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#41
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"Porky" wrote in message
... The point is that everyone hears the Doppler shift in a train whistle, No, they don't. Everyone hears just a sound. The only way to know that the sound is shifted is to compare it to the original unshifted sound either by hearing it or deriving the shift mathematically by knowing the relative motions and the source frequencies. There's no way the brain or the most advanced mathematical models can look at a *single* sound and tell if it's distorted. but when comparing a "live" sound to it's replica being reproduced by a very accurate loudspeaker system, under the closest to ideal conditions possible, If it were otherwise, we wouldn't be having this discussion! :-) Sure we would. This has always been about theory, not application. As to it being an esoteric discussion, it has certainly become one, but the original question was "Do speakers create Doppler distortion when producing both a HF tone and an LF tone at the same time?", Esoteric simply means limited to small group of people. That has always been the case. Ask a thousand people if they care about the above question and maybe one will say yes. occur in a speaker, then there must a number of ways to minimize it, Nobody has done it even though it has been "known" for decades. It would require *every* speaker in the chain to be free of Doppler distortion. All those guitar licks run through an amp and speaker are distorted to begin with. |
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#42
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"Jim Carr" wrote in message news:JWp8d.12173$mS1.9564@fed1read05...
What bothered me was that I could not (and still cannot) see how a speaker really works. Yeh, I can describe the mechanics involved, but I still don't fully understand the exact physics where the diaphragm creates the sound wave. Is it at the start of the throw? The end? The middle? If it's in the middle of a long throw for a loud low frequency, how does it make the higher frequencies at the same time? That's odd. I don't find this hard to imagine, slowing it down in my mind. The highest speed of the diaphragm forward will result in the highest "sweeping up" of air molecules, resulting in the highest local density. In this mental image, then, the middle of the throw (for a fundamental tone) would produce the peak of the oscillation in the medium. As far as the harmonics, simply remind yourself of the waveform of, say, a flute note, as seen on an oscilloscope, periodic but not sinusoidal. According to the above picture, that's the velocity profile of the diaphragm. This is perhaps why a square wave is hard to push through a speaker, because it demands instantaneous accelerations of the diaphragm. PD |
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#43
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"The Ghost" wrote in message . 6... "Daydream Electric" wrote in : Your abuse of the English language is disgusts me. When's the last time you heard music performed inside a tube? Never? Exactly. We all familiar what can happen (statistically speaking) if enough typewriters are put in the hands of enough monkeys. "Your abuse of the English language IS disgusts me?" Right on, moron! The topic of discussion involves a theoretically ideal situation of a piston in a tube. Since you clearly have **** for brains, I don't expect you to understand why that theoretically ideal situation is being discussed. Nonetheless, why don't you do both of us a favor and simply not read my posts, you stupid idiot. Actually, I had beer for brains that night. **** for brains most of the time, though. |
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#45
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zigoteau wrote: Bob Cain wrote in message ... Hi, Bob, Just a minor correction. Please notice the difference between what I wrote and what Zigoteau wrote. His approach, a first order approximation using a M-T power series, yields (using common symbols and frames of reference): Vp(d,t) = Vd(t - (d - Sd(t-d/c))/(c - Vd(t-d/c))) Mine, which involves no approximation, yields: Vp(d,t) = Vd(t - (d - Sd(t-d/c))/c) I am sorry, Bob, but the second equation does involve an approximation. I was afraid you were going to say something like that. :-) From the logic I used to get it could you please show me the trap I've fallen into? It goes as follows: The fluid velocity (in units of distance/time) of a test particle (a tiny, zero mass thing) located a distance d from the driver face is given by Vp(d,t) = Vd(t-d/c) 1) Where Vd() is the velocity of the driver. The position of the particle relative to its rest position is Sp(d,t) = Sd(t-d/c) 2) Where Sd() is the position of the driver relative to its rest position. What then is the velocity of a second test particle that is located at the first particle's rest position? The distance between the two test particles is Sp(d,t) so that the velocity of the second particle should be given by Vf(d,t) = Vp(d-Sp(d,t),t) 3) Where the d on the LHS is the distance from the driver rest position to the second particle and the d on the right hand side is the distance from the driver face to the first particle. These have the same value. Substituting 1) into 3) gives Vf(d,t) = Vd(t-(d-Sp(d,t))/c) 4) Substituting 2) into 4) gives Vf(d,t) = Vd(t-(d-Sd(t-d/c))/c) 5) I'm afraid I can't see anything in that chain of logic that makes 5) an approximate solution and would appreciate some help with that if you are certain that it is approximate. Thanks, Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
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#46
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"Paul Draper" wrote in message
m... That's odd. I don't find this hard to imagine, slowing it down in my mind. The highest speed of the diaphragm forward will result in the highest "sweeping up" of air molecules, resulting in the highest local density. In this mental image, then, the middle of the throw (for a fundamental tone) would produce the peak of the oscillation in the medium. Thanks for the feedback. Take me through a simple sine wave. It starts at the zero line. It does a semicircle above the line an a semicircle below the line. Describe for me the motion of the speaker in relation to that and at what point the wave is created. I will note that even though this is a contentious thread, I am making a sincere request to help me envision this. Maybe I'm thinking too hard or have some sort of mental block on this issue. This is perhaps why a square wave is hard to push through a speaker, because it demands instantaneous accelerations of the diaphragm. I always though it was because most speakers are round! Nyuk! |
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#47
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"Jim Carr" wrote in message news:vbK8d.13387$mS1.11043@fed1read05... "Paul Draper" wrote in message m... That's odd. I don't find this hard to imagine, slowing it down in my mind. The highest speed of the diaphragm forward will result in the highest "sweeping up" of air molecules, resulting in the highest local density. In this mental image, then, the middle of the throw (for a fundamental tone) would produce the peak of the oscillation in the medium. Thanks for the feedback. Take me through a simple sine wave. It starts at the zero line. It does a semicircle above the line an a semicircle below the line. Describe for me the motion of the speaker in relation to that and at what point the wave is created. Actually it's an ellipse, not a semicircle, and that does make a difference. The cone starts from zero at a fairly rapid rate of acceleration, and the rate slows gradually until it reaches the peak excursion where it reverses direction and gradually starts accelerating in the other direction, the rate of acceleration increases to a peak value as it passes through zero again, then the rate starts decreasing again until it again reverses direction again at the "bottom" of the cycle, where the rate starts increasing again until it again reaches zero, then the whole thing repeats again. Due to the inertia and compressability of air, the air pressure at the cone's surface will vary in proportion to the cone's speed, and this pressure variation becomes a sound wave. Personally, I think the "virtual" source point of the sound wave being radiated by the speaker in the rest point of the cone at the center of the excursion limits, but this has been a subject of debate for a long time, and there are quite a few other theories. One other theory is that the instantaneous virtual sound source point is the position of the cone at any given point in time, which is where the notion of Doppler distortion in a speaker originates. Note that if the virtual source is really the rest point of the cone, then there is no Doppler shift in a loud speaker because the virtual source is not moving with respect to the listener. If the latter theory is correct, then Doppler shift is produced by a loudspeaker because the virtual source is moving with respect to the listener. I will note that even though this is a contentious thread, I am making a sincere request to help me envision this. Maybe I'm thinking too hard or have some sort of mental block on this issue. This is perhaps why a square wave is hard to push through a speaker, because it demands instantaneous accelerations of the diaphragm. I always though it was because most speakers are round! Nyuk! Nope, you can push a square wave through a speaker, but you can only do it once because the sharp edges will tear up the cone. :-) Seriously, some very good speakers will produce a pretty good approximation of a square wave at higher frequencies, but none will do it at lower frequencies, and no speaker will reproduce a true square wave at any frequency because a reproducing a true square wave would require that the cone travel instantaneously from one excursion limit to the other, and as long as the cone has mass, that ain't gonna happen! |
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#48
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"Bob Cain" wrote in message ... Porky wrote: If we get into Doppler shift due to motion in air molecules, I suspect we're getting down to "the bumble bee doesn't really fly because the math says it can't" point. Huh? That's exactly what Doppler distortion is due to. There is a mild non-linear relationship everywhere in a soundfield between particle velocity about a point and the fluid velocity at that point. That was my epiphany. Not much as epiphanies go but, hey, they get fewer and fewer every year. :-) Doppler shift occurs because as the source moves toward the listener, the source's motion causes the apparent wavelength of the sound to shorten, and as it moves away it causes the apparent wavelength of the sound to lengthen. I say "apparent wavelength" because anyone traveling along with the source hears it as a steady tone, meaning the actual wavelength does not change. The point I was trying to make (I think) is that soundwaves travel at the speed of sound but the individual air molecules hardly travel at all. If the equations show that all that much Doppler distortion in a speaker, why can't we hear it? Who says we can't? It's sorta hard to get rid of to do an ABX test on. As I said, some speaker companies have done ABX testing between a reproduced sound and a live sound source, and under the best conditions, even expert listeners had trouble relaibly telling the difference. They weren't trying to test whether Doppler shift existed in a loudspeaker, but if Doppler shift in a speaker were all that audible, why didn't it clue the listeners in to which was the speaker and which was live? If you say that the live source produced Doppler shift too, then if it is audible, it would have to be at similar levels in both the live and reproduced sound, and that would mean that Doppler shift is a natural part of everything we hear, so in a speaker, it could not be considered distortion at all! By Occam's Rasor, either our hearing mechanism has built in compensation, so Doppler distortion doesn't matter, or the math is wrong and it needs to be revised. That isn't to say that it doesn't happen in the piston-in-an-infinite-tube model, it just means that the speaker/room model is a totally different animal. Sure it is but it should get signifigantly worse with a driver in an enclosure rather than in a tube because of the large excursions demanded at the low frequencies to couple anything from an enclosed speaker to a room. You can get real high SPL low frequencies in a tube without much excursion, which is what causes it, but not so with an enclosed speaker in a room. You have to push a whole lot of LF air up close to get what's in the signal to reach you at any distance. It all depends on if the virtual sound source is the rest point of the speaker cone, or if the instantaneous virtual sound source is the surface of the cone at any given point in time. If it is the rest point of the cone, then no Doppler shift occurs because that point is stationary relative to the listener. If it is the surface of the cone at any given point, then Doppler shift does occur in a speaker, but it also occurs in a guitar string when it is plucked, in a drum head when it is struck, in a trumpet when a note is being played, in everything that makes a sound by vibration (meaning everything!) Dunno what you mean by built in compensation nor why there would be anything like that. It's not the kind of thing evolution would have devoted much energy to. There weren't many broadband sound sources to work with even if it had been deemed important for some reason. There are a huge number of natural broadband sound sources, thunder, wind, the ocean, a babbling brook, a river, a herd of stampeding elephants, etc. But I said "either there is some sort of built in compensation, or the Doppler shift isn't audible." The "either" is the key word there, and I think it is the latter, not the former. |
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#49
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"TonyP" wrote in message u... "Porky" wrote in message ... "TonyP" wrote in message It seems you've missed my point. The brain "compensates" for the auditory system itself, because you have NO other point of reference. Sure you do, ever hear of bone conduction? That bypasses the eardrum totally. Not at all. It is concurrent with the eardrum. Audioligists must use large masking signals just to get some figure for bone conduction, but it is less than via the eardrum. I know of no way to seperate the two for comparison purposes, do you? (The worlds audiologists await your reply :-) There is also considerable evidence that we can "hear" the audio in audio modulated RF at certain frequencies, which would seem to bypass the physical hearing mechanism entirely. Some level of diode detection has been demonstated in some cases, usually connected with metal fillings. This couples audio signals via bone conduction. I'm not talking about a filling acting as a diode, some recent research has indicated that the nervous system may, in some cases, be able to directly receive certain modulated rf signals and pick up the audio. The article I read didn't go into much detail, but I believe it indicated that the spinal cord was invloved in the recption process. However, the whole point I was trying to make is that any Doppler shift, if it does occur in a mic or an eardrum, is at such a low level as to be totally inaudible under any normal conditions. However, my point was that if one can tell the difference between a "live" sound and the same sound reproduced on a very high quality sound system, then either there is no audible distortion present, or our ears have a mechanism that compensated for whatever distortion is present, including any Doppler distortion. ??? I meant, "If one can't tell the difference...." And again, the key word is "either" I don't doubt that there isn't a mechanism in our hearing that compensates for Doppler distortion, I think Doppler distortion, if it does occur, is inaudible under anything approaching normal conditions. |
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#50
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"Bob Cain" wrote in message ... Porky wrote: It seems that you missed the point, which was that if there is Doppler shift in a mic because of diaphragm motion, then there must also be Doppler shift in the human ear because of eardrum motion. If that is true then the hearing mechanism must have a method of compensation for it. There is none of _any_ signifigance with either. Excursions in either case are more than a few orders of magnitude away from signifigance. That was precisely my main point. It seemed that some folks were worrying about audible Doppler shift in a microphone, and they weren't groking that any that might exist would be absolutely, totally inaudible. I was merely trying to point out that the alternatives weren't perticularly viable. |
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#51
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"Jim Carr" wrote in message news:qLA8d.13074$mS1.257@fed1read05... "Porky" wrote in message ... The point is that everyone hears the Doppler shift in a train whistle, No, they don't. Everyone hears just a sound. The only way to know that the sound is shifted is to compare it to the original unshifted sound either by hearing it or deriving the shift mathematically by knowing the relative motions and the source frequencies. There's no way the brain or the most advanced mathematical models can look at a *single* sound and tell if it's distorted. When we hear a train whistle or car motor, or siren for more than a second or so, we can tell if it is moving, if it is moving toward us or away from us, and even approximately how fast it's going. It's true that this is a learned experience, but most of us are aware of this even as children. If the *single* sound lasts long enough for the brain to identify it, then the brain can indeed tell whether it's distorted or not. Once again, this is a learned talent, but that is not relevant to this discussion. but when comparing a "live" sound to it's replica being reproduced by a very accurate loudspeaker system, under the closest to ideal conditions possible, If it were otherwise, we wouldn't be having this discussion! :-) Sure we would. This has always been about theory, not application. As to it being an esoteric discussion, it has certainly become one, but the original question was "Do speakers create Doppler distortion when producing both a HF tone and an LF tone at the same time?", Esoteric simply means limited to small group of people. That has always been the case. Ask a thousand people if they care about the above question and maybe one will say yes. Agreed, though the true ratio might be closer to one in ten thousand. :-) occur in a speaker, then there must a number of ways to minimize it, Nobody has done it even though it has been "known" for decades. It would require *every* speaker in the chain to be free of Doppler distortion. All those guitar licks run through an amp and speaker are distorted to begin with. The argument about Doppler distortion in a speaker has been going on for at least fifty years, and probably longer than that, and I have yet to see any empirical proof either way, or even any way of actually testing whether it exists or not. I just thought of a possible test! I think everyone agrees that if you physically move the source, Doppler shift occurs. So, how about taking a source producing a single hf tone and mounting it on a motor, vibration table, etc that is oscillating at a given low frequency, and feeding a loudspeaker with a mix of precisely the same tones at precisely the same levels, such that the LF excursion of the speaker is the same as the LF excursion of the motor or vibration table. Record both using exactly the same equipment and technique, and compare and analyze them to see if there is any difference, if there isn't, then I think we have to agree that Doppler shift does occur in a loudspeaker. If there is a difference, then that would provide some level of empirical proof. |
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#52
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"Porky" wrote in message
... Doppler shift occurs because as the source moves toward the listener, the source's motion causes the apparent wavelength of the sound to shorten, and as it moves away it causes the apparent wavelength of the sound to lengthen. I say "apparent wavelength" because anyone traveling along with the source hears it as a steady tone, meaning the actual wavelength does not change. I gotta disagree with you here. The wavelength is the distance between the waves, right? The easiest way to measure something that's moving is if I know it's speed. So, using the whistle on the train as an example, I can measure the time interval between two waves at the source and calculate the distance between those waves at the source since I know the speed of sound. Now, if the train is moving in the same direction as the waves are being emitted, I must subtract from that distance I just calculated the distance that I know the source traveled between the waves. Therefore, the distance is shorter than when it was stationary. If I'm moving away, it's longer. The stationary listener hears this as a higher or lower pitch. If the receiver is moving relative to the source, it's measurements are "off" so to speak. The ear lacks the ability to add or subtract to compensate for the motion. It perceives a time interval between the waves and the brain recognizes this at a certain pitch. The brain has a fixed value for the speed of sound so to speak. The physical distance between the waves didn't change. Rather the receiver changed distances between the waves but the brain didn't compensate. The key here is that if I performed the *same* measuring techniques when the receiver was moving as I did at the source when the source was moving (taking into account the distance I traveled), I would come up with the same distance between the waves as I did when the source did when it was stationary. It might be fair to say that if the source is moving relative to the listener that there is a real shift in wavelength. If the listener is moving relative to the source the ear fails to compensate for the movement and the receiver perceives an apparent shift in wavelength. I know this sounds like a bunch of relativistic mumbo-jumbo, but it makes sense to me if I use objective techniques to measure distances, not the subject measurements of my ear-brain configuration. As I said, some speaker companies have done ABX testing between a reproduced sound and a live sound source, and under the best conditions, even expert listeners had trouble relaibly telling the difference. Can you cite a reference? There are a huge number of natural broadband sound sources, thunder, wind, the ocean, a babbling brook, a river, a herd of stampeding elephants, etc. But I said "either there is some sort of built in compensation, or the Doppler shift isn't audible." The "either" is the key word there, and I think it is the latter, not the former. Please explain how it is possible to detect any type of distortion in a single sound. You can pick any one sound, just explain how one can measure the distortion. |
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#53
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"Porky" wrote in message
t... I'm not talking about a filling acting as a diode, some recent research has indicated that the nervous system may, in some cases, be able to directly receive certain modulated rf signals and pick up the audio. Can you cite a reference? I meant, "If one can't tell the difference...." And again, the key word is "either" I don't doubt that there isn't a mechanism in our hearing that compensates for Doppler distortion, I think Doppler distortion, if it does occur, is inaudible under anything approaching normal conditions. Just admit you were wrong about the brain compensating for it. Your backtracking with "either" being some "key word" is like saying, "either it's not audible or fairies are sprinkling fairy dust to cast magic spells so we can't hear it." I'm not arguing whether it's audible or not, but the brain compensation thing is just silly. |
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#54
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Porky wrote: Doppler shift occurs because as the source moves toward the listener, the source's motion causes the apparent wavelength of the sound to shorten, and as it moves away it causes the apparent wavelength of the sound to lengthen. Have you seen the derivation of it that I wrote for Zigoteau today (10/05/04)? It is not equivalent to what you say here. Plugging in the motion of a driver comprised of a LF and a HF sinusoid, generating a discrete time signal from that forumula, and eliminating the fundamentals via the FFT and looking graphically at the result shows that the distortion amplitude is a minimum when the LF source velocity is at a maximum and vice versa. Maybe it's not correct to call it Doppler distortion because that would seem to imply the opposite but that name seems to be what we are stuck with. If you say that the live source produced Doppler shift too, then if it is audible, it would have to be at similar levels in both the live and reproduced sound, and that would mean that Doppler shift is a natural part of everything we hear, so in a speaker, it could not be considered distortion at all! The sound is measured at a *point* with a pressure mic, or pressure gradient mic (which is another ball of wax.) If we transform that signal to the *motion* of a driver, the pressure at a distance won't be what we originally measured. That's Doppler distortion. There are a huge number of natural broadband sound sources, thunder, wind, the ocean, a babbling brook, a river, a herd of stampeding elephants, etc. But I said "either there is some sort of built in compensation, or the Doppler shift isn't audible." The "either" is the key word there, and I think it is the latter, not the former. In nature, LF sounds are made by *big* things. The excursion of whatever generates them need not be very big so that the pressure measured at a point, by a mic or an eardrum, is not much distorted. When we try to generate LF sounds with little things like loudspeakers we have to compensate for their smallness by making them move with much bigger excursions than what you are going to get from nature's generators. These bigger excursions of the synthetic device makes more of this kind of distortion than nature does. Evolution would have no reason to take notice of the small natural effect, much less to correct for it. This is not an argument for the audibility of the Doppler distortion of a speaker, I'm just trying to disabuse you of the notion that the ear/brain would have had any reason to correct for it. Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
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#55
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"Porky" wrote in message
... When we hear a train whistle or car motor, or siren for more than a second or so, we can tell if it is moving, if it is moving toward us or away from us, Because we are comparing it against a second sound, namely the sound of a siren when the source and receiver are stationary. If you did not know what it was *supposed* to sound like you would never know anything was amiss. By definition we are not dealing with a single sound. How can the brain "compensate" for a sound when it requires knowing the sound in advance? Nobody is arguing that the brain cannot compare two sounds. However, your notion that somehow the brain could possibly mask Doppler distortion in such a way as to make it inaudible strikes me as silly. |
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#56
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On Tue, 5 Oct 2004 23:50:59 -0700, "Jim Carr"
wrote: How can the brain "compensate" for a sound when it requires knowing the sound in advance? Nobody is arguing that the brain cannot compare two sounds. However, your notion that somehow the brain could possibly mask Doppler distortion in such a way as to make it inaudible strikes me as silly. The brain is remarkably good at focusing on data and ignoring distortion. I'm sure Doppler distortion isn't immune to this principle. CubaseFAQ www.laurencepayne.co.uk/CubaseFAQ.htm "Possibly the world's least impressive web site": George Perfect |
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#57
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"Jim Carr" wrote in message news:F8M8d.13722$mS1.13474@fed1read05... "Porky" wrote in message ... Doppler shift occurs because as the source moves toward the listener, the source's motion causes the apparent wavelength of the sound to shorten, and as it moves away it causes the apparent wavelength of the sound to lengthen. I say "apparent wavelength" because anyone traveling along with the source hears it as a steady tone, meaning the actual wavelength does not change. I gotta disagree with you here. The wavelength is the distance between the waves, right? The easiest way to measure something that's moving is if I know it's speed. So, using the whistle on the train as an example, I can measure the time interval between two waves at the source and calculate the distance between those waves at the source since I know the speed of sound. Or you can measure the apparent frequency and if you know the actual frequency, you can calculate the speed of the source. "Speed of propagation / Frequency" gives you the wave length, but you have to know the actual frequency or wavelength before you can calculate the speed. Whichever way you do it, the wavelength you measure depends on (a) the speed and direction of the source, relative to you, (b) your speed and direction relative to the source, and (c) the actual frequency of the source. Since the wavelength you measure will vary depending on (a) and (b), even if (c) is constant, which measurement is the real one? Actually all of them are, but I used the term "apparent" as meaning as it appears to you, the listener, and "actual" to mean as it appears relative to the source. I think we are actually in agreement, I just didn't make myself clear enough. Now, if the train is moving in the same direction as the waves are being emitted, I must subtract from that distance I just calculated the distance that I know the source traveled between the waves. Therefore, the distance is shorter than when it was stationary. If I'm moving away, it's longer. The stationary listener hears this as a higher or lower pitch. Actually, the waves will propagate in all three dimensions. The whistle isn't particularly all that directional, and it wouldn't matter if it were. Everything is relative to the source and the listener. If the source is moving toward the listener the waves traveling between them will appear shortened to the listener, resulting in an increase in pitch. If the train is moving away from the listener, the waves traveling between them will appear to be lengthened, resulting in a decrease in pitch. However, once again, in order to calculate the source's speed, you must know the actual frequency of the source. If the train is taking a path that will take it past you, you can measure the apparent frequency and at the point when it passes and the frequency stops rising and starts decreasing, you have a very good approximation of the actual frequency (I say approximation because the point at which the train is stationary relative to you is instantaneous so the half cycle on one side of that point will be shortened and the half cycle on the other side will be lengthened, but measuring the whole cycle gives a very close approximation assuming the source is traveling at a steady speed), and you can then calculate the speed of the train relative to you. If the receiver is moving relative to the source, it's measurements are "off" so to speak. The ear lacks the ability to add or subtract to compensate for the motion. It perceives a time interval between the waves and the brain recognizes this at a certain pitch. The brain has a fixed value for the speed of sound so to speak. The physical distance between the waves didn't change. Rather the receiver changed distances between the waves but the brain didn't compensate. There is nothing for the brain to compensate for, since instrumentation at the listener's position will measure exactly the same frequency the listener hears, so the apparent frequancy is actually the real frequency at that point. The key here is that if I performed the *same* measuring techniques when the receiver was moving as I did at the source when the source was moving (taking into account the distance I traveled), I would come up with the same distance between the waves as I did when the source did when it was stationary. It might be fair to say that if the source is moving relative to the listener that there is a real shift in wavelength. If the listener is moving relative to the source the ear fails to compensate for the movement and the receiver perceives an apparent shift in wavelength. No, I think we must come up with an acoustic theory of relativity for this one. It can be stated thusly. "The difference between the measured and actual frequency of any sound source is relative to the relative velocity and direction of travel between the source and the measuring instrument." The fact that the instrument measures exactly the frequency the listener hears proves that it is a real shift in frequency, but the fact that an instrument carried by another listener moving at some different velocity relative to the source will measure (and the listener will hear) a different frequency, proves that it's all relative. I know this sounds like a bunch of relativistic mumbo-jumbo, but it makes sense to me if I use objective techniques to measure distances, not the subject measurements of my ear-brain configuration. Actually since the frequency you hear is exactly the same as that measured by the instrument you carry, there's nothing subjective about it. It's real to you, and to anybody standing beside you, it's just not what the guy in the car going past you is hearing and what his instrument is measuring. Now, here's where it gets really confusing, if the train is traveling faster than the speed of sound, you won't even hear it until it is past you, and when you do hear it, the frequency will be rising at first, even though it's moving away from you, of course as the sound catches up, the frequency will start falling. As I said, some speaker companies have done ABX testing between a reproduced sound and a live sound source, and under the best conditions, even expert listeners had trouble relaibly telling the difference. Can you cite a reference? I know JBL, EV, and Altech have all done this in the past, and I believe it was JBL who did a series of public demonstrations all over the country several years ago. I'm pretty sure it was pre-Internet, but you might find something on it at those companies' web sites. There are a huge number of natural broadband sound sources, thunder, wind, the ocean, a babbling brook, a river, a herd of stampeding elephants, etc. But I said "either there is some sort of built in compensation, or the Doppler shift isn't audible." The "either" is the key word there, and I think it is the latter, not the former. Please explain how it is possible to detect any type of distortion in a single sound. You can pick any one sound, just explain how one can measure the distortion. You probably can't measure it, but you can certainly hear it, if you've ever heard that sound before. As I said, it's a matter of experience. However, if it's the first time you've ever heard that particular sound, of course you have no way of telling if it's distorted or not. It's just that as adults, we hear relatively few "new" sounds that we have no basis of comparison for. Chances are you've heard something that you can use for comparison to judge approximate distortion. |
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#58
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Bob Cain wrote in message ...
Hi, Bob, Mine, which involves no approximation, yields: Vp(d,t) = Vd(t - (d - Sd(t-d/c))/c) I am sorry, Bob, but the second equation does involve an approximation. I was afraid you were going to say something like that. :-) From the logic I used to get it could you please show me the trap I've fallen into? It goes as follows: The fluid velocity (in units of distance/time) of a test particle (a tiny, zero mass thing) located a distance d from the driver face is given by Vp(d,t) = Vd(t-d/c) 1) No, that's your mistake. It certainly follows from the linear wave equation and the assumed boundary condition at infinity that: Vp[d,t] = Vp[x, t+(x-d)/c] , Sd=xd Your equation effectively firstly assumes that the diaphragm is at x=0 and then that it is at x=Sd(t), whichever is most convenient at the time. It is not a bad approximation, but it is an approximation. The other postings casting doubt on the boundary condition between the diaphragm and the air are way off-beam. It is quite legitimate to assume that, at a mesoscopic length scale, the air in contact with the diaphragm is moving at the velocity of the diaphragm, i.e. no slip. Sure, this is an average over a few million molecules. The individual molecules are moving at high speed, of order of the speed of sound, in all possible directions. Each molecule scatters a small amount, and it is Doppler-shifted, with a different Doppler shift for each molecule. Overall it leads to attenuation of the sound wave. We know that in fact sound can travel for miles, so that it is not a large effect, and it is fully taken into account in an effective medium approximation by this small attenuation. Cheers, Zigoteau. |
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"Bob Cain" wrote in message ... Porky wrote: Doppler shift occurs because as the source moves toward the listener, the source's motion causes the apparent wavelength of the sound to shorten, and as it moves away it causes the apparent wavelength of the sound to lengthen. Have you seen the derivation of it that I wrote for Zigoteau today (10/05/04)? It is not equivalent to what you say here. Plugging in the motion of a driver comprised of a LF and a HF sinusoid, generating a discrete time signal from that forumula, and eliminating the fundamentals via the FFT and looking graphically at the result shows that the distortion amplitude is a minimum when the LF source velocity is at a maximum and vice versa. My statement above is pretty much exactly what Doppler postulated in his theory of Doppler shift, so if it is not equivalent, you must be describing something other than Doppler shift. Maybe it's not correct to call it Doppler distortion because that would seem to imply the opposite but that name seems to be what we are stuck with. The use of inappropriate labels has probably been the cause of more disagreements over the course of human history than all other causes combined. So, let's not be stuck with it, define it and give it an appropriate label. Speakers create several forms of audible distortion, I just don't think Doppler shift is the root of any of them. If you say that the live source produced Doppler shift too, then if it is audible, it would have to be at similar levels in both the live and reproduced sound, and that would mean that Doppler shift is a natural part of everything we hear, so in a speaker, it could not be considered distortion at all! The sound is measured at a *point* with a pressure mic, or pressure gradient mic (which is another ball of wax.) If we transform that signal to the *motion* of a driver, the pressure at a distance won't be what we originally measured. That's Doppler distortion. I don't think so, I'm not arguing that the distortion you're defining doesn't exist, I'm saying it's the result of a phenomonen other than Doppler shift. There are a huge number of natural broadband sound sources, thunder, wind, the ocean, a babbling brook, a river, a herd of stampeding elephants, etc. But I said "either there is some sort of built in compensation, or the Doppler shift isn't audible." The "either" is the key word there, and I think it is the latter, not the former. In nature, LF sounds are made by *big* things. The excursion of whatever generates them need not be very big so that the pressure measured at a point, by a mic or an eardrum, is not much distorted. An elephant fart may exceed 90 dB SPL at ten feet, and the elephant's anus is smaller in diameter than an eight inch woofer. You'll have to measure excursion, I ain't gonna get close enough.:-) Ok, I'm getting silly now, but my point is that unless the sound measures something more than 140 dB SPL at the listener's position, it isn't going to cause distortion in the hearing mechanism When we try to generate LF sounds with little things like loudspeakers we have to compensate for their smallness by making them move with much bigger excursions than what you are going to get from nature's generators. These bigger excursions of the synthetic device makes more of this kind of distortion than nature does. Evolution would have no reason to take notice of the small natural effect, much less to correct for it. Yes, but my position is that the distortion you're describing isn't Doppler related, or not directly, anyway. There may be some subtle relation, however. This is not an argument for the audibility of the Doppler distortion of a speaker, I'm just trying to disabuse you of the notion that the ear/brain would have had any reason to correct for it. I never actually thought it did in any specific sense, however, if every sound source creates Doppler shift in the natrual mechanism of creating the sound, no matter how small, then it's a part of the natural order of sound, and a part of what we hear. I understand that you're saying, because of the size of the speaker relative to the wave length of the sound, some added level of distortion takes place when we use that speaker to generate low frequency sounds. Let's look at a speaker reproducing a 40 Hz sine wave. If you look at the cone's excursion, to reproduce higher volumes, the cone has to move further back and forth, therefore it's frequency of oscillation remains at 40 Hz but its velocity increases because it has to move further back and forth in the same amount of time. At some point, either the linear range of excursion will be exceeded and distortion will occur because of the nonlinear motion, or the motive source will be unable to overcome the inertia and the load of the cone and the air it's pushing, and again, distortion will occur due to the cone's nonlinear motion. Both of these types of distortion have to do with exceeding the linear limits of the speaker or it's motive force. Neither of these types of distortion is what you're talking about. Now, the speaker moving back and forth creates high and low pressure areas in front of the cone, but these pressure waves are not sound waves, they're just molecules being moved back and forth by the cone and by the air's natural tendency to seek a uniform pressure. At 40 Hz, a cone movement of a few millimeters must generate a soundwave nearly thirty feet in length. This is possible because the sound wave generated by the moving cone propagates at a velocity much higher than that of the cone. The frequency of oscillation of the cone is directly related to the wavelength of the sound being reproduced and equal to the soundwave frequency, but the velocity of the cone is not related at all to either frequency or the speed of propagation, and the distance of excursion is not related at all to the wavelength, frequency, or propagation speed of the soundwave being generated. Cone velocity and excursion are directly related to each other for any given frequency, and they are related to the soundwave only as a function of amplitude. If the relationship of cone excursion to soundwave amplitude is not linear then distortion will occur due to the speaker's inefficiency at low frequncies. This will also introduce distortion in any hf tone being generated at by the cone, but it isn't due to Doppler shift, it's due to the non-linear relationship of excursion to amplitude. However, if the relationship of cone excursion to amplitude is linear at any given frequency, then no distortion will result, either at the low frequency being generated, or at any higher frequency tone simultaneously being generated by the cone. This would tend to explain why a well designed woofer in the proper enclosure will have less distortion that a larger woofer in an enclosure not well suited to it. I have no idea what all this might have to do with "Doppler distortion", I just thought some of it might have some relevance to some part of this discussion. |
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"Jim Carr" wrote in message news:0cM8d.13723$mS1.11835@fed1read05... "Porky" wrote in message t... I'm not talking about a filling acting as a diode, some recent research has indicated that the nervous system may, in some cases, be able to directly receive certain modulated rf signals and pick up the audio. Can you cite a reference? I had seen it several years ago, and then there was a brief article about it, on MSNBC I think, in the last few days. I meant, "If one can't tell the difference...." And again, the key word is "either" I don't doubt that there isn't a mechanism in our hearing that compensates for Doppler distortion, I think Doppler distortion, if it does occur, is inaudible under anything approaching normal conditions. Just admit you were wrong about the brain compensating for it. Your backtracking with "either" being some "key word" is like saying, "either it's not audible or fairies are sprinkling fairy dust to cast magic spells so we can't hear it." I'm not arguing whether it's audible or not, but the brain compensation thing is just silly. No, I said if Doppler distortion exists, EITHER there is is something in the hearing mecanism to compensate for it, OR it isn't audible. I believe it's the latter, but if it turns out to exist at a high enough level to be audible and you still don't hear it, then it must be the former. :-) If all reasonable explanations are exausted, it's time to consider the unreasonable... |
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"Jim Carr" wrote in message news:0yM8d.13727$mS1.6687@fed1read05... "Porky" wrote in message ... When we hear a train whistle or car motor, or siren for more than a second or so, we can tell if it is moving, if it is moving toward us or away from us, Because we are comparing it against a second sound, namely the sound of a siren when the source and receiver are stationary. If you did not know what it was *supposed* to sound like you would never know anything was amiss. By definition we are not dealing with a single sound. I agree when you state it like that. I would substitute *singular* for single, in the sense of a *singularity* something that only happens once, but it doesn't matter because I think we're both on the same page now. How can the brain "compensate" for a sound when it requires knowing the sound in advance? Nobody is arguing that the brain cannot compare two sounds. However, your notion that somehow the brain could possibly mask Doppler distortion in such a way as to make it inaudible strikes me as silly. It strikes me as silly too, but if Doppler distortion exists at the level that's been postulated here, if should be very audible, and if it is and you still can't hear it, what other explanation would you suggest? Personally, I prefer the explanation that we don't hear it because if it does exist, it's at a level so low as to be inaudible. |
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"Laurence Payne" wrote in message ... On Tue, 5 Oct 2004 23:50:59 -0700, "Jim Carr" wrote: How can the brain "compensate" for a sound when it requires knowing the sound in advance? Nobody is arguing that the brain cannot compare two sounds. However, your notion that somehow the brain could possibly mask Doppler distortion in such a way as to make it inaudible strikes me as silly. The brain is remarkably good at focusing on data and ignoring distortion. I'm sure Doppler distortion isn't immune to this principle. That's true, and one can even hear a conversation that's below the ambient level across a crowded and noisy room, if one is interested enough to concentrate on it. However, I really wasn't postulation that such a mechanism actually existed, I was speculating on why we couldn't hear it if it really was at the high levels suggested. |
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On Wed, 6 Oct 2004 05:43:41 -0500, "Porky" wrote:
How can the brain "compensate" for a sound when it requires knowing the sound in advance? Nobody is arguing that the brain cannot compare two sounds. However, your notion that somehow the brain could possibly mask Doppler distortion in such a way as to make it inaudible strikes me as silly. The brain is remarkably good at focusing on data and ignoring distortion. I'm sure Doppler distortion isn't immune to this principle. That's true, and one can even hear a conversation that's below the ambient level across a crowded and noisy room, if one is interested enough to concentrate on it. However, I really wasn't postulation that such a mechanism actually existed, I was speculating on why we couldn't hear it if it really was at the high levels suggested. In which case, that's what you should have said :-) Why do you say one can "even" hear below the noise floor? A louder sound doesn't mask a softer one, especially if they have different tonal characteristics. Just because the brass come in, it doesn't mean the string players can pack up and go home :-) CubaseFAQ www.laurencepayne.co.uk/CubaseFAQ.htm "Possibly the world's least impressive web site": George Perfect |
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"Jim Carr" wrote in message news:0cM8d.13723$mS1.11835@fed1read05...
"Porky" wrote in message t... I'm not talking about a filling acting as a diode, some recent research has indicated that the nervous system may, in some cases, be able to directly receive certain modulated rf signals and pick up the audio. Can you cite a reference? I meant, "If one can't tell the difference...." And again, the key word is "either" I don't doubt that there isn't a mechanism in our hearing that compensates for Doppler distortion, I think Doppler distortion, if it does occur, is inaudible under anything approaching normal conditions. Just admit you were wrong about the brain compensating for it. Your backtracking with "either" being some "key word" is like saying, "either it's not audible or fairies are sprinkling fairy dust to cast magic spells so we can't hear it." I'm not arguing whether it's audible or not, but the brain compensation thing is just silly. Not "the" Jim Carr, by any chance? The physicist? Myxococcus xanthus |
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"Laurence Payne" wrote in message ... On Wed, 6 Oct 2004 05:43:41 -0500, "Porky" wrote: How can the brain "compensate" for a sound when it requires knowing the sound in advance? Nobody is arguing that the brain cannot compare two sounds. However, your notion that somehow the brain could possibly mask Doppler distortion in such a way as to make it inaudible strikes me as silly. The brain is remarkably good at focusing on data and ignoring distortion. I'm sure Doppler distortion isn't immune to this principle. That's true, and one can even hear a conversation that's below the ambient level across a crowded and noisy room, if one is interested enough to concentrate on it. However, I really wasn't postulation that such a mechanism actually existed, I was speculating on why we couldn't hear it if it really was at the high levels suggested. In which case, that's what you should have said :-) I did, that was why I used an "either, or" statement. :-) Why do you say one can "even" hear below the noise floor? A louder sound doesn't mask a softer one, especially if they have different tonal characteristics. Just because the brass come in, it doesn't mean the string players can pack up and go home :-) In a crowded room with many conversations going on, the voices pretty much fall into the relatively narrow range of normal conversation, so everything falls into the same frequency range. This does provide a pretty good masking level when trying to make out a low conversation all the way across the room. Part of the reason we can do it is that we don't have to hear every word, we just have to pick up enough key words to get the gist of it, and another part of it is that we have a very good built in selective filter when we concentrate. |
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"Porky" wrote in message t...
"Jim Carr" wrote in message news:vbK8d.13387$mS1.11043@fed1read05... "Paul Draper" wrote in message m... That's odd. I don't find this hard to imagine, slowing it down in my mind. The highest speed of the diaphragm forward will result in the highest "sweeping up" of air molecules, resulting in the highest local density. In this mental image, then, the middle of the throw (for a fundamental tone) would produce the peak of the oscillation in the medium. Thanks for the feedback. Take me through a simple sine wave. It starts at the zero line. It does a semicircle above the line an a semicircle below the line. Describe for me the motion of the speaker in relation to that and at what point the wave is created. Actually it's an ellipse, not a semicircle, and that does make a difference. The cone starts from zero at a fairly rapid rate of acceleration, and the rate slows gradually until it reaches the peak excursion where it reverses direction and gradually starts accelerating in the other direction, the rate of acceleration increases to a peak value as it passes through zero again, then the rate starts decreasing again until it again reverses direction again at the "bottom" of the cycle, where the rate starts increasing again until it again reaches zero, then the whole thing repeats again. Due to the inertia and compressability of air, the air pressure at the cone's surface will vary in proportion to the cone's speed, and this pressure variation becomes a sound wave. Personally, I think the "virtual" source point of the sound wave being radiated by the speaker in the rest point of the cone at the center of the excursion limits, but this has been a subject of debate for a long time, and there are quite a few other theories. One other theory is that the instantaneous virtual sound source point is the position of the cone at any given point in time, which is where the notion of Doppler distortion in a speaker originates. Note that if the virtual source is really the rest point of the cone, then there is no Doppler shift in a loud speaker because the virtual source is not moving with respect to the listener. If the latter theory is correct, then Doppler shift is produced by a loudspeaker because the virtual source is moving with respect to the listener. I will note that even though this is a contentious thread, I am making a sincere request to help me envision this. Maybe I'm thinking too hard or have some sort of mental block on this issue. This is perhaps why a square wave is hard to push through a speaker, because it demands instantaneous accelerations of the diaphragm. I always though it was because most speakers are round! Nyuk! Nope, you can push a square wave through a speaker, but you can only do it once because the sharp edges will tear up the cone. :-) Seriously, some very good speakers will produce a pretty good approximation of a square wave at higher frequencies, but none will do it at lower frequencies, and no speaker will reproduce a true square wave at any frequency because a reproducing a true square wave would require that the cone travel instantaneously from one excursion limit to the other, and as long as the cone has mass, that ain't gonna happen! Actually, this isn't quite right. The pressure fluctuations that correspond to a sound wave don't match with the *position* profile of the speaker, but with the *velocity* profile of the speaker. Thus the requirement of a square wave (in pressure) is that the speaker instantaneously accelerate to its maximum velocity, proceed from one end of its throw to the other at constant (maximum) velocity, and then at the other end of the throw instantaneously accelerate to maximum negative velocity. The way to think about the motion under such a circumstance is like the little ball in a Pong game. For just the reason you mention, real speakers won't achieve that hard, instantaneous bounce at the extrema, but this sure isn't the same as the even more extreme motion you describe. PD |
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"Jim Carr" wrote in message news:F8M8d.13722$mS1.13474@fed1read05...
"Porky" wrote in message ... Doppler shift occurs because as the source moves toward the listener, the source's motion causes the apparent wavelength of the sound to shorten, and as it moves away it causes the apparent wavelength of the sound to lengthen. I say "apparent wavelength" because anyone traveling along with the source hears it as a steady tone, meaning the actual wavelength does not change. I gotta disagree with you here. The wavelength is the distance between the waves, right? The easiest way to measure something that's moving is if I know it's speed. So, using the whistle on the train as an example, I can measure the time interval between two waves at the source and calculate the distance between those waves at the source since I know the speed of sound. Now, if the train is moving in the same direction as the waves are being emitted, I must subtract from that distance I just calculated the distance that I know the source traveled between the waves. Therefore, the distance is shorter than when it was stationary. If I'm moving away, it's longer. The stationary listener hears this as a higher or lower pitch. If the receiver is moving relative to the source, it's measurements are "off" so to speak. The ear lacks the ability to add or subtract to compensate for the motion. It perceives a time interval between the waves and the brain recognizes this at a certain pitch. The brain has a fixed value for the speed of sound so to speak. The physical distance between the waves didn't change. Rather the receiver changed distances between the waves but the brain didn't compensate. The key here is that if I performed the *same* measuring techniques when the receiver was moving as I did at the source when the source was moving (taking into account the distance I traveled), I would come up with the same distance between the waves as I did when the source did when it was stationary. It might be fair to say that if the source is moving relative to the listener that there is a real shift in wavelength. If the listener is moving relative to the source the ear fails to compensate for the movement and the receiver perceives an apparent shift in wavelength. I know this sounds like a bunch of relativistic mumbo-jumbo, but it makes sense to me if I use objective techniques to measure distances, not the subject measurements of my ear-brain configuration. It is all relativistic mumno-jumbo, from Einstein's train experiment to the speaker diaphrams. The brain has nothing to do with light or sound. It's the geometric relationship bewteen eye sockets, ear sockets, and wind that create the sound. Since sound is a reverberation, that actually has nothing to do with waves. It only concerns echo chambers. Which only produce QM standing waves, not waves. As I said, some speaker companies have done ABX testing between a reproduced sound and a live sound source, and under the best conditions, even expert listeners had trouble relaibly telling the difference. Can you cite a reference? There are a huge number of natural broadband sound sources, thunder, wind, the ocean, a babbling brook, a river, a herd of stampeding elephants, etc. But I said "either there is some sort of built in compensation, or the Doppler shift isn't audible." The "either" is the key word there, and I think it is the latter, not the former. Please explain how it is possible to detect any type of distortion in a single sound. You can pick any one sound, just explain how one can measure the distortion. |
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On Mon, 04 Oct 2004 13:25:30 +0200, Vladan wrote:
After so many words spent on the effect in regard to speaker, when are you to start examining it in regard to (dynamic) microphone. And don't forget the air. Molecules are moving. There must be some dopler there, too. And so much more after these above. Before I knew several legs were pulled. Is it o.K. to tell you I was only joking? Guess it is. I have opinion on the subject, though. I think there must be differentiation made btw Doppler effect, we all learned about in high scholl Physics classes (and experienced in everyday life) and Doppler distortion (?), which seam to be unfortunately choosen engineering term. Even more important, to produce sound, source does not have to travel, but only to oscilate. That's one huge difference. Also, when explaining Doppler effect, Sound Source is always considered to be small (dot, or point, or whatever), at least few magnitudes smaller than trajectory it travells towards the listener , or from him. Further, it's looked at as one compact thing. No moving parts in it. When we substitute loudspeaker for dot source, we get into one another teritory. Real life. However, physics is all about describing real life, therefore it should be expected two to coincide, as usualy it is the case. As presented here, it seams there should be some ammount of doppler (or is it Doepler?) effect present even if the source (loudspeaker) is not travelling. Obviously, as physics says, source must travel in order dopler effect to be produced. Therefore we can freely say, ther's no Doppler effect due to speaker cone movement, as it does not travell. it simply oscilates. That's how sound is made. The Distance is distance from the neutral position (of speaker cone) to listener. That much for Doppler effect (in acoustics). Fact is, as some clever (?) people noticed, cone is sometimes closer to the listener, sometimes further, depending of the position of the oscilating cone in regard to neutral position. Unforuneately for them, as source does not travel, but merely oscilate (it has to oscilate, or there would not be sound) wavelength of the sound the listener hears is all the same all the time, no matter if cone is pushing or pulling at that exact moment. Due to fact above, term Doppler distortion was invented by some people, whoever they are, to mud waters and let them discuss about nothing. Longer the better. Get some money for a project or two? All that can happen in regarded situation is slight phase difference. Same one we can hear due to polarity reversal, if the speaker is in special position (near the wall), or we could hear in the open if we were size of a bug with ears of a bat and positioned few mm from the cone it self (preferably orthogonal to direction of cone movement ???, or it wouldn't matter). So, why they refer to this as to Dopler distortin, instead to simply call it modulation, phase difference, shift,... or whatever, is beyond me. Back to sleep. |
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"Laurence Payne" wrote in message
... On Tue, 5 Oct 2004 23:50:59 -0700, "Jim Carr" wrote: How can the brain "compensate" for a sound when it requires knowing the sound in advance? Nobody is arguing that the brain cannot compare two sounds. However, your notion that somehow the brain could possibly mask Doppler distortion in such a way as to make it inaudible strikes me as silly. The brain is remarkably good at focusing on data and ignoring distortion. I'm sure Doppler distortion isn't immune to this principle. There's a difference between critical listening masking out sounds and Porky's rather dubious "either/or" claim that the reason we can't hear Doppler distortion is because the brain is automatically compensating for it. The former is a matter of concentration and experience. The latter is magic. |
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"Myxococcus xanthus" wrote in message
om... Not "the" Jim Carr, by any chance? The physicist? How could you possibly think that with my amateurish discussions of physics? :-) |
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"ZZBunker" wrote in message
om... It is all relativistic mumno-jumbo, from Einstein's train experiment to the speaker diaphrams. The brain has nothing to do with light or sound. It's the geometric relationship bewteen eye sockets, ear sockets, and wind that create the sound. I disagree. The speed of sound is so slow when compared to relativistic speeds that we're entitled to ignore Einstein's theory that our measuring sticks will be affected. Furthemore, if the receiver is moving faster than the speed of sound away from the source the listener will not hear any sounds at all. What's the measurement of the wavelength at that point? Is there now no Doppler shift? There's still a distance between the waves. We can't hear them, but we can certainly apply my measuring technique by taking into account our greater than mach speed. In Einstein's theory we can never move faster than the speed of light. It's a whole different ball game. Since sound is a reverberation, that actually has nothing to do with waves. Can you please elaborate? |
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Porky wrote: I don't think so, I'm not arguing that the distortion you're defining doesn't exist, I'm saying it's the result of a phenomonen other than Doppler shift. It is the only distortion phenomenon that follows from the motion of the source, so call it what you wish. That its magnitude is a minimum, zero, when the velocity due to the LF is a maximum just belies the usual picture people have of what is going on and the way it is often described. An elephant fart may exceed 90 dB SPL at ten feet, and the elephant's anus is smaller in diameter than an eight inch woofer. You'll have to measure excursion, I ain't gonna get close enough.:-) There's a very signifigant excursion there if you consider its equivalent displacement of air. :-) Ok, I'm getting silly now, but my point is that unless the sound measures something more than 140 dB SPL at the listener's position, it isn't going to cause distortion in the hearing mechanism Distortion at the biological level sets in much sooner than your numbers indicate. Yes, but my position is that the distortion you're describing isn't Doppler related, or not directly, anyway. There may be some subtle relation, however. Whatever it is, it's all there is. [snip] I have no idea what all this might have to do with "Doppler distortion", I just thought some of it might have some relevance to some part of this discussion. None. We are discussing IM distortion occuring in a linear medium due to source motion. Distortion due to being in a regime where the pressure and velocity of air are not related linearly is a whole 'nuther discussion. Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
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zigoteau wrote: The fluid velocity (in units of distance/time) of a test particle (a tiny, zero mass thing) located a distance d from the driver face is given by Vp(d,t) = Vd(t-d/c) 1) No, that's your mistake. It certainly follows from the linear wave equation and the assumed boundary condition at infinity that: Vp[d,t] = Vp[x, t+(x-d)/c] , Sd=xd Your equation effectively firstly assumes that the diaphragm is at x=0 and then that it is at x=Sd(t), whichever is most convenient at the time. It is not a bad approximation, but it is an approximation. Yes, I understand. The way I stated it implies that at time t it is a distance d from where the face is at the same time t. Clearly wrong. 1) and 2) are a poorly stated attempt to establish that the motion of a particle whose rest position is a distance d from the driver's rest position is the same as that of the driver at a time d/c in the past, with which I don't think you will disagree other than what you said about the small attenuation effect. If I properly establish 1) and 2) I still think my conclusion follows but I need to go away again and think about it. The other postings casting doubt on the boundary condition between the diaphragm and the air are way off-beam. It is quite legitimate to assume that, at a mesoscopic length scale, the air in contact with the diaphragm is moving at the velocity of the diaphragm, i.e. no slip. Right, this and the wave equation justify the condition I describe in the last paragraph. As always, Thank you. Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
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On Wed, 06 Oct 2004 11:26:51 -0700, Bob Cain
wrote: It is the only distortion phenomenon that follows from the motion of the source, so call it what you wish. That its magnitude is a minimum, zero, when the velocity due to the LF is a maximum just belies the usual picture people have of what is going on and the way it is often described. The Elliot Effect? Read his "paper" again and try to find where he directly records position or velocity. Failing that, look for any asumptions he has made about the position of the cone in relation to the recorded signals and see whether there is any argument or evidence put forward to support his assumptions. Most of his fans are too dozy to notice any flaws or don't care. . . |
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Goofball_star_dot_etal wrote: On Wed, 06 Oct 2004 11:26:51 -0700, Bob Cain wrote: It is the only distortion phenomenon that follows from the motion of the source, so call it what you wish. That its magnitude is a minimum, zero, when the velocity due to the LF is a maximum just belies the usual picture people have of what is going on and the way it is often described. The Elliot Effect? Yes. IIRC, his test shows exactly that. Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
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"Porky" wrote in message news:NIP8d.267327$% doesn't
An elephant fart may exceed 90 dB SPL at ten feet, I would guess that you reached that conclusion from personal experience and that it is based on numerous observations. and the elephant's anus is smaller in diameter than an eight inch woofer. I wouldn't be surprised if the diameter of an elephant's anus is about the same diameter as your mouth. Furthermore, it would seem that diameter isn't the only thing that your mouth and an elephant's anus have in common. |
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"Jim Carr" wrote in message news:S%V8d.19$_a3.8@fed1read05...
"ZZBunker" wrote in message om... It is all relativistic mumno-jumbo, from Einstein's train experiment to the speaker diaphrams. The brain has nothing to do with light or sound. It's the geometric relationship bewteen eye sockets, ear sockets, and wind that create the sound. I disagree. The speed of sound is so slow when compared to relativistic speeds that we're entitled to ignore Einstein's theory that our measuring sticks will be affected. Furthemore, if the receiver is moving faster than the speed of sound away from the source the listener will not hear any sounds at all. What's the measurement of the wavelength at that point? Is there now no Doppler shift? There's still a distance between the waves. We can't hear them, but we can certainly apply my measuring technique by taking into account our greater than mach speed. I think you talking about music though, not sound though. The "speed" of sound, and the Doppler Shift only exist in the context of music, where the medium is locally linear. Most mediums, including the Earth's crust, the Ocean Bottom, clouds, and the sun's photosphere don't have a fixed speed of sound. So the "wavelength" in those media doesn't really exist in the Classical Fourier sense. Which is why you need complex frequencies in those cases. And the imaginary part of the frequency is going to be traveling through a different state of matter than the real part of the frequency. And it's going to travel infinitely faster than the real part. Which is where matrix mechanics and the Uncertainty Prinicple came from. And what the wavelength is in a matrix, nobody knows. And Einstein was right about the measuring sticks. They're only useful where you have a source and receiver. But the Mercury Perehelion measurement doesn't have a source receiver mode. The only way to even derive the GTR mass Equivalency Principle is just the way he did it, with the Elevator experiment, where there are no Doppler Shifts. And because sound is slow, doesn't mean acoustic waves are slow. The acoustic pressure waves from solar flares, Nuclear Reactor Cores, and Hydrogen bombs are too fast for even the fastest acoustician to keep up with. Which is why both Digital Sound, TV, and Electric Guitars and Laser Pickups work so much better than "acoustics". And the receiver will hear the source sound, after loud after he breaks the speed of sound. Since the pressure wave is going to reflect off the upper atmosophere and be heard as a constant rumble, until he comes back below the speed of sound. But what he hears won't intelligible to him, because all the source frequency are nonlinearly shifted. But you can unshift them, in many sitution, which is the main reason that Stealth Aircraft work. In Einstein's theory we can never move faster than the speed of light. It's a whole different ball game. That's hardly true. The only reason refraction of light through a telescope even works at all is because Newton could move infinitely faster through glass, than Einstein or QM people will ever even approximatelty catch up to. Since sound is a reverberation, that actually has nothing to do with waves. Can you please elaborate? Sound needs an incident pressure to even exist. but the only part of the pressure wave that is "hearable" is the frequency band that it's tuned to hear. But the tuning operation itself makes the wave "stop", so at those moments, the sound it no longer apparently a wave. It's fixed in space as a mathematical function that has definite visible edges, at a *location* is space, rather than apparently travelling though space. |
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"ZZBunker" wrote in message
om... I am snipping your explanation not out of disprespect but simply because I don't understand a lot of it and I think it exceeds the discussion. Porky wrote, "the source's motion causes the apparent wavelength of the sound to shorten." I disagreed. I will try to state my disagreement more simply: The apparent *frequency* shifts. However, the wavelength either shifts or it doesn't depending on whether the source if moving. Suppose I have a machine that sends a ball rolling in a straight line at 10 feet per second. I launch one ball per second. I think we would agree that the balls are 10 feet apart. Now, if I move my machine at 5 feet per second in the same direction as the balls, then each ball will be five feet apart. That's the wavelength. Frequency is a different matter. If there is a wall some distance away, the balls launched from the stationary machine will hit the wall a frequency of one per second. In the second case the balls will strike the wall at one per half second until eventually my machine crashes into the wall and I lose my government grant. If in the first scenario I move the wall at 2 feet per second towards the oncoming balls, the balls will strike the wall at a frequency of one every 0.8 seconds. I still say the balls are 10 feet apart. The frequency changed because the wall was moving, but distance between the balls didn't change. If the balls traveled down a lane that had a line painted once per foot, and I took a snapshot of the balls, I could clearly see that they were 10 feet apart. I think that to describe Doppler shift accurately one should state that the apparent frequency shifts in all cases but that the wavelength only changes if the source is moving. Our ears only measure frequency so the net effect is the same. Would you agree or disagree with this explanation? |
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"Paul Draper" wrote in message om... "Porky" wrote in message t... "Jim Carr" wrote in message news:vbK8d.13387$mS1.11043@fed1read05... "Paul Draper" wrote in message m... That's odd. I don't find this hard to imagine, slowing it down in my mind. The highest speed of the diaphragm forward will result in the highest "sweeping up" of air molecules, resulting in the highest local density. In this mental image, then, the middle of the throw (for a fundamental tone) would produce the peak of the oscillation in the medium. Thanks for the feedback. Take me through a simple sine wave. It starts at the zero line. It does a semicircle above the line an a semicircle below the line. Describe for me the motion of the speaker in relation to that and at what point the wave is created. Actually it's an ellipse, not a semicircle, and that does make a difference. The cone starts from zero at a fairly rapid rate of acceleration, and the rate slows gradually until it reaches the peak excursion where it reverses direction and gradually starts accelerating in the other direction, the rate of acceleration increases to a peak value as it passes through zero again, then the rate starts decreasing again until it again reverses direction again at the "bottom" of the cycle, where the rate starts increasing again until it again reaches zero, then the whole thing repeats again. Due to the inertia and compressability of air, the air pressure at the cone's surface will vary in proportion to the cone's speed, and this pressure variation becomes a sound wave. Personally, I think the "virtual" source point of the sound wave being radiated by the speaker in the rest point of the cone at the center of the excursion limits, but this has been a subject of debate for a long time, and there are quite a few other theories. One other theory is that the instantaneous virtual sound source point is the position of the cone at any given point in time, which is where the notion of Doppler distortion in a speaker originates. Note that if the virtual source is really the rest point of the cone, then there is no Doppler shift in a loud speaker because the virtual source is not moving with respect to the listener. If the latter theory is correct, then Doppler shift is produced by a loudspeaker because the virtual source is moving with respect to the listener. I will note that even though this is a contentious thread, I am making a sincere request to help me envision this. Maybe I'm thinking too hard or have some sort of mental block on this issue. This is perhaps why a square wave is hard to push through a speaker, because it demands instantaneous accelerations of the diaphragm. I always though it was because most speakers are round! Nyuk! Nope, you can push a square wave through a speaker, but you can only do it once because the sharp edges will tear up the cone. :-) Seriously, some very good speakers will produce a pretty good approximation of a square wave at higher frequencies, but none will do it at lower frequencies, and no speaker will reproduce a true square wave at any frequency because a reproducing a true square wave would require that the cone travel instantaneously from one excursion limit to the other, and as long as the cone has mass, that ain't gonna happen! Actually, this isn't quite right. The pressure fluctuations that correspond to a sound wave don't match with the *position* profile of the speaker, but with the *velocity* profile of the speaker. Thus the requirement of a square wave (in pressure) is that the speaker instantaneously accelerate to its maximum velocity, proceed from one end of its throw to the other at constant (maximum) velocity, and then at the other end of the throw instantaneously accelerate to maximum negative velocity. The way to think about the motion under such a circumstance is like the little ball in a Pong game. For just the reason you mention, real speakers won't achieve that hard, instantaneous bounce at the extrema, but this sure isn't the same as the even more extreme motion you describe. In order to accurately reproduce a square wave, the time required for the cone to travel from its negative excursion to its positive excursion must equal the rise time of the square wave, and since a perfect square wave has a rise time of zero the travel would have to be instantaneous. By "excursion limit" in my previous post, I did not mean the speaker's excursion limits, I meant the amount of excursion necessary to reproduce the wave at the desired volume. |
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"ZZBunker" wrote in message .. . "Jim Carr" wrote in message news:S%V8d.19$_a3.8@fed1read05... "ZZBunker" wrote in message om... It is all relativistic mumno-jumbo, from Einstein's train experiment to the speaker diaphrams. The brain has nothing to do with light or sound. It's the geometric relationship bewteen eye sockets, ear sockets, and wind that create the sound. It was Doppler's train experiment, not Einstein's. I think you talking about music though, not sound though. The "speed" of sound, and the Doppler Shift only exist in the context of music, where the medium is locally linear. Most mediums, including the Earth's crust, the Ocean Bottom, clouds, and the sun's photosphere don't have a fixed speed of sound. So the "wavelength" in those media doesn't really exist in the Classical Fourier sense. Which is why you need complex frequencies in those cases. And the imaginary part of the frequency is going to be traveling through a different state of matter than the real part of the frequency. And it's going to travel infinitely faster than the real part. Which is where matrix mechanics and the Uncertainty Prinicple came from. The Heisenberg uncertainty principle pertains to subatomic particle physics. It has nothing to do with sound. And what the wavelength is in a matrix, nobody knows. Again, nothing to do with the real world. And Einstein was right about the measuring sticks. They're only useful where you have a source and receiver. But the Mercury Perehelion measurement doesn't have a source receiver mode. The Mercury Perehelion measurement has nothing to do with audio either. The only way to even derive the GTR mass Equivalency Principle is just the way he did it, with the Elevator experiment, where there are no Doppler Shifts. Again, nothing to do wuith audio. And because sound is slow, doesn't mean acoustic waves are slow. The acoustic pressure waves from solar flares, Nuclear Reactor Cores, and Hydrogen bombs are too fast for even the fastest acoustician to keep up with. Which is why both Digital Sound, TV, and Electric Guitars and Laser Pickups work so much better than "acoustics". Acoustic pressure waves have nothing to do with digital sound, TV, electric guitars (the pickups work with magnetic induction, not acoustic pressure) or laser pickups. And the receiver will hear the source sound, after loud after he breaks the speed of sound. Since the pressure wave is going to reflect off the upper atmosophere and be heard as a constant rumble, until he comes back below the speed of sound. But what he hears won't intelligible to him, because all the source frequency are nonlinearly shifted. But you can unshift them, in many sitution, which is the main reason that Stealth Aircraft work. The reasons stealth aircraft work have nothing to do with the speed of sound. There are both subsonic and supersonic stealth aircraft, and the term "stealth" when used with them primarily has to do with very small radar and thermal footprints. I took alt.sci.physics out of this, because for the purpose of this discussion, we aren't concerned with the speed of sound in the sun's photosphere, the ocean's botton, etc. Talk about your "mumbo jumbo", this post is so far out in left field as to have no relevance whatever. In Einstein's theory we can never move faster than the speed of light. It's a whole different ball game. That's hardly true. The only reason refraction of light through a telescope even works at all is because Newton could move infinitely faster through glass, than Einstein or QM people will ever even approximatelty catch up to. ??????! Since sound is a reverberation, that actually has nothing to do with waves. Can you please elaborate? Sound needs an incident pressure to even exist. but the only part of the pressure wave that is "hearable" is the frequency band that it's tuned to hear. But the tuning operation itself makes the wave "stop", so at those moments, the sound it no longer apparently a wave. It's fixed in space as a mathematical function that has definite visible edges, at a *location* is space, rather than apparently travelling though space. Out here in the real world, sound is a wave-based phenomenon. While a single measurement does take a "picture" of an instantaneous peice of time, it doesn't actually stop anything. Over here at home-studio, we're trying to use math to describe what actually happens, it's a means to an end, not an end in itself. |
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