion.
However, the driver, even under the best of circumstances, does not do that.
It converts, approximately, to displacement.

Displacement is not equivalent to air pressure.
>
> Practical -- It should be trivial to compute the sidebands produced by
combining
> (say) a 60Hz signal and 6000 Hz signal. By looking at the phase of the
> sidebands, it should be possible to determine what part of them is IM
distortion
> (AM components) and what part is Doppler (FM components).
>

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"Scott Dorsey" <kludge@panix.com> wrote in message
news:cfde31$2nj$1@panix2.panix.com
> Arny Krueger <arnyk@hotpop.com> wrote:
>> "Scott Dorsey" <kludge@panix.com> wrote in message
>> news:cfdaf6$41b$1@panix2.panix.com
>>
>>> Arny Krueger <arnyk@hotpop.com> wrote:
>>>> "Scott Dorsey" <kludge@panix.com> wrote in message
>>
>>>>> Well, surprisingly enough, Phil actually made the good point that
>>>>> the woofer position does not directly follow the input signal, but
>>>>> that the excursion at lower frequencies is exaggerated. This is
>>>>> indeed the reason that we get Doppler distortion.
>>
>>>> I would say that the exaggerated excursion at low frequencies is a
>>>> contributing cause for Doppler distortion, but not the only cause.
>>
>>> On a typical full-range speaker _not_ breaking up, what other good
>>> causes are there?
>>
>> My point is that you don't need exaggerated excursion, just any
>> substantial excursion at all.

> No.

Yes.

>If the motion of the cone perfectly follows the waveform, and
> the air is not compressable, the pressure waveform that results in
> the air will be a perfect representation of the original wave.

Scott, I think you've missed a critical point. A speaker is a transducer,
and that means that in the process of following the waveform ideally, the
results in the air can be a little different than what was intended.

> If you can somehow arrange for perfect coupling so that the woofer
> excursion perfectly matches the input signal, doppler effects should
> be a non-issue.

No. Imperfect woofer excursion is more along the line of AM effects.

> Unfortunately this does not go along well with
> accurate frequency response in the real world.

I think that this issue of imperfect cone motion has some light shed on it
by contemplating a system with a tweeter, versus a system that lacks one. In
both cases the respective signals are transduced into the air with ideal
waveforms. The Doppler comes from the fact that the HF gets transduced into
the air from a platform with large-scale motion due to some other signal.
The tweeter does not have this situation.

It's like the train whistle. The whistle itself need not be affected by its
motion through the air. The fact that the whistle is moving w/r/t the
listener is the root cause of the Doppler effect.

>> For small modulation indices, the sidebands related to AM and FM
>> follow similar rules. One difference is the fact that the modulation
>> index for FM rises with carrier frequency. AM sidebands maintain a
>> constant relative amplitude as the carrier frequency rises, all
>> other things being equal.

> Right, but what you are seeing are not just fixed sidebands, but
> sidebands that are modulated by another signal.

Well, if there was no modulation, there would be no sidebands. So, the
presence of sidebands are *always* due to that other signal that is
modulating the carrier. When you FM modulate with a continuous wave, the
sidebands are fixed in the frequency spectrum just like the ones for AM. The
FM effect comes from the vector sum of the sidebands and the carrier. In AM,
the AM effect comes from a similar vector sum.

> If you've got a fixed sideband caused by constant modulation, the
audibility of it is
> predictable and there are some good studies on audibility. If it's
> not caused by constant modulation, all that becomes meaningless.

I wouldn't call it meaningless, I'd call it potentially very nasty to
analyze and characterize.

>>> Right, because you have pretty much one dominant distortion source,
>>> and it is an easy one to model.

>> But, that dominant modulation source seems to want to be AM, not FM.

> So?

Which means that while we can't deny that the FM is there, we assign it a
lower importance until we fix the AM distortion a whole lot more.

>>> You should be able to plug and chug
>>> and get a simple value for Doppler distortion due to increased
>>> excursion at low frequencies, knowing little more than the tone
>>> frequencies and the driver excursion for a given cabinet.

>> There's quite a bit about that in the technical literature. I cited
>> some of the JAES papers about a week ago in another forum.

> Right. It's very easy to model that effect. The questions are how
> audible it is, and whether there are any other effects involved. The
> latter is easier to answer than it seems, the former is probably more
> difficult.

The BIG other effect is AM.

The audibility of AM and FM is similar. The sidebands are heard as a varying
carrier when they are very close to the carrrier. They are heard as
*roughness* for sidebands that are moderately close to the carrier. When the
sidebands get widely dispersed they eventually crawl out from under the
masking curve and are heard as separate tones.

>>> How does
>>> that compare with the measured Doppler modulation on that cabinet?
>>> That will tell you if there are any other hidden effects to worry
>>> about.
>
>> Theory and practice seem to correspond pretty well.

> If that is the case, then there should really be no other doppler
> sources you need to worry about.

Frankly, I'm not all that worried about the sources we know about. The other
sources are known to be even smaller.

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Doppler distortion obviously exists. The question is one of how audible it is.

My feelings are "not very." You don't hear people who own full-range
electrostatics complaining about Doppler distortion.

Consider the following. Suppose an electrostatic speaker is reproducting 60Hz at
a peak-to-peak excursion of 0.25". That means its maximum velocity would be
around 30 inches per second. That's less than 1/4 of 1% of the speed of sound!

I really, really doubt that's audible.

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>> Consider the following. Suppose an electrostatic speaker is
>> reproducing 60Hz at a peak-to-peak excursion of 0.25".

>> That means its maximum velocity would be around 30 inches
>> per second. That's less than 1/4 of 1% of the speed of sound!

> It appears that a practical inter-electrode gap for an electrostatic
> speaker might be 2 mm or about 0.05". I believe that bad things
> might happen if the diaphragm traversed a great deal of that gap.

> In the +20 example posted at http://www.pcavtech.com/techtalk/doppler,
> the cone excursion was very roughly on the order of 1/8". There was
> a ton of distortion, almost all of which was AM distortion, not FM.

I deliberately chose an obviously extreme (!!!) situation to make the point.


> I admit it, my interest in Doppler distortion was peaked by someone
> who had doubts about high-Xmax woofers because of the exposure
> to Doppler distortion.

piqued

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William Sommerwerck <williams@nwlink.com> wrote:
>Doppler distortion obviously exists. The question is one of how audible it is.
>
>My feelings are "not very." You don't hear people who own full-range
>electrostatics complaining about Doppler distortion.
>
>Consider the following. Suppose an electrostatic speaker is reproducting 60Hz at
>a peak-to-peak excursion of 0.25". That means its maximum velocity would be
>around 30 inches per second. That's less than 1/4 of 1% of the speed of sound!

Right, but this is a speaker that has a huge surface area and therefore has
a very low total excursion. This is a _good_ thing. When your woofer
excursion starts getting to be an order of magnitude larger, the numbers
change. But the large surface area of an electrostatic panel means you can
get considerable bass without substantial excursion... which is good because
nonlinearities in the field become a big issue when there is substantial
excursion.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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Robert Morein wrote:


> If the driver converted voltage to air pressure, there could be no Doppler
> distortion.
> However, the driver, even under the best of circumstances, does not do that.
> It converts, approximately, to displacement.

I'm afraid this is incorrect. The heuristics usually used
to describe this effect apply equally well to an ideal
system where a massless, infinitely compliant and lossless
piston is driven by a force function.


Bob
--

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

A. Einstein

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Arny Krueger wrote:

> Fool that I am, I'm kinda stuck down here in the real world. Forget the
> math,

Then we are whistling in the dark. I admire your
experimentalist approach. Experiment trumps theory, always.

I just ask that you draw no conclusions from a system that
contains measurable non-linearity in the transducer itself.


Bob
--

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

A. Einstein

Archived from groups: rec.audio.pro (More info?)

>> Doppler distortion obviously exists. The question is one of how audible it
is.
>> My feelings are "not very." You don't hear people who own full-range
>> electrostatics complaining about Doppler distortion.

>> Consider the following. Suppose an electrostatic speaker is reproducting 60Hz
at
>> a peak-to-peak excursion of 0.25". That means its maximum velocity would be
>> around 30 inches per second. That's less than 1/4 of 1% of the speed of
sound!

> Right, but this is a speaker that has a huge surface area and therefore has
> a very low total excursion. This is a _good_ thing. When your woofer
> excursion starts getting to be an order of magnitude larger, the numbers
> change. But the large surface area of an electrostatic panel means you can
> get considerable bass without substantial excursion... which is good because
> nonlinearities in the field become a big issue when there is substantial
> excursion.

All correct, but multi-way dynamic systems with such large excursions eliminate
much of the potential for Doppler distortion, simply because the higher
frequencies are reproduced through a separate driver.

Note, also, that such a large excursion would usually occur on a bass transient,
not during "normal" (???) music.

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Scott Dorsey wrote:


> No. If the motion of the cone perfectly follows the waveform, and the air
> is not compressable, the pressure waveform that results in the air will be
> a perfect representation of the original wave. If you can somehow arrange
> for perfect coupling so that the woofer excursion perfectly matches the
> input signal, doppler effects should be a non-issue. Unfortunately this
> does not go along well with accurate frequency response in the real world.

However, for it to be a real, non-linear effect it must be
demonstrable in a hyperlinear transducer. All linear
imperfections in the system can be cancelled by a suitable
linear function block between the input and the transducer
such that the cone follows the waveform. If there are
non-linearities in the transducer all bets are off because
we don't really know what is the cause of the resulting
non-linear output.

[theory alert]
The reason that it must be a non-linear effect is that any
linear system has sinusoids as eigenfunctions.
Eigenfunctions are those functions which when presented to
the system as input, in any summation, result in an output
that contains only complex scalings of the magnitudes of the
input eigenfunctions, which are called the eigenvalues.
Scaling zero results in zero which means that no non-zero
eigenvalues can result in the output which were zero in the
input. The hypothetical Doppler distortion fails this test.
[end alert]


> Right. It's very easy to model that effect.

Actually, for an arbitrary input it has not been done.
There is no model even that will quantitatively predict the
measured result of experiments with two tones.


Bob
--

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

A. Einstein

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"Bob Cain" <arcane@arcanemethods.com> wrote in message
news:cfdljp018ck@enews4.newsguy.com...
>
>
> Robert Morein wrote:
>
>
> > If the driver converted voltage to air pressure, there could be no
Doppler
> > distortion.
> > However, the driver, even under the best of circumstances, does not do
that.
> > It converts, approximately, to displacement.
>
> I'm afraid this is incorrect. The heuristics usually used
> to describe this effect apply equally well to an ideal
> system where a massless, infinitely compliant and lossless
> piston is driven by a force function.
>
1. You can't contradict what I said with a heuristic. See
http://www.hyperdictionary.com/dictionary/heuristic

2. The "heuristic", by which you probably mean approximation, is exactly
that and no more, a very useful approximation.

3. The subject under discussion is whether the "heuristic" breaks down in a
meaningful fashion. This is the very core of the discussion. If Doppler
effect influences the sound output, then, to the extent which it does, the
"heuristic" is invalid.

4. I do not imply by the above that Doppler is important, or is not. But
please understand that the following is an equivalence relationship:

a. If Doppler is unimportant, then the heuristic you mention is, for all
practical purposes, a very good one.
b. If Doppler is in some situation important, then the heuristic cannot
be used in that situation.

Because the two are equivalent, we need to be wary of saying things like:
"Doppler is unimportant because the approximation we use that excluded
Doppler says Doppler is unimportant."

That would be circular reasoning.
>
> Bob
> --
>
> "Things should be described as simply as possible, but no
> simpler."
>
> A. Einstein

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On Wed, 11 Aug 2004 11:47:23 -0400, "Arny Krueger" <arnyk@hotpop.com>
wrote:

>"Scott Dorsey" <kludge@panix.com> wrote in message
>news:cfde31$2nj$1@panix2.panix.com

>>If the motion of the cone perfectly follows the waveform, and
>> the air is not compressable,

Air is compressible, even water is compressible, and that's why the
speed of sound in air and water are finite.

>> the pressure waveform that results in
>> the air will be a perfect representation of the original wave.
>
>Scott, I think you've missed a critical point. A speaker is a transducer,
>and that means that in the process of following the waveform ideally, the
>results in the air can be a little different than what was intended.
>
>> If you can somehow arrange for perfect coupling so that the woofer
>> excursion perfectly matches the input signal, doppler effects should
>> be a non-issue.

If you made a transducer that increases and decreases the air
pressure without using physical movement, there would be no doppler
distortion. You could have two valves that alternately open and close,
connected to a source of compressed air and the other to a vacuum,
which would cause increase and decrease in air pressure without
movement, but this doesn't seem practical for good audio reproduction.
The problem is that the cone moves, and its movement is significant
in relation to the speed of sound in air.

>No. Imperfect woofer excursion is more along the line of AM effects.
>
>> Unfortunately this does not go along well with
>> accurate frequency response in the real world.
>
>I think that this issue of imperfect cone motion has some light shed on it
>by contemplating a system with a tweeter, versus a system that lacks one. In
>both cases the respective signals are transduced into the air with ideal
>waveforms. The Doppler comes from the fact that the HF gets transduced into
>the air from a platform with large-scale motion due to some other signal.
>The tweeter does not have this situation.
>
>It's like the train whistle. The whistle itself need not be affected by its
>motion through the air. The fact that the whistle is moving w/r/t the
>listener

AND that the speed of sound is finite, and that the speed of the
train is a significant percentage of the speed of sound...

>is the root cause of the Doppler effect.

A lot of stuff has already been covered (and uncovered and
discovered and recovered...) on "another forum." If anyone wants to
read some "background info" before posting further (I suggest it just
to see what has already been rehashed), read these threads "Drum dB's"
and "Doppler Distoriton?" on alt.music.home-studio:

http://groups.google.com/groups?dq [...] e%2BSearch
or
http://makeashorterlink.com/?R2DD35609

http://groups.google.com/groups?dq [...] e%2BSearch
or
http://makeashorterlink.com/?H2ED62609

And a heads up, "Porky" over there is quite similar in demeanor to
"Phil Allison" here on RAP.

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

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Arny Krueger wrote:

>>I just ask that you draw no conclusions from a system that
>>contains measurable non-linearity in the transducer itself.
>
>
> I think that's being too restrictive. We have at least two ways to
> distinguish AM from FM. The fact that we're finding so much AM is probably a
> guide to the most practical answer.

Arny, when you start mixing distributed non-linearities such
as that in the surround, that of cone distortion, that of
the magnetic circuit, etc. It is not generally possible to
describe the resulting form of distortion. It most likely
involves recursion and thus results in chaotic effects. I
can see no reason why FM cannot occur as a consequence of
these mixed factors. Just about any form of non-linearity
can result from them.

In fact, when I simulated a simple model of the described
effect, the distortion produced was chaotic and broadband,
not isolated spectral lines.

The only way to experimentally learn about the effect we are
discussing is to isolate it.

Until that can be done the effect described remains
hypothetical from an experimental standpoint.


Bob

--

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

A. Einstein

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Robert Morein wrote:


>>I'm afraid this is incorrect. The heuristics usually used
>>to describe this effect apply equally well to an ideal
>>system where a massless, infinitely compliant and lossless
>>piston is driven by a force function.
>>
>
> 1. You can't contradict what I said with a heuristic. See
> http://www.hyperdictionary.com/dictionary/heuristic

I'm not trying to; I'm trying to cast doubt on the heuristic
description. All too often when this is attempted, the
result is a failure of intuition.

>
> 2. The "heuristic", by which you probably mean approximation, is exactly
> that and no more, a very useful approximation.

Can you state an expression for it? That, if it is
justifiable from first principles, eliminates heuristics.

>
> 3. The subject under discussion is whether the "heuristic" breaks down in a
> meaningful fashion. This is the very core of the discussion. If Doppler
> effect influences the sound output, then, to the extent which it does, the
> "heuristic" is invalid.

No, it is whether it has any validity at all.

>
> 4. I do not imply by the above that Doppler is important, or is not. But
> please understand that the following is an equivalence relationship:
>
> a. If Doppler is unimportant, then the heuristic you mention is, for all
> practical purposes, a very good one.

It is just hand waving to this point.


Bob
--

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

A. Einstein

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Ben Bradley wrote:


>
> If you made a transducer that increases and decreases the air
> pressure without using physical movement, there would be no doppler
> distortion.

Impossible on first principles of acoustics. Increasing and
decreasing the air pressure results in totally predictable
changes in the velocity of the air. The are simply
proportional through the (real) characteristic impedence of air.

If the SPL is high enough, yes, nonlinearities occur in the
air and the above isn't true but you have to get pretty
darned high for that to have any signifigance. At the
levels we listen to, air is highly linear.

My argument is simply that if you can reproduce velocity of
air then by the above, the pressure has no choice but to
remain in phase and proportional if it remains in the linear
regime. If you can measure it you can reproduce it by
moving a piston with the measured velocity. Exactly. The
resulting pressure wave contains no distortion.

The above argument stands whether we are talking about
reproducing pressure or velocity because in air they are in
phase and proportional in a plane wave and deviations from
planarity only have linear consequences.


> And a heads up, "Porky" over there is quite similar in demeanor to
> "Phil Allison" here on RAP.

Actually, Porky has been nothing but congenial and careful
of late. It was on that tentative basis that I chose to
address his post.


Bob
--

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

A. Einstein

Archived from groups: rec.audio.pro (More info?)

"Bob Cain" <arcane@arcanemethods.com> wrote in message
news:cfe0nk0261j@enews2.newsguy.com
> Arny Krueger wrote:
>
>>> I just ask that you draw no conclusions from a system that
>>> contains measurable non-linearity in the transducer itself.
>>
>>
>> I think that's being too restrictive. We have at least two ways to
>> distinguish AM from FM. The fact that we're finding so much AM is
>> probably a guide to the most practical answer.
>
> Arny, when you start mixing distributed non-linearities such
> as that in the surround, that of cone distortion, that of
> the magnetic circuit, etc. It is not generally possible to
> describe the resulting form of distortion.

What, whether it is AM or FM or what proportion of which?

Sources don't matter, all that matter is a clean enough signal to analyze.

> It most likely
> involves recursion and thus results in chaotic effects. I
> can see no reason why FM cannot occur as a consequence of
> these mixed factors. Just about any form of non-linearity
> can result from them.

Lets go down your list:

(1) that in the surround - doesn't matter where the Doppler comes from,
just that it is.
(2) that of cone distortion - doesn't matter where the Doppler comes from,
just that it is.
(3) the magnetic circuit - not moving, so it can't cause Doppler


> In fact, when I simulated a simple model of the described
> effect, the distortion produced was chaotic and broadband,
> not isolated spectral lines.

We get pretty clean isolated spectral lines from real-world measurements.
Guess what that says about the simulation?

>Until that can be done the effect described remains hypothetical from an
experimental standpoint.

We don't need a working theory to have believable experimental results.

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""Granma" Dave Schein II, CSO" <granmadave@yahoo.com> wrote in message
news:F0wSc.5602$EQ5.3679@nwrddc03.gnilink.net

> In layman's terms, what, exactly, is Doppler Distortion?

http://www.sweetwater.com/insync/word.php?find=Doppler

The Doppler effect, named after a German physicist (how come things are
always named after a German physicist?), is the apparent change in pitch of
the sound that occurs when the source of the sound is moving relative to the
listener. For example: A car horn will sound higher in pitch as it
approaches, and lower in pitch after it passes us. This is one principle
that is employed in a rotating speaker system like a Leslie. The rapid
movement of the horn to and away from the listener creates a sort of vibrato
effect. There are many modern effects units that simulate the Leslie sound,
and also offer other types of Doppler effects.

If a loudspeaker is producing both low and high frequencies, the low
frequencies will cause the cone to move alternatingly toward and away from
the listener (obviously high frequencies do this too, but the lows are much
more pronounced). As this is happening the perceived pitch of the higher
frequency sounds rise and fall at a rate (or rates) equal to the low
frequencies moving the cone. This is actually Frequency Modulation of the
high frequency by the low frequency, and is called "Doppler Distortion." It
manifests itself as a sort of "muddiness" (subjective audio term #108) of
the sound.

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"Bob Cain" <arcane@arcanemethods.com> wrote in message
news:cfe100026b8@enews2.newsguy.com...
>
>
> Robert Morein wrote:
>
>
> >>I'm afraid this is incorrect. The heuristics usually used
> >>to describe this effect apply equally well to an ideal
> >>system where a massless, infinitely compliant and lossless
> >>piston is driven by a force function.
> >>
> >
> > 1. You can't contradict what I said with a heuristic. See
> > http://www.hyperdictionary.com/dictionary/heuristic
>
> I'm not trying to; I'm trying to cast doubt on the heuristic
> description. All too often when this is attempted, the
> result is a failure of intuition.
>
Well, then, no argument on that account!
> >
> > 2. The "heuristic", by which you probably mean approximation, is exactly
> > that and no more, a very useful approximation.
>
> Can you state an expression for it? That, if it is
> justifiable from first principles, eliminates heuristics.
>
No, I can't, and I don't think it's been done.
The heuristic you mention is a linear approximation, which is useful because
it allows the "transfer function" methodology.
The Doppler effect is, like most things considered distortions, nonlinear.
The transfer function methodology is not extensible to nonlinear processes.
Hence, if the Doppler effect is considered important in a particular
instance, the machinery of linear systems cannot be used.

An additional way it breaks down is thus:
We usually consider a particular distortion in terms of percentages of the
total signal strength.
But the Doppler effect shifts ALL signals coming out of the driver, to some
degree.
Usually, one mentally organizes the problem in terms of a low frequency
signal, and a high frequency signal, and the peturbation in frequency caused
by the low frequency signal on the high frequency signal. However, this is
artificial. Two signals, one at 50 Hz and another at 51 Hz, theoretically
intermodulate due to the Doppler effect.

Unless some arbitrary standard is used, there's nothing left of the original
signal at all! Ridiculous, of course, but this shows the weakness of
thinking in linear terms about a nonlinear system. A percentage of Doppler
distortion cannot be recognized unless arbitrary criteria are used to define
it.

Consequently, I wave my hand at you (in a friendly gesture .

> >
> > 3. The subject under discussion is whether the "heuristic" breaks down
in a
> > meaningful fashion. This is the very core of the discussion. If Doppler
> > effect influences the sound output, then, to the extent which it does,
the
> > "heuristic" is invalid.
>
> No, it is whether it has any validity at all.
>
> >
> > 4. I do not imply by the above that Doppler is important, or is not.
But
> > please understand that the following is an equivalence relationship:
> >
> > a. If Doppler is unimportant, then the heuristic you mention is, for
all
> > practical purposes, a very good one.
>
> It is just hand waving to this point.
>
>
> Bob
> --
>
> "Things should be described as simply as possible, but no
> simpler."
>
> A. Einstein

Archived from groups: rec.audio.pro (More info?)

On 11 Aug 2004 18:14:09 -0400, kludge@panix.com (Scott Dorsey) wrote:

>\"Granma\" Dave Schein II, CSO <granmadave@yahoo.com> wrote:
>>wow! NEAT!
>>
>>A question about the Leslie, though: I thought the Leslie had two speakers,
>>one high and one low, rotating at user-defined rates. Would that cause
>>Doppler, or simply a tremolo effect based around the directionality of the
>>speaker? I.e., if the speaker is pointed away from the microphone (or ear),
>>the volume would be softer, and vice-versa?
>
>This is true, but the pitch also changes as the thing rotates. Play a
>note, and you not only hear tremolo caused by changing amplitude, you
>also hear vibrato caused by changing frequency. This is part of why the
>Leslie is so hard to model accurately and why most of the Leslie simulators
>don't sound like the real thing.
>--scott

So I've often wondered whether a Leslie was more acoustical than
electrical instrument <g>.

Edi Zubovic, Crikvenica, Croatia

Archived from groups: rec.audio.pro (More info?)

> The Doppler effect, named after a German physicist (how come
> things are always named after a German physicist?)...

They aren't. I own lots of Land cameras, and they're named after a
Russian/American physicist.