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Doppler Distortion - Fact or Fiction
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. I've asked numerous places for that, including alt.sci.acoustics, sci.physics and sci.physics.research and have looked hard for a solution in the literature. Nothing to date. I think there is a good reason for that; the force and pressure in the wave are simply proportional and thus there is no such thing as Doppler distortion. At least that is what my reasoning from first principles says. So I'm issuing a challenge to anyone here that thinks they might be able to analyze it and produce an equation that isn't a simple proportionality and is non-linear, as it must be for the frequency modulation required of this so called Doppler distortion. If you do it and it withstands peer scrutiny, you get the pleasure of knowing that I have a leather hat meal awaiting me (and the strong possiblity that you've gone where no one else has gone before.) :-) No heuristic arguments involving two tones, please, but a real (or complex) equation that applies to any signal. Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
#2
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"Bob Cain" I've got an argument that so far has withstood some scrutiny which shows that Doppler distortion in a myth. ** Doppler distortion from what cause ??????? The air ? Some moving cone driver ???? ........... Phil |
#3
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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). |
#4
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"William Sommerwerck" 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? ** No driver ever does that - the excursion quadruples every octave lower you go for the same voltage input. 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). ** Bob Cain never mentioned drivers at all. He may well be referring to Doppler in the air itself. .............. Phil |
#5
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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. |
#6
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"Carey Carlan" wrote in message
. 203 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. That's it. 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. Yes, that's it. Just like the whistle on a busy train engine in a switch yard. No amplitude modulation distortion required. I probably have an easier time than most with this sort of thing because of my long-ago tour with Uncle Sam as a Doppler Radar technican. If some of the arguments presented were taken to their logical conclusion, the whole missle system I worked on would have never worked. While high tech military toys of that era tended to be dodgy, the one I worked on could at least partially work, and did I see the most important parts of it work from incoming bogey to big bang in the sky. |
#7
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"Bob Cain" 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. A couple of us have been pursuing the measurement route, and as often the case the results are practically speaking, not all that startling. First off, we've found that actually measuring Doppler distortion is not all that easy. This is complexified by the fact that speakers have lots of distortion of many kinds, and at most loudspeaker Doppler distortion is relatively small. BTW, along the way, I've found reason to doubt a lot of published jitter measurements. They don't seem to distinguish AM from FM. But bottom line, we think we are measuring some Doppler distortion. However, there's so many other kinds of distortion of a similar nature happening at the same time, that it's practically a non-issue. People who like looking at raw evidence can visit: http://www.pcavtech.com/techtalk/doppler/ . It might be most informative to compare these two sets of graphs and data: "Triple Tone Lab Measurements - 316 millivolts RMS" and "Triple Tone Lab Measurements - 10 Volts RMS" Triple Tone Lab Measurements - 316 millivolts RMS shows a speaker operating at a 1 meter SPL of about 78 dB. The background noise in the room actually masks harmonics generated by the 50 Hz fundamental. Most visible distortion products are 60 dB or more down (0.1%). Of course 0.1% is an absolutely rediculously huge amount of distortion compared to the 0.003% THD that some on RAP seem to want to worry about. Triple Tone Lab Measurements - 10 Volts RMS shows a speaker operating at a 1 meter SPL of about 105 dB SPL. Don't be confused by the 1 meter SPL of 105 dB, out in the room the SPL is loud enough, but fairly modest (under 100 dB) by modern standards. Frankly, with 10 volts applied to its voice coil, this speaker has been turned just about every which way but loose. 50 Hz THD is some place around 10%. IM is around 3%. It sounds pretty badly stressed, in real life. The driver under test is a Peerless 6.5" speaker that is similar to what you might find in one of the better near-field monitors. I would imagine that a speaker like this would be rated for a maximum 1 meter SPL in excess of 110 dB. You can imagine how distorted it is under those conditions! |
#8
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Arny Krueger wrote:
"Bob Cain" 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." |
#9
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"Scott Dorsey" wrote in message
Arny Krueger wrote: "Bob Cain" 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. I would say that the exaggerated excursion at low frequencies is a contributing cause for Doppler distortion, but not the only cause. 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. Double the diaphragm area, and you get the same benefit. Subwoofers make even more sense! BTW, this effect is the justification for the triple-tone tests posted at http://www.pcavtech.com/techtalk/doppler/ . The FM-related sidebands on the uppermost tone (4.25 KHz) will be about 4 times larger, compared to the carrier, as those on the middle tone (1.0 KHz). The two tones are not even multiples of each other so that the sidebands from each tone will not be unlikely to land on top the sidebands of the other. 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. 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. 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. |
#10
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Arny Krueger wrote:
"Scott Dorsey" 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." |
#11
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"Bob Cain" 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. I would be willing to wager that it's damn near unmeasurable and impossible to hear, compared to the other types of distortion loudspeakers introduce. Of course "It can be shown" that it exists for any mechanical transducer--effectively a moving sound source, and be relatively easy to calculate--it's a fairly straightforward manipulation of the wave function for velocity, then make the velocity a function of the input signal . . . "Easy" to set up . . . but the algebra and trig gets a smidgeon knotty. Anyone have Maple or Mathematica handy? |
#12
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"William Sommerwerck" wrote in message ... 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). |
#13
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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. I think they traded one problem for another. By building the horn flare out they may have avoided modulating the HF by the 15" cone, but in so doing they also placed a substantial acoustic mask in front of the woofer. Still, the UREI implementation was more successful than the Altec device it replaced, with its hard edges, sectoral dividers, & straight sides. Scott Fraser |
#14
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I recorded some violins, viola & cello through a nice speaker last week, a
lovely lacquered maple cabinet made by somebody named Leslie. I'll be damned if there wasn't a ton of distortion AND doppler shifting. Just couldn't get rid of it. It kind of made everybody seasick, but they all loved it anyway. Scott Fraser |
#15
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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. |
#16
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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 |
#17
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so, anybody see where bob went...?
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#18
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William Sommerwerck wrote: 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? Precisely, and an argument by reciprocity shows that to be the case. If you measure the particle (voxel if you don't like discrete) velocity and then make the reproducing system follow that velocity function then what goes out as a wave will be the same as what was measured. 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). Non-linearities mix in very wierd ways. The only real experimental test would require a super-linear driver and those are hard to find. Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
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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 |
#20
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William Sommerwerck 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." |
#21
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Carey Carlan wrote: 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. That's the definition. What is needed to put the question to bed is a general dynamical equation for what happens at a piston-air interface which will yield that result when applied to a sum of such sinusiods. No such equation has been forthcoming in places where it should be a trivial exercise for those in attendance. Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
#22
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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 |
#23
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"William Sommerwerck" wrote in message ... 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. Electrostatics should be less prone to doppler than many other speakers, because of the large diaphram size. 1/4" is an impossible excursion for an electrostatic, because electric force is much weaker than magnetic force. The diaphrams of these speakers move minutely, inversely proportional to the size of the diaphram for a given SPL. The most serious examples would be: 1. a two-way, with a bass/mid driver that's really pumping 2. A three-way with a small driver optimized for dispersion, in which case both the bass and mid drivers might be stressed. I'm not commenting on the audibility of Doppler, only that an electrostat is not a good example. I do own Acoustat 2+2's, and I do not complain about Doppler distortion Unless it were introduced to me in a laboratory setting, I would have no way of recognizing it. |
#24
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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 |
#25
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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! 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. |
#26
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On Wed, 11 Aug 2004 10:56:27 -0700, Bob Cain
wrote: 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. That does not seem fair. . . Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
#27
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"Bob Cain" wrote in message
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. 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. |
#28
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William Sommerwerck wrote:
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. In most cases, yes. When Doppler distortion becomes a big problem is in systems like the Lowther when you have both large excursions and wide bandwidth through a driver. Or, in coaxial systems, where the seperate driver is still using the bass driver cone. In typical multi-way systems, the issue is much smaller. Note, also, that such a large excursion would usually occur on a bass transient, not during "normal" (???) music. Right. But what if if the string section is playing a nice long note that is held for a while, and there is a hit on the tympani? Can you hear the strings being modulated? The "Ondekoza" track I submitted to one of the RAP CDS should be a real torture test since it has some clean flute notes combined with heavy low end . I can't hear any modulation at all on the Magnepans, but I can hear lots on my father's old Wharfdales (which have 6 dB/octave crossovers on the top and bottom and run the midrange full range). --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#29
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On Wed, 11 Aug 2004 11:47:23 -0400, "Arny Krueger"
wrote: "Scott Dorsey" wrote in message 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=&...oogle%2BSearch or http://makeashorterlink.com/?R2DD35609 http://groups.google.com/groups?dq=&...oogle%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 |
#30
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U-CDK_CHARLES\Charles wrote: Of course "It can be shown" that it exists for any mechanical transducer--effectively a moving sound source, and be relatively easy to calculate--it's a fairly straightforward manipulation of the wave function for velocity, then make the velocity a function of the input signal . . . "Easy" to set up . . . but the algebra and trig gets a smidgeon knotty. Anyone have Maple or Mathematica handy? Nope, or I'd lend it to you. If it can be "shown" in a fully general way that withstands scrutiny I really want to know the answer. Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
#31
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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 |
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"Bob Cain" wrote in message
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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On Wed, 11 Aug 2004 10:56:27 -0700, Bob Cain
wrote: I just ask that you draw no conclusions from a system that contains measurable non-linearity in the transducer itself. No amplitude non-linearity is needed to generate phase or frequency modulation. I'm not following this discussion at all. Are you asking if a train whistle's pitch changes as it passes by? Or that a recorded train whistle played through a speaker doesn't change pitch as it passes by? Chris Hornbeck |
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""Granma" Dave Schein II, CSO" wrote in message
In layman's terms, what, exactly, is Doppler Distortion? http://www.swee****er.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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wow! NEAT!
It makes total sense, but I had never thought about it in terms of Doppler. I have found this to be a problem with my home stereo, and I've solved it in my own experience by using crossovers and filters to isolate the output of the speakers. 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? Thank you for your information, -gran -- Dave Schein II, CSO Printergy, Inc. - Moving a Million Documents to the Web! www.printergy.com DOCHighway, Inc. - Wherever You Are! Wireless! www.dochighway.com CDs, DVDs, Scanning, Document Management, Knowledge Sharing... ...The Future! 2066 York. Rd. Suite 205 Baltimore, MD 21093 410-561-8436 - TEL 410-561-1220 - FAX 443-803-2119 - Direct "Arny Krueger" wrote in message ... ""Granma" Dave Schein II, CSO" wrote in message In layman's terms, what, exactly, is Doppler Distortion? http://www.swee****er.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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\"Granma\" Dave Schein II, CSO 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 -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
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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. Point... My memory of modulation theory is that the only difference between AM and weak FM is the phase of the sidebands. (This is how modern high-powered AM transmitters are built -- the carrier is weakly FM modulated, then amplified, then goes through a phase shifter. Or something like that.) So... If you analyze the sideband frequencies into their AM (in-phase) and FM (quadrature) components, you have the relative amounts of IM and Doppler distortion. The FM component is, by definition, Doppler distortion. (Right? ???) So its source or cause doesn't matter. |
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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. |
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