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#41
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On Wed, 11 Aug 2004 01:31:04 -0700, Bob Cain
wrote: 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 I think I understand what you're getting at, so let me restate it non-mathermatically, for those of us who are sound techs rather than audio engineers. Example 1: I take a tiny 2" speaker, and mount in on the center of an 18" high-excursion driver. The tiny speaker has tiny wires leading to a tiny amplifier. I drive it with 4KHz; it reproduces the tone. Now I drive the 18" driver with 50Hz at maximum excursion. I hear a 50Hz vibrato on the 4KHz tone. This is Doppler distortion. Example 2: I generate a 4 KHz tone and a 50Hz tone. I sum them, and feed them to a full-range speaker through an amplifier with low IM distortion. From the speaker, I hear 4KHz and 50Hz. No vibrato, because the speaker is accurately reproducing the waveform that is the sum of the two tones. No Doppler distortion. Example 3: I take the full range speaker which is accurately reproducing the two-tone waveform, and shake it rapidly back and forth, toward and away from the listener. The listener hears variations in pitch. Dopppler distortion. Mike T. |
#42
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Example 2:
I generate a 4 KHz tone and a 50Hz tone. I sum them, and feed them to a full-range speaker through an amplifier with low IM distortion. From the speaker, I hear 4KHz and 50Hz. No vibrato, because the speaker is accurately reproducing the waveform that is the sum of the two tones. No Doppler distortion. Well... No. The 4kHz signal is being reproduced from a source that is moving with respect to the listener. |
#43
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Arny Krueger wrote: Why? Separation of variables is essential to an experiment attempting to measure the consequences of one effect. The reason is quite clear. The question at hand is about loudspeaker Doppler distortion. All known loudspeakers have copious amounts of measurable nonlinearity.. If we disallow experimental results from loudspeakers that have measurable non-linearity, we disallow all experiments with loudspeakers. And rightfully so. Please believe that I'm not trying to be right here but just correct. I will be just as happy if someone can come up with the formal theoretical underpinning of this hypothetical phenomenon as I will if it is found that there isn't one. So far no physicist or acoustician that reads usenet has even tried in public. What's up with that? It's not like it's an uninteresting problem. Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
#44
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A question about the Leslie, though: I thought the Leslie had two
speakers, Yes. one high and one low, rotating at user-defined rates. There are two speeds, slow & fast, controllable from the organ console. You can trim these speeds somewhat within the unit, although it isn't a realtime performance control. And, there are two motors, so some customization is possible by offsetting the low & high frequency slow & fast speeds. BTW, they rotate in opposite directions. 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? It's complex. There is amplitude modulation, frequency modulation, phase modulation, timbre modulation, varying amounts of overdive distortion from the tube amps, & a whole lot of reflections inside the cabinet. In a word, they sound fantastic. Scott Fraser |
#46
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Arny Krueger wrote: 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? Whether it is even formally describable. Sources don't matter, all that matter is a clean enough signal to analyze. The hell they don't. If what is generating the data to be measured cannot be characterized then neither can the data. Lets go down your list: (1) that in the surround - doesn't matter where the Doppler comes from, just that it is. I don't follow this. (2) that of cone distortion - doesn't matter where the Doppler comes from, just that it is.' This either. (3) the magnetic circuit - not moving, so it can't cause Doppler I understand this one. The question remains whether FM can be ruled out of an active system that has these forms of distortion in a distributed and interacting fashion. Can it? 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? What's it say about the system under test? What does it specifically say about Doppler distortion? We don't need a working theory to have believable experimental results. Absolutely agreed, but to have a believable experimental result all factors that can contribute to the data in the same way that the phenomenon being investigated can must either be completely characterized or eliminated. This is fundamental. Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
#47
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"William Sommerwerck" wrote in message
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.) Agreed. 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. Agreed. And what we find is a mixture of AM distortion and FM distortion. A number of other tests pass, as well. The FM component is, by definition, Doppler distortion. (Right? ???) Agreed. So its source or cause doesn't matter. |
#48
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On Wed, 11 Aug 2004 01:31:04 -0700, Bob Cain
wrote: 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. OK, now I see the problem. Frequency modulation doesn't require and, in fact, is independent of, non-linearity in the sense used here. FM sidebands are Bessel functions generated by expanding the right hand side of an expression that includes angular velocities and frequencies *only*. IOW, "linear". Chris Hornbeck |
#49
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Mike Tulley wrote: I think I understand what you're getting at, so let me restate it non-mathermatically, for those of us who are sound techs rather than audio engineers. Disqualified. :-) Example 1: I take a tiny 2" speaker, and mount in on the center of an 18" high-excursion driver. The tiny speaker has tiny wires leading to a tiny amplifier. I drive it with 4KHz; it reproduces the tone. Now I drive the 18" driver with 50Hz at maximum excursion. I hear a 50Hz vibrato on the 4KHz tone. This is Doppler distortion. No you won't and I'm going to explain why in a response to my original post since this argument in various forms has come up frequently and was in fact the original motivation for thinking there was such a thing. The flaw in this and the traditional reasoning finally came to me just now with a response to you in progress and pending while I watched the news. Stay tuned to this thread. :-) Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
#50
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I've been really stuck finding the flaw in the traditional arguement for Doppler distortion and in some of the scenarios that have been presented here in argument for it. It finally just came to me. It's really simple and even easy to understand. Doppler shift is a phenomenon that occurs when a source is moving with respect to the medium or _through_ the medium in which it is generating a wave. In the case of a loudspeaker, or a little one mounted on a big one, or whatever, it is not moving with respect to the medium, it is moving the medium. There is a fundamental difference. In the situation presented of a little speaker mounted on a big one, you will get Doppler distortion only if the big one is acoustically transparent. When it is acoustically rigid you have an entirely different situation and no Doppler distortion will occur. Doppler distortion in loudspeakers is a dead issue. It does not exist. I cannot explain the posted data other than to wonder if the effect was accidently in the input data. Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
#51
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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. ** Try modulating a 2000 Hz tone so the frequency shifts up and down by 5 Hz - see how wrong you are. BTW ES speakers have large diaphragm areas and small excursions so they have less Doppler than cone speakers. ............... Phil |
#52
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On Wed, 11 Aug 2004 16:58:54 -0700, Bob Cain
wrote: No amplitude non-linearity is needed to generate phase or frequency modulation. I don't know what amplitude non-linearity is. I described what linearity is in another post. When distributed linear and non-linear factors are all mixed up with an energy source driving the whole thing there isn't a whole lot that can be said about what can come out of it. By amplitude non-linearity I only mean the stuffs in the transfer function errors that don't include time. Harmonic and intermodulation distortions; like that. FM doesn't require any amplitude non-linearity. That may be the source of several posters' confusions. It's an interesting topic. (May you live in interesting topics!) Chris Hornbeck |
#53
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On Wed, 11 Aug 2004 18:26:42 -0700, "William Sommerwerck"
wrote: Question: How does the air in front of the speaker "distinguish" between the cone moving back and forth, and the driver as a whole being moved back and forth (without any signal applied to the voice coil) at the same rate and amplitude? Exactly right. This can be restated as "air is very low impedance". Conventional speakers operate into something close to a short circuit. Chris Hornbeck |
#54
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Chris Hornbeck wrote:
On Wed, 11 Aug 2004 01:31:04 -0700, Bob Cain wrote: 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. OK, now I see the problem. Frequency modulation doesn't require and, in fact, is independent of, non-linearity in the sense used here. FM sidebands are Bessel functions generated by expanding the right hand side of an expression that includes angular velocities and frequencies *only*. IOW, "linear". Right. The whole system is completely linear (if you assume perfect drivers and noncompressable air), and can be modelled in a linear fashion. The easiest way to do it is to take a coaxial speaker as your example. You got two parts, first a way to determine the woofer excursion as an instantaneous function of input signal, which is easy to do and a matter of some simple box modelling, and secondly a way to take the output of that and use it to modulate the tweeter signal. And that is just a Bessel function. You should be able to knock this out in Matlab in fairly short order. Doing it with a full-range driver is harder because you can't easily just draw a line and say below this point is modulating signal and above this point is modulated. But yes, I don't see any reason why we have to assume anything but a linear model. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#55
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Bob Cain wrote in
: In the situation presented of a little speaker mounted on a big one, you will get Doppler distortion only if the big one is acoustically transparent. When it is acoustically rigid you have an entirely different situation and no Doppler distortion will occur. That equates to saying that a train whistle moving toward you at 20 mph will have no pitch shift if it's in a 20 mph tailwind. Try again. |
#56
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On Wed, 11 Aug 2004 14:18:01 -0700, Bob Cain
wrote: 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. Doesn't a whistle do this, make sound without any movement of anything but the air itself? Or are we talking past each other... Increasing and decreasing the air pressure results in totally predictable changes in the velocity of the air. Right. See the second "pressure =" equation in the other post I just posted... The are simply proportional through the (real) characteristic impedence of air. Yes, if you detect it as I describe below. 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. Agreed, and I am assuming SPL levels that are not unusually high. 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. To measure it with no distortion, you would have to 'follow' the pressure wave: (this is impractical except for the lowest frequencies, it ignores the mass of the mic diaphragm [as well as the rest of the mic!], and lots of other detailed problems, but bear with me) have a mic mounted on a servo (such as a voice coil of a loudspeaker) that moves the mic back when it senses an increase in pressure and forward when it senses a decrease, so there is practically no change in the pressure sensed by the mic diaphragm. The servo signal to move the mic will reproduce the acoustic wave impressing on the mic. 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. Okay, I was obviously going on previous experience. Posts from AMHS were nonexistent for a month or two (I can only imagine Bellsouth's feed for AMHS dried up), then I all of a sudden saw them showing up again in the last few days. Bob ----- http://mindspring.com/~benbradley |
#57
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On Wed, 11 Aug 2004 22:28:36 -0400, 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. Doesn't a whistle do this, make sound without any movement of anything but the air itself? Or are we talking past each other... Also the modulated arc "ion" speakers and the modulated flame (I **** thee not) speakers used in Vietnam for propaganda flights. The latter weren't high fi but were loud enough to allow you to fly high enough to not be seen. A definite advantage given the mood of the crowd. Chris Hornbeck |
#58
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#59
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William Sommerwerck wrote: It's really simple and even easy to understand. Doppler shift is a phenomenon that occurs when a source is moving with respect to the medium or _through_ the medium in which it is generating a wave. In the case of a loudspeaker, or a little one mounted on a big one, or whatever, it is not moving with respect to the medium, it is moving the medium. There is a fundamental difference. Interesting. (Sounds like one of my own posts.) Question: How does the air in front of the speaker "distinguish" between the cone moving back and forth, and the driver as a whole being moved back and forth (without any signal applied to the voice coil) at the same rate and amplitude? I don't think a question of how it distinguishes is meaninful. The physics is simply different if the generator is moving within the medium or moving it. Notice the difference I alluded to between a speaker just moving back and forth in a medium by itself and mounted to the face of a plane that is moving the same way and itself generating a plane wave. Does it not seem logical that there would be a difference? I say that the difference is that when it is mounted on a plane that is moving it just adds to the velocity/pressure of the wave that is generated in a linear fashion and when it is moving the same way by itself, without being part of a larger generator, the result is "Doppler distortion" of whatever it is generating. Thing is, though, that the latter doesn't describe the physics of loudspeakers we use. Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
#60
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Carey Carlan wrote: Bob Cain wrote in : In the situation presented of a little speaker mounted on a big one, you will get Doppler distortion only if the big one is acoustically transparent. When it is acoustically rigid you have an entirely different situation and no Doppler distortion will occur. That equates to saying that a train whistle moving toward you at 20 mph will have no pitch shift if it's in a 20 mph tailwind. Try again. Ok. If the whistle is moving at 20 miles an hour, and so is the wind, and you are standing on the ground then the medium is moving with respect to you. That's the same physics as being on the moving train listening to a stationary whistle. Doppler shift will result. Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
#61
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On Thu, 12 Aug 2004 02:56:06 GMT, Chris Hornbeck
wrote: I wonder how significant it may be in the context of related questions of audibility. On second thought, this is irrelevant ("It's not irrelevant; it's a hippopatamus!"). Note: you have to say the quote above with the cheesiest possible faux Viennese accent. Anyway, just ignore me; I'm an idiot. Chris Hornbeck |
#62
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One other assumption for "perfect" linearity is infinite bandwidth.
It's a given for our models, but I wonder how significant it may be in the context of related questions of audibility. Not so. Linearity and bandwidth are not related. To put it another way, lack of infinite bandwidth is not considered "distortion." |
#63
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On Wed, 11 Aug 2004 21:00:38 -0700, "William Sommerwerck"
wrote: One other assumption for "perfect" linearity is infinite bandwidth. It's a given for our models, but I wonder how significant it may be in the context of related questions of audibility. Not so. Linearity and bandwidth are not related. To put it another way, lack of infinite bandwidth is not considered "distortion." For FM they're (only) related in the context of a complete modulation and demodulation. That's a poor fit to the models we're discussing. Best to just ignore my ravings. OTOH..... Nahhh... Chris Hornbeck |
#64
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William Sommerwerck wrote: Question: How does the air in front of the speaker "distinguish" between the cone moving back and forth, and the driver as a whole being moved back and forth (without any signal applied to the voice coil) at the same rate and amplitude? I don't think a question of how it distinguishes is meaninful. The physics is simply different if the generator is moving within the medium or moving it. That's the problem. There is no difference. Ah, but there is. Notice the difference I alluded to between a speaker just moving back and forth in a medium by itself and mounted to the face of a plane that is moving the same way and itself generating a plane wave. Does it not seem logical that there would be a difference? Nope, I'm afraid Newtonian Relativity says otherwise. Hmmm, do you mean that these two quite different physical systems are going to behave in the same way? Please explain what Newtonial relativity says about these different systems that makes them indistinguishable. Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
#65
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Kurt Riemann "Arny Krueger" It is this added velocity that causes the Doppler distortion. Except that I disagree that there IS any velocity added to the higher wave. The wave IS velocity. No more, no less. Complex but coherent. ** The source of the wave has a velocity - you moron. That alters the frequency in the air. The wave velocity in air is fixed. Given the degree to which Doppler shift is a measurable phenomena regulated by the speed of sound in air, and is very auduble from objects traveling at relatively low velocities AND Given that a speaker moves at a high rate of speed, ** Nope - 1 m/S at most. About 2 mph. Shouldn't doppler distortion be garishly apparent? ** No - you jerk off. If it is so subtle that it has escaped everyone's attention, it must be violating some of Doppler's own physical laws. ** False assertion - thence a false conclusion. Even a Sawtooth wave would theoretically show doppler distortion. Not only that, but if you were to take the harmonic components of a sawtooth wave and play them all out of phase with each other, there would be no difference in sound, but they would theoretically be modulating each other. ** Ignorant bull****. Equates summing with multiplying. Anyway, I stand by my assertions. ** Where the lunatic stands is a place for others to instantly evacuate. ............. Phil |
#66
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On Wed, 11 Aug 2004 21:45:56 -0700, Bob Cain
wrote: William Sommerwerck wrote: Question: How does the air in front of the speaker "distinguish" between the cone moving back and forth, and the driver as a whole being moved back and forth (without any signal applied to the voice coil) at the same rate and amplitude? I don't think a question of how it distinguishes is meaninful. The physics is simply different if the generator is moving within the medium or moving it. How so, Bob? That's the problem. There is no difference. Ah, but there is. Bob, what's the difference if the cone is moved by an electrically supeimposed signal of two sine waves, and if it is moved electrically by one sine wave and the whole speaker frame is moved mechanically by another sine wave? Does the cone not go through the same motion in both cases? Notice the difference I alluded to between a speaker just moving back and forth in a medium by itself and mounted to the face of a plane that is moving the same way and itself generating a plane wave. Does it not seem logical that there would be a difference? The plane will have more surface area than the speaker cone, so the plane would make a louder sound for the same depth of motion. But if the speaker were emitting a high frequency signal, it would be emitted on a surface that's moving back and forth at a lower frequency to a large depth (whether it's just the cone or the driver and plane), and it would be frequency-modulated just the same. Nope, I'm afraid Newtonian Relativity says otherwise. Hmmm, do you mean that these two quite different physical systems are going to behave in the same way? Please explain what Newtonial relativity says about these different systems that makes them indistinguishable. Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein ----- http://mindspring.com/~benbradley |
#67
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Haiku for my new newsgroup stalker
-------------------------------------------------------- I wake up each day Happy and refreshed For I am not Phil A great stereo must never be listened to only analyzed innacuracies in all trivial matters make his head explode The burden of his knowledge Drives him to lash out oddly, at Rivers no social graces and yet he knows everything alone in his cult Australia is home To Phil and the vast outback Thank God for oceans His perfect knowlege Hidden in his dark replies Never sees the point His perfect newsgroup Would be all Phil Allisons All noise? Or silence? His nasty retorts drive away the questioners Now he will be Plonked ---------------------------------------------------------- I am Kurt Riemann No longer seeing that jerk On my newsreader |
#68
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"Bob Cain" wrote in message
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. Can't air pressure be changed by thermal means? |
#69
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Kurt Riemann wrote in message
On Wed, 11 Aug 2004 21:51:48 -0400, "Arny Krueger" wrote: Are you thinking the 50Hz is a modulator? Yes, the 50 Hz is the modulator. It isn't moving identically to a pure 50Hz tone, it has 4k *mixed* in which means that the speaker changes it's excursion to represent the 4k tone. Agreed. If there is no IM distortion, then the two waveforms merely follow the waveform. That's what the cone does. However you must look at the situation from the viewpoint of the receiver/listener, not the viewpoint of the cone. Interesting. The receiver/listener *sees* a 4 KHz source that is moving back and forth in accordance with the 50 Hz sine wave. The position of the cone as a function of time is correct at all times. However, the 4 KHz source is apparently moving from the viewpoint of the listener. Therefore it is Doppler-shifted. It is not "pushing" the waves of the 4k tone. No, but it is moving he source of the 4 KHz tone. It is here where great minds disagree. Disagreement can be good. The position of the cone is analogous to the amplitude of the cone. The position of the cone is correct, so there is no amplitude distortion and no amplitude distortion. However, when the cone correctly follows the electrical energy applied to it, the cone moves a 4 KHz acoustical source (itself) back and forth with respect to the receiver/listener at a 50 Hz rate. It is this added velocity that causes the Doppler distortion. Except that I disagree that there IS any velocity added to the higher wave. The wave IS velocity. No more, no less. Complex but coherent. You're right, there is no velocity added to the wave in the air. It moves at the same speed as any other sound. It's the source that picks up the added velocity. Given the degree to which Doppler shift is a measurable phenomena regulated by the speed of sound in air, and is very auduble from objects traveling at relatively low velocities AND Given that a speaker moves at a high rate of speed, Shouldn't doppler distortion be garishly apparent? Depends on the situation. I don't think that a speaker cone motion due to bass is really all that fast. A woofer cone operating at 50 Hz will move at a peak speed of about 314 inches per second, or about 27 feet per second. This is only about 2% of the speed of sound. If it is so subtle that it has escaped everyone's attention, it must be violating some of Doppler's own physical laws. Doppler has not escaped the attention of the technical community. There are a number of JAES papers about it. Audio Glossaries on the web contain up-to-date information about it. Thing is, Doppler is a relatively simple thing, and the issue is thought to be more-or-less settled within learned circles. I think the last paper I found about it in the JAES archive was from the early 1980s. Even a Sawtooth wave would theoretically show doppler distortion. Yes. Not only that, but if you were to take the harmonic components of a sawtooth wave and play them all out of phase with each other, there would be no difference in sound, but they would theoretically be modulating each other. I don't know about the "no difference in sound" in every case. What we have learned is that Doppler FM distortion always seems to create artifacts that in reasonable worst case real-world speakers, are always submerged in AM effects. Perhaps when we reduce AM effects by an order of magnitude, we'll have to look more seriously at Doppler-related effects. |
#70
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"Phil Allison" wrote in message
Given that a speaker moves at a high rate of speed, ** Nope - 1 m/S at most. About 2 mph. Do the math Phil. The largest stroke woofers around have about 2" linear stroke which can happen at 50 Hz. The frequency at which the maximum stroke can be achieved is limited, because the woofer becomes heavily mass-loaded at higher frequencies, but 50 Hz can be below that frequency for woofers like these. 50*2*pi*2 = 628 ips = about 50 fps = a peak cone velocity about 15 m/S In my other example, I used 1" stroke, and still came up with 27 FPS or about 8 m/S In fact there are commercial woofers with a lot more than 25.6 mm Xmax, and some of them can go a bit higher than 50Hz without becoming so mass-loaded that they can't do their full linear stroke. Speaker cones can also sometimes substantially exceed their Xmax. Bottom line Phil, you're off by about an order of magnitude on maximum cone velocity. But your heart is in the right place - these aren't what could be reasonably called "a high rate of speed". |
#71
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"philicorda" wrote in message
news On Wed, 11 Aug 2004 16:21:24 -0700, William Sommerwerck wrote: Example 2: I generate a 4 KHz tone and a 50Hz tone. I sum them, and feed them to a full-range speaker through an amplifier with low IM distortion. From the speaker, I hear 4KHz and 50Hz. No vibrato, because the speaker is accurately reproducing the waveform that is the sum of the two tones. No Doppler distortion. Well... No. The 4kHz signal is being reproduced from a source that is moving with respect to the listener. Say I have a diaphram like a bass drum, and hit it, surely the same thing is going on (tones at higher multiple frquencies+the skin moving slowly at the fundamental pitch). I believe so. So, would a speaker cone not have to do the same thing to reproduce it? Remember, that a speaker does not reproduce the motion of a drum diaphragm, it reproduces that which was picked up by a microphone that was in the sound field of the drum. Your example would be more valid if we were in the habit of putting transducers on bass drum diaphragms. Is there 'doppler distortion' on acoustic instruments? So it would seem. Could the same thing be said about microphone diaphrams? By picking up a low frequency and high frequency sound at the same time, would the same effect not apply? So it would seem. However, in the case of mics the motion of the diaphragm is so small that its Doppler distoriton is really small. Sorry for all the questions. Well, the big lesson is that in the current context, Doppler FM distortion is submerged by the large amounts of AM distortion in speakers. |
#72
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"William Sommerwerck" wrote in message
One other assumption for "perfect" linearity is infinite bandwidth. It's a given for our models, but I wonder how significant it may be in the context of related questions of audibility. Not so. Linearity and bandwidth are not related. To put it another way, lack of infinite bandwidth is not considered "distortion." Agreed. However, the phrase "Linear Distortion" applies to things like lack of bandwidth. |
#73
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"Chris Hornbeck" wrote in message
On Wed, 11 Aug 2004 18:26:42 -0700, "William Sommerwerck" wrote: Question: How does the air in front of the speaker "distinguish" between the cone moving back and forth, and the driver as a whole being moved back and forth (without any signal applied to the voice coil) at the same rate and amplitude? Exactly right. This can be restated as "air is very low impedance". Conventional speakers operate into something close to a short circuit. I don't think so. Speakers are suspension and/or enclosure air compliance loaded below resonance. They usually become cone mass-loaded around and above resonance. Loudspeaker horns are acoustical transformers that match the high compliance of the air to the relatively low source impedance of most conventional loudspeaker drivers. Horn-loaded drivers are the only ones that operate into an acoustical impedance low enough to be called anything like a matched impedance, and it still isn't anything like an acoustical short. A driver operating into an acoustical short circuit would be motionless. |
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"Ben Bradley" wrote in message
Bob, what's the difference if the cone is moved by an electrically supeimposed signal of two sine waves, and if it is moved electrically by one sine wave and the whole speaker frame is moved mechanically by another sine wave? Does the cone not go through the same motion in both cases? Good point. |
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"Chris Hornbeck" wrote in message
On Wed, 11 Aug 2004 18:08:28 -0700, Bob Cain wrote: Doppler shift is a phenomenon that occurs when a source is moving with respect to the medium or _through_ the medium in which it is generating a wave. In the case of a loudspeaker, or a little one mounted on a big one, or whatever, it is not moving with respect to the medium, it is moving the medium. There is a fundamental difference. I see two flaws here. First is that the FM exists *at the diaphragm* and is independent of media. The FM exists at the receiver or listener. If the speaker and the listener have no relative velocity, no Doppler. People who ride on trains don't hear the whistle of their train as being Doppler-shifted. Been there, done that. Second and lesser is in a way just a restatement of the observation that conventional diaphragms are high impedance and air is low impedance. Low acoustic impedance drivers exhibit low FM distortions (by definition, in this backwards description). Agreed. At some point here we're going to need to talk about horns, but I'm dreading it. Folk get all riled up. Sad too, its really pretty simple - a horn is an acoustical impedance matching transformer. |
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"Bob Cain" wrote in message
Arny Krueger wrote: 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? Whether it is even formally describable. It's formally describable! Sources don't matter, all that matter is a clean enough signal to analyze. The hell they don't. If what is generating the data to be measured cannot be characterized then neither can the data. Well, all we need to know is "This is the sound that is coming out of the front of the speaker". Lets go down your list: (1) that in the surround - doesn't matter where the Doppler comes from, just that it is. I don't follow this. What does a speaker do? It makes sound. What do we do with speakers? We put them in boxes. To make things simple let's consider a sealed box. The sealed box is there to ensure that the only sound we hear comes out of the front of the speaker, not its back. Therefore, all we need to do is characterize the sound that is coming out of the front of the speaker. (2) that of cone distortion - doesn't matter where the Doppler comes from, just that it is.' This either. Same story. All we need to do is characterize the sound coming out of the front of the speaker, regardles of its source. (3) the magnetic circuit - not moving, so it can't cause Doppler I understand this one. The question remains whether FM can be ruled out of an active system that has these forms of distortion in a distributed and interacting fashion. Can it? The general case is that there is both AM & FM distortion. The purpose of the measurement is to determine where or not there is FM distortion. 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? What's it say about the system under test? It's performing a lot different than the simulation that predicts chaos. What does it specifically say about Doppler distortion? Our test finds some Doppler distortion. We don't need a working theory to have believable experimental results. Absolutely agreed, but to have a believable experimental result all factors that can contribute to the data in the same way that the phenomenon being investigated can must either be completely characterized or eliminated. This is fundamental. It's not a problem here because we do have a working, believeable theory about loudspeaker Doppler. |
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"Phil Allison" wrote in message
"Bob Cain" I've got an argument that so far has withstood some scrutiny which shows that Doppler distortion in a myth. ** This article has all the maths re the Doppler effect in woofers. http://www.geocities.com/kreskovs/Doppler1.html I still haven't reviewed it thoroughly, but it looks a lot like some of the JAES papers I've cited recently. But, he blew the experiment, because his results could be and probably are dominated by AM effects. The expeirment part of the article was deconstructed last week in that other forum you participate in, Phil. Forgot? |
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On Wed, 11 Aug 2004 16:21:24 -0700, William Sommerwerck wrote:
Example 2: I generate a 4 KHz tone and a 50Hz tone. I sum them, and feed them to a full-range speaker through an amplifier with low IM distortion. From the speaker, I hear 4KHz and 50Hz. No vibrato, because the speaker is accurately reproducing the waveform that is the sum of the two tones. No Doppler distortion. Well... No. The 4kHz signal is being reproduced from a source that is moving with respect to the listener. Say I have a diaphram like a bass drum, and hit it, surely the same thing is going on (tones at higher multiple frquencies+the skin moving slowly at the fundamental pitch). So, would a speaker cone not have to do the same thing to reproduce it? Is there 'doppler distortion' on acoustic instruments? Could the same thing be said about microphone diaphrams? By picking up a low frequency and high frequency sound at the same time, would the same effect not apply? Sorry for all the questions. |
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"Arny Krueger" "Phil Allison" Given that a speaker moves at a high rate of speed, ** Nope - 1 m/S at most. About 2 mph. Do the math Phil. ** Go pull your tiny dick - Arny. The largest stroke woofers around have about 2" linear stroke which can happen at 50 Hz. ** The OP mentioned simply "speaker" - not sub woofer. You come back with an excursion number for the most extreme sub woofer that exists. Subs are not used to produce frequencies in the kHz range. 50*2*pi*2 = 628 ips = about 50 fps = a peak cone velocity about 15 m/S ** WRONG: V = 2*pi*f * X-max. In my other example, I used 1" stroke, and still came up with 27 FPS or about 8 m/S ** WRONG. Bottom line Phil, you're off by about an order of magnitude on maximum cone velocity. ** But I was talking of a "speaker" - just as the OP was. Speakers have an X-max of about 6mm which is reached at about 30 Hz. 2*pi*.006*30 = 1.13 m/S .............. Phil |
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"Phil Allison" wrote in message
"Arny Krueger" "Phil Allison" Given that a speaker moves at a high rate of speed, ** Nope - 1 m/S at most. About 2 mph. Do the math Phil. ** Go pull your tiny dick - Arny. How do you know what size it is, Phil? Been fantasizing about me? ;-) The largest stroke woofers around have about 2" linear stroke which can happen at 50 Hz. ** The OP mentioned simply "speaker" - not sub woofer. Oh Phil are you saying that subwoofers aren't speakers? You come back with an excursion number for the most extreme sub woofer that exists. No, that one has Xmax that is about 40% more than 1" - about 36 mm if I recollect properly. I was giving you a break! Subs are not used to produce frequencies in the kHz range. Agreed, but due to their long stroke, some people think they might be candidates for Doppler distortion. 50*2*pi*2 = 628 ips = about 50 fps = a peak cone velocity about 15 m/S ** WRONG: V = 2*pi*f * X-max. Agreed, so now we're back to my original example of about 7 m/S Bottom line Phil, you're off by about an order of magnitude on maximum cone velocity. ** But I was talking of a "speaker" - just as the OP was. Phi,l are you saying that subwoofers aren't speakers? Speakers have an X-max of about 6mm which is reached at about 30 Hz. Phil, are you saying that subwoofers aren't speakers? 2*pi*.006*30 = 1.13 m/S But to quote you Phil, "Nope - 1 m/S at most." Phil, even your example, as limited as it is, is 1 m/S |
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