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#521
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Mark Simonetti writes:
The distortions you are talking about in this thread are comparitively small, please correct me if I'm wrong. That being the case, I was basically stating that you often find people are not even bothered about the larger distortions from sub standard systems, and that I felt it is unfortunate. What are these "larger" distortions you are referring to? Seems to me, as I read someone else state here recently, that the typical high-end audio freak laments issues that are several orders of magnitude below something like Doppler distortion in speakers. Not that Doppler distortion is the largest problem facing sound reproduction, but comparitively, it seems to be much more worthy of our attention. -- Randy Yates Sony Ericsson Mobile Communications Research Triangle Park, NC, USA , 919-472-1124 |
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#522
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Bob Cain wrote in message ...
Mark wrote: Bob, What makes you think Doppler does not occur in an infinite terminated tube. That the radiation impedence seen by the piston is identically that of the characteristic impedence of the gas. Consider the implcations of that. The implication is that there is no reflected wave and no standing wave and all the power propogates forward. This has nothing to do with the Doppler effect. This is a similar case to a sliding RF transmission line. A radio wave in a transmission line that is changing length will experience Doppler shift. Well, yes if the reciever is in motion, other things begin to happen. I'm only, at this point, considering the situation where the transmitter and reciever are at a constant distance from each other. If the vernacular argument for Doppler distortion predicts the phenomenon for that case then it is incorrect, and it does. I agree, if the Rx and Tx are at a CONSTANT distance, there is no Doppler effect. But in the speaker case being discussed, as far as the 4 kHz tone is concerend the Tx cone and Rx ear are NOT at a constant distance. The distance is changing sinusoidaly because the cone is also vibrating at 50 Hz. The Tx cone is moving closer and further from the Rx ear at 50Hz and this imparts Doppler onto the 4 kHz. Mark |
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#523
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Arny Krueger wrote:
You did more than express yourself poorly, you cast aspersons on other people. Hmm, it certainly came across wrong then ! Appologies if I offended anyone, it was not my intention. -- Mark Simonetti. Freelance Software Engineer. |
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#524
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William Sommerwerck wrote:
No, Mark, your point of view (though perhaps not geramaine to the discussion) was perfectly clear. Arny deliberately "misunderstood" you. He does this all the time. It's his snide way of attacking people he disagrees with, rather than directly confronting the issue. (He has a high IQ, but little insight.) You're not the only person who's been on the receiving end. Possibly but re-reading my original post, I was fairly unclear, but I stand by what I said, without meaning to offend to anyone. My later post clarifies what I meant. Cheers, -- Mark Simonetti. Freelance Software Engineer. |
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#525
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Unfortunately I've not had the oppertunity to A/B test cables of such
different caliber. As for the phase difference of 2 degrees, again its not something I've tried on purpose, so I couldn't say for sure whether I think its bogus or not. I think it would certainly be interesting to blind test a group of people to see if they really can tell the difference, and the same goes for the expensive speaking cable. If its been done, I'd be interested to see the results. Cheers, Mark. -- Porky wrote: I agree wholeheartedly with that statement, however, what the traind and experienced discerning listener, and what the self-proclaimed "golden-eared audiophile" claim to hear are often totally different things. The difference between the two is that what the trained and experinced discerning listener claims to hear can be verified by scientific double-blind tests, and what the self-proclaimed "golden-eared audiophile" claims to hear often cannot. That isn't to say that all audiophiles are bogus, that isn't true, I know of audiophiles who are exceptionally accurate and discerning listeners. However the "audiophile" who buys $400 a foot speaker cable and claims to hear "a big difference" over good quality regular speaker cable, or the "audiophile" who claims to hear a phase difference of 2 degrees at far field in a live room, is full of it!:-) -- Mark Simonetti. Freelance Software Engineer. |
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#526
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"William Sommerwerck" wrote in message
Arny deliberately "misunderstood" you. This accurately discerned by what means? |
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#527
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"Mark Simonetti" wrote in message
Unfortunately I've not had the oppertunity to A/B test cables of such different caliber. As for the phase difference of 2 degrees, again its not something I've tried on purpose, so I couldn't say for sure whether I think its bogus or not. I think it would certainly be interesting to blind test a group of people to see if they really can tell the difference, and the same goes for the expensive speaking cable. If its been done, I'd be interested to see the results. The results of cables tests of electrically similar (RLC) audio (speaker, line lvel) cables of reasonable length are universally negative for audible differences. |
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#528
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"William Sommerwerck"
Arny deliberately "misunderstood" you. This accurately discerned by what means? The objective fact that I understood exactly what he meant, even though it lacked a tight connection to the topic under discussion. As you and I are of comparable intelligence and verbal skills, it therefore follows, etc, etc, etc. |
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#529
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"William Sommerwerck" wrote in message
"William Sommerwerck" Arny deliberately "misunderstood" you. This accurately discerned by what means? The objective fact that I understood exactly what he meant, even though it lacked a tight connection to the topic under discussion. As you and I are of comparable intelligence and verbal skills, it therefore follows, etc, etc, etc. So Bill, you're unfamiliar with the idea that someone's perceptions are strongly influenced by their past experiences? My relevant past experiences are that often when I start talking saying things like: "Speaker Doppler as insignificant as it is, is positively huge compared to the errors that a common nasty old 5532 or TL072 makes in most audio circuits" ....an attack on my hearing acuity and self-praise for the writer's hearing acuity often follows. |
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#530
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Arny Krueger wrote: It was my annoyance and disbelief in all of this that motivated me to look hard at Doppler distortion and find a way to quantify it. The rest, as they say, is history. :-) It's there! In the far field. Near field tests that show any measurable amount of it are still highly suspect. The point is that AM distortion, which dominates and is relatively large in speakers, and often pretty audible, is only present in good electronics in far smaller quantities. Masking rules, we hear distortion in speakers far more so than in non-clipping, non-noisy electronics. OTOH, aren't the AM products that speakers generate mostly the "warmth" generating kind that people seem to like in small amounts? That's not to say that it still isn't swamping the smaller effects. Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
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#531
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Arny Krueger wrote: "Mark Simonetti" wrote in message I think perhaps I didn't write that very well, you've midunderstood me, sorry. You did more than express yourself poorly, you cast aspersons on other people. Arny, I think you read the fellow completely wrong. I sensed none of that with respect to local participants in reading it. Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
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#532
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Porky wrote: I agree wholeheartedly with that statement, however, what the traind and experienced discerning listener, and what the self-proclaimed "golden-eared audiophile" claim to hear are often totally different things. The guys I'm talking about are far more than self-proclaimed. You'd definitely know their names. Nonetheless... Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
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#533
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Mark wrote: Bob Cain wrote in message ... Mark wrote: Bob, What makes you think Doppler does not occur in an infinite terminated tube. That the radiation impedence seen by the piston is identically that of the characteristic impedence of the gas. Consider the implcations of that. The implication is that there is no reflected wave and no standing wave and all the power propogates forward. This has nothing to do with the Doppler effect. It also implies error free transmission. But in the speaker case being discussed, as far as the 4 kHz tone is concerend the Tx cone and Rx ear are NOT at a constant distance. The distance is changing sinusoidaly because the cone is also vibrating at 50 Hz. The Tx cone is moving closer and further from the Rx ear at 50Hz and this imparts Doppler onto the 4 kHz. This is the part I'm working hard to debunk. If what I've said isn't enough there's more to come. Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
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#534
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From: "Arny Krueger"
...an attack on my hearing acuity and self-praise for the writer's hearing acuity often follows. Yep you very much mis-interpreted my post ! Read it from the point of view that I am actually supporting what you are saying. I was infact stating that it is a sad state of affairs that people do not care about problems that are much worse than these in cheap mainstream audio systems. I realise I wasn't overly clear in that post. Anyway, enough said. Mark. -- "Arny Krueger" wrote in message ... "William Sommerwerck" wrote in message "William Sommerwerck" Arny deliberately "misunderstood" you. This accurately discerned by what means? The objective fact that I understood exactly what he meant, even though it lacked a tight connection to the topic under discussion. As you and I are of comparable intelligence and verbal skills, it therefore follows, etc, etc, etc. So Bill, you're unfamiliar with the idea that someone's perceptions are strongly influenced by their past experiences? My relevant past experiences are that often when I start talking saying things like: "Speaker Doppler as insignificant as it is, is positively huge compared to the errors that a common nasty old 5532 or TL072 makes in most audio circuits" ...an attack on my hearing acuity and self-praise for the writer's hearing acuity often follows. |
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#535
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"Bob Cain" wrote in message
The point is that AM distortion, which dominates and is relatively large in speakers, and often pretty audible, is only present in good electronics in far smaller quantities. Masking rules, we hear distortion in speakers far more so than in non-clipping, non-noisy electronics. OTOH, aren't the AM products that speakers generate mostly the "warmth" generating kind that people seem to like in small amounts? I await your theoretical proof of your claim, Bob. ;-) That's not to say that it still isn't swamping the smaller effects. To say the least. I think the way this warm distortion thing works is that people who don't do well-designed listening tests, have all these percptions that they are hearing things they theorize or measure, without regard to the important scientific principle called quantification. They perceive that they can sort amplifiers using speakers with far more distortion, so they need yet another operational theory to *explain* how they accomplish what they claim. Back in the real world there are few if any people who have done well-designed listening tests that have the belief that they can wine-taste good amplifiers. These people are noticable by their circumspect language when amplifier audible differences are being discussed. |
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#536
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In this place, Arny Krueger was recorded saying ...
[.. snip ..] The point is that AM distortion, which dominates and is relatively large in speakers, and often pretty audible, is only present in good electronics in far smaller quantities. Masking rules, we hear distortion in speakers far more so than in non-clipping, non-noisy electronics. Just a while ago Arny, you were proclaiming the awful sound of a coupe of well known power amplifiers. If speaker distortions so outweigh those contributed by electronics, does that mean (a) that you can't, in fact, tell the difference between two differing power amps connected to the same speakers or (b) that you have joined the "golden ears" set? ;^) -- George Newcastle, England Problems worthy of attack Prove their worth, by hitting back [Piet Hein] |
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#537
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In this place, Bob Cain was recorded saying ...
FWIW, the full expression of it is entirely dependant on the physical configuration of the speaker and how it is thus coupled to the air as well as the position in space from which the phenomenon is measured. Which just happens to be what I (and a good few other folks) have been saying all along. -- George Newcastle, England Problems worthy of attack Prove their worth, by hitting back [Piet Hein] |
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#538
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In this place, Bob Cain was recorded saying ...
I no longer believe that Doppler distortion is non-existant, When do the pictures of you eating your hat appear, then? vbg -- George Newcastle, England Problems worthy of attack Prove their worth, by hitting back [Piet Hein] |
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#539
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George Perfect wrote: In this place, Bob Cain was recorded saying ... FWIW, the full expression of it is entirely dependant on the physical configuration of the speaker and how it is thus coupled to the air as well as the position in space from which the phenomenon is measured. Which just happens to be what I (and a good few other folks) have been saying all along. Then you, and they, have agreed all along that it doesn't happen in the case of a piston in a terminated tube? Somehow I missed that. Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
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#540
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George Perfect wrote: In this place, Bob Cain was recorded saying ... I no longer believe that Doppler distortion is non-existant, When do the pictures of you eating your hat appear, then? vbg This is as tricky as what has turned up as the truth of the matter. The reasons commonly stated for it are utterly wrong and there are definitely simple conditions in which it doesn't occur. :-0 How about a small piece of my hat? ;-) Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
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#541
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Bob Cain wrote: But in the speaker case being discussed, as far as the 4 kHz tone is concerend the Tx cone and Rx ear are NOT at a constant distance. The distance is changing sinusoidaly because the cone is also vibrating at 50 Hz. The Tx cone is moving closer and further from the Rx ear at 50Hz and this imparts Doppler onto the 4 kHz. This is the part I'm working hard to debunk. If what I've said isn't enough there's more to come. FWIW, I've received confirmation from Art Ludwig and from a theoretical physicist on sci.physics.research that my analysis is correct. Dopper distortion as commonly described doesn't happen but it does happen in various circumstances for the other reasons that I've stated. Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
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#542
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"George Perfect" wrote in message
o.uk In this place, Arny Krueger was recorded saying ... [.. snip ..] The point is that AM distortion, which dominates and is relatively large in speakers, and often pretty audible, is only present in good electronics in far smaller quantities. Masking rules, we hear distortion in speakers far more so than in non-clipping, non-noisy electronics. Just a while ago Arny, you were proclaiming the awful sound of a coupe of well known power amplifiers. Say what? If speaker distortions so outweigh those contributed by electronics, does that mean (a) that you can't, in fact, tell the difference between two differing power amps connected to the same speakers or Nope (b) that you have joined the "golden ears" set? ;^) Wrong again. Some amps sound different, as is proven by my HFNRR article of some years ago. Some don't. |
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#543
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Some amps sound different, as is proven by my HFN&RR article
of some years ago. Some don't. I'm getting off-topic here, but I don't subscribe to HFN&RR and didn't see it. Did all the differences correspond to measurable distortion or frequency-response errors, or were there differences that didn't have immediately obvious correlations? |
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#544
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Bob Cain wrote:
Then you, and they, have agreed all along that it doesn't happen in the case of a piston in a terminated tube? Somehow I missed that. It is the result of the motion of the speaker not being the same as the motion of the microphone diaphragm. IF the piston is terminated such that the speaker motion is identical to the microphone motion, the effect goes away. If you start out with a synthesized signal, you can demonstrate an effect with any speaker configuration, but in a reciprocal arrangement the effects in the microphone and speaker cancel one another out. If you are going to talk about it in this way, I think you have to split out the three cases of a microphone driving a speaker with reciprocal movement (no effect), a microphone driving a speaker with nonreciprocal movement (effect), and a speaker reproducing a synthetic tone (effect). --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
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#545
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Scott Dorsey wrote: Bob Cain wrote: Then you, and they, have agreed all along that it doesn't happen in the case of a piston in a terminated tube? Somehow I missed that. It is the result of the motion of the speaker not being the same as the motion of the microphone diaphragm. Scott, the effects of Doppler distortion, or its absence, exist in the air itself and are not dependant on there being anything to measure it. Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
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#546
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"Porky" wrote in message ...
Methinks The Ghost has a compulsive need to repeat everything three times. Maybe he labors under the misapprehension that repeating a falsehood enough will make it true.  Methinks Porky has pig fat for brains. |
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#547
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Bob Cain wrote in message ...
Bob Cain wrote: But in the speaker case being discussed, as far as the 4 kHz tone is concerend the Tx cone and Rx ear are NOT at a constant distance. The distance is changing sinusoidaly because the cone is also vibrating at 50 Hz. The Tx cone is moving closer and further from the Rx ear at 50Hz and this imparts Doppler onto the 4 kHz. This is the part I'm working hard to debunk. If what I've said isn't enough there's more to come. FWIW, I've received confirmation from Art Ludwig and from a theoretical physicist on sci.physics.research that my analysis is correct. Dopper distortion as commonly described doesn't happen but it does happen in various circumstances for the other reasons that I've stated. Art Ludwig DOES NOT agree with your position on this issue, and you need to post the retraction/correction as he has requested. As for your your unnamed theoretical physicist, tell him that I said that he doesn't know squat about the fundamentals of acoustics, and that being an expert in one area of theoretical physics does not authorize him to pontificate in another. |
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#548
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The Ghost wrote: Art Ludwig DOES NOT agree with your position on this issue, and you need to post the retraction/correction as he has requested. Not sure where you got that. The last email I have from him (yesterday) tells me we are in substantial agreement. Yes, there were still some disagreements but the essentials of what I've posted here were corroborated. If that has changed, he has not told me about it (or if it is posted here I haven't seen it yet) and if he does, I will most certainly report that here. Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
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#549
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"William Sommerwerck" wrote in message
Some amps sound different, as is proven by my HFN&RR article of some years ago. Some don't. I'm getting off-topic here, but I don't subscribe to HFN&RR and didn't see it. Did all the differences correspond to measurable distortion or frequency-response errors, or were there differences that didn't have immediately obvious correlations? There was very readily measureable nonlinear distortion that was detected after the listening test. It's appearance was unexpected because it was generated by an amplifier that had just been highly reviewed by The Absolute Sound, and the amplifier was being operated well below (maybe 6 dB) the point predicted by the amp's specifications and resistive load testing. |
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#550
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"Phil Allison" wrote in message ...
** Bob Cain is a not just a blatant troll, but a nut, a slanderer and a probably a desperate reject from the Flat Earth Society. Glad to know that there's at least one other person out there who recognizes Bob Cain for who and what he really is. He is unquestionably the most technically inept jackass and contemptable piece of human waste that I have ever encountered in all of my years on this plannet. He has been slandering me for the last 3-4 years in alt.sci.physics.acoustics, and now it's payback time. His unlimited stupidity and uninhibited arrogance has finally brought him into a direct confrontation with mother nature, and he is going loose on this one big time. I look forward to that end and I intend to joyfully participate in the celebration. |
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#552
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"Randy Yates" (The Ghost) writes: "Phil Allison" ** Bob Cain is a not just a blatant troll, but a nut, a slanderer and a probably a desperate reject from the Flat Earth Society. Glad to know that there's at least one other person out there who recognizes Bob Cain for who and what he really is. He is unquestionably the most technically inept jackass and contemptable piece of human waste that I have ever encountered in all of my years on this plannet. He has been slandering me for the last 3-4 years in alt.sci.physics.acoustics, and now it's payback time. His unlimited stupidity and uninhibited arrogance has finally brought him into a direct confrontation with mother nature, and he is going loose on this one big time. I look forward to that end and I intend to joyfully participate in the celebration. Although I vehemently disagree with Bob on the Doppler issue, the over-riding question in my mind is, "What is the reality of the situation?" These agressive and viscious ad-hominem attacks do nothing to discover the answer to this question. ** The person originating all such attacks is *** BOB CAIN *** !!!!!!!!!! Then his porcine pal followed suit. I have posted here test my test procedure and results in detail - did you see them ??? ( No contentious spectrum analyser involved ) ** QUOTE: " ** So this is what all the Doppler Distortion fuss is about ???? A tiny bit of phase jitter, which at 5 kHz rarely amounts to more than a few degrees ?? I was looking at it on my scope yesterday: 1. A 5 inch woofer, in box, driven by an amp fed from with two sine wave generators with outputs summed. 2. A condenser mic feeding a pre-amp and followed by a 12 dB/oct HPF at 2 kHz thence to the scope. 3. The high frequency generator output is also linked to the scope which operates in X-Y mode. 4. Park mic in front of woofer fed with a circa 5000 Hz sine wave at about 10 watts. ( I used ear muffs) 5. Adjust scope and exact mic position to get a straight, diagonal line traced on the scope screen - note that adjusting the 5000 Hz amplitude affects the angle of the diagonal line only (ie makes it easy to visually distinguish amplitude modulation ). 6. Turn up low frequency generator, set to say 40 Hz, and watch the line open out to form a narrow ellipse indicating that the phase is changing as the cone moves closer and further away from the mic. 7. Sweep low frequency generator up and down and note that cone excursion alone controls the size of the ellipse - it never opens out more than about 15 degrees for a linear cone excursion of 3 mm. 8. Try hard to imagine that this is the notorious, evil, Doppler distortion before your eyes. Wow. " ............. Phil |
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#553
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With all this sturm and drang no one has yet produced a mathematical expression for the sound pressure at some chosen distance from a velocity controled piston in a tube (to keep the situation as simple as possible) given a signal containing the sum of some chosen pair of frequencies at a chosen relative magnitude. Odd, that. Sounds simple enough. Several have said they could if they wanted to but don't. Odd, that. I have, but almost no one likes it. :-) Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
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#554
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"Phil Allison" writes:
"Randy Yates" (The Ghost) writes: "Phil Allison" ** Bob Cain is a not just a blatant troll, but a nut, a slanderer and a probably a desperate reject from the Flat Earth Society. Glad to know that there's at least one other person out there who recognizes Bob Cain for who and what he really is. He is unquestionably the most technically inept jackass and contemptable piece of human waste that I have ever encountered in all of my years on this plannet. He has been slandering me for the last 3-4 years in alt.sci.physics.acoustics, and now it's payback time. His unlimited stupidity and uninhibited arrogance has finally brought him into a direct confrontation with mother nature, and he is going loose on this one big time. I look forward to that end and I intend to joyfully participate in the celebration. Although I vehemently disagree with Bob on the Doppler issue, the over-riding question in my mind is, "What is the reality of the situation?" These agressive and viscious ad-hominem attacks do nothing to discover the answer to this question. ** The person originating all such attacks is *** BOB CAIN *** !!!!!!!!!! Then his porcine pal followed suit. I have posted here test my test procedure and results in detail - did you see them ??? ( No contentious spectrum analyser involved ) ** QUOTE: " ** So this is what all the Doppler Distortion fuss is about ???? A tiny bit of phase jitter, which at 5 kHz rarely amounts to more than a few degrees ?? I was looking at it on my scope yesterday: 1. A 5 inch woofer, in box, driven by an amp fed from with two sine wave generators with outputs summed. 2. A condenser mic feeding a pre-amp and followed by a 12 dB/oct HPF at 2 kHz thence to the scope. 3. The high frequency generator output is also linked to the scope which operates in X-Y mode. 4. Park mic in front of woofer fed with a circa 5000 Hz sine wave at about 10 watts. ( I used ear muffs) 5. Adjust scope and exact mic position to get a straight, diagonal line traced on the scope screen - note that adjusting the 5000 Hz amplitude affects the angle of the diagonal line only (ie makes it easy to visually distinguish amplitude modulation ). 6. Turn up low frequency generator, set to say 40 Hz, and watch the line open out to form a narrow ellipse indicating that the phase is changing as the cone moves closer and further away from the mic. If the line turned into an allipse, then the phenomenom is a simple phase delay. For a specific amplitude from the low frequency generator, a varying ellipse width and angle would be predicted for FM. Perhaps you could clarify. But this is a neat idea for an experiment! -- % Randy Yates % "The dreamer, the unwoken fool - %% Fuquay-Varina, NC % in dreams, no pain will kiss the brow..." %%% 919-577-9882 % %%%% % 'Eldorado Overture', *Eldorado*, ELO http://home.earthlink.net/~yatescr |
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#555
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Bob Cain writes:
With all this sturm and drang no one has yet produced a mathematical expression for the sound pressure at some chosen distance from a velocity controled piston in a tube (to keep the situation as simple as possible) given a signal containing the sum of some chosen pair of frequencies at a chosen relative magnitude. Odd, that. Sounds simple enough. You know Bob, you are an irritating ****. This is in every physics book and dozens if not hundreds of web sites. To terminate this drivel, I'll type it in for you here. The perceived frequency, or Doppler frequency, fd, of a frequency f due to a relative velocity v between source and observer is given as fd = f*c/(c + v), where c is the speed of the medium (about 1100 ft/second for sound at reasonable temperatures at sea level). The instantaneous velocity v(t) of a speaker cone that is reproducing a sine wave A*sin(2*pi*fl*t) at frequency fl (f low) and at an excursion of A (meters, inches or whatever) is v(t) = A*2*pi*fl*cos(2*pi*fl*t). Put these two facts together and you get the dynamic doppler shift in a speaker: fd = f*c/(c + A*2*pi*fl*cos(2*pi*fl*t)). Of course units have to match, but that's up to the person applying this equation. Now what???? -- % Randy Yates % "Remember the good old 1980's, when %% Fuquay-Varina, NC % things were so uncomplicated?" %%% 919-577-9882 % 'Ticket To The Moon' %%%% % *Time*, Electric Light Orchestra http://home.earthlink.net/~yatescr |
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#556
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"Randy Yates" "Phil Allison" I have posted here test my test procedure and results in detail - did you see them ??? ( No contentious spectrum analyser involved ) ** QUOTE: " ** So this is what all the Doppler Distortion fuss is about ???? A tiny bit of phase jitter, which at 5 kHz rarely amounts to more than a few degrees ?? I was looking at it on my scope yesterday: 1. A 5 inch woofer, in box, driven by an amp fed from with two sine wave generators with outputs summed. 2. A condenser mic feeding a pre-amp and followed by a 12 dB/oct HPF at 2 kHz thence to the scope. 3. The high frequency generator output is also linked to the scope which operates in X-Y mode. 4. Park mic in front of woofer fed with a circa 5000 Hz sine wave at about 10 watts. ( I used ear muffs) 5. Adjust scope and exact mic position to get a straight, diagonal line traced on the scope screen - note that adjusting the 5000 Hz amplitude affects the angle of the diagonal line only (ie makes it easy to visually distinguish amplitude modulation ). 6. Turn up low frequency generator, set to say 40 Hz, and watch the line open out to form a narrow ellipse indicating that the phase is changing as the cone moves closer and further away from the mic. If the line turned into an ellipse, then the phenomenom is a simple phase delay. ** No - it was a blurred or oscillating ellipse. But the maxima were easy to see. For a specific amplitude from the low frequency generator, a varying ellipse width and angle would be predicted for FM. Perhaps you could clarify. ** The oscillating ellipse demonstrates rapidly varying phase shift. The peak angle is known from the scope pattern - ie the max dimensions of the ellipse. The low frequency is known during the test - and that it is of sine wave shape. The max instantaneous frequency deviation of the 5000 Hz tone can be easily calculated from the fact that Frequency = rate change of phase. So, if the low frequency is 50 Hz and the phase shift seen in the 5000 Hz tone is maxing at +/- 15 degrees ( corresponding to a +/- 3 mm come excursion) then the peak rate of phase change is : 2.pi.15.50 = 4712 digress /S Now, the rate of phase change for a steady 5000 Hz sine wave is: 360 . 5000 = 1.8 exp6 degrees/S As a percentage that becomes: 4712 . 100 / 1.8 exp6 = 0.26 % ( or 13 Hz) Which is the **exact** same figure predicted by the Doppler effect. QED. Game over - all go home please. .............. Phil |
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#557
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Randy Yates wrote: Bob Cain writes: With all this sturm and drang no one has yet produced a mathematical expression for the sound pressure at some chosen distance from a velocity controled piston in a tube (to keep the situation as simple as possible) given a signal containing the sum of some chosen pair of frequencies at a chosen relative magnitude. Odd, that. Sounds simple enough. You know Bob, you are an irritating ****. As always, irritation is a personal choice. I prefer the more objective word, tenacious. :-) This is in every physics book and dozens if not hundreds of web sites. To terminate this drivel, I'll type it in for you here. The perceived frequency, or Doppler frequency, fd, of a frequency f due to a relative velocity v between source and observer is given as fd = f*c/(c + v), Randy, that equation is only defined for a static v. where c is the speed of the medium (about 1100 ft/second for sound at reasonable temperatures at sea level). The instantaneous velocity v(t) of a speaker cone that is reproducing a sine wave A*sin(2*pi*fl*t) at frequency fl (f low) and at an excursion of A (meters, inches or whatever) is v(t) = A*2*pi*fl*cos(2*pi*fl*t). Put these two facts together and you get the dynamic doppler shift in a speaker: fd = f*c/(c + A*2*pi*fl*cos(2*pi*fl*t)). Of course units have to match, but that's up to the person applying this equation. Now what???? Try again, 'cause it's not correct. You cannot simply subsitiute a v(t) into an equation for a static v, especially without considering the conditions under which it holds, and expect correct results. Did you see my post to William where I explained that? I'm sure it would be no news to you. To get a dynamic equation you must do a fully dynamic analysis of a specific system, including some set of boundry conditions. That's why I like the tube which is what is assumed in what follows. The boundry conditions lead to a much easier analysis. It's just a section of the infinite plane wave and a piston creating it. Instead of starting from the acoustic wave equation and solving it with the given boundry conditions (something I could have done once but which would be a real struggle now if I could do it at all), I chose to assume a solution and see if it leads to any contradictions. This approach is not uncommon in physics or math. It is further assumed that the air remains in its linear regime, i.e. it's instantaneous velocity is always signifigantly less than the speed of sound. If that is not true the relationship between the pressure and velocity of air will yield all kinds of disortions without appeal to Doppler. My assumed solution is: p(x(t)+d,t+d/C) = v(x(t),t)*Ra where v() and x() are the instantaneous controlled velocity and resulting position of the piston at the same time t, d is any displacement from the rest position of the piston, Ra is the characteristic impedence of air, C is the speed of sound and p() is instantaneous pressure at d. This is a formulaic way of stating that the position of the piston is always appropriate to the velocity being imparted so as to propegate that condition down the tube as a wave at a speed C. It says that the piston follows the pressure/position profile of the traveling wave at its position in space. If instead of driving the wave at that point, a test piston were placed there in the path of the same wave traveling by, it would have the same pressure/position profile in time. If the assumed solution is absurd, here's the place to say why. From form alone, does the above not require that p'(X+d,t+d/C) = v'(X,t)*Ra for any X in the tube including the changing position of the piston? If so, we can let X=0 and get: p'(d,t+d/c) = v'(0,t)*Ra This says that the pressure at a point d from the piston is the delayed volume velocity at the rest position of the piston times the characteristic impedence of the air. It remains to connect the volume velocity at the rest postion to the velocity of the piston as it moves and here I appeal to the reciprocity argument I stated above. The final expression is independant of the nature of the controled velocity and is purely linear. If its consequences as I've stated them are logically correct, does my assumption lead to any contradictions in physics that can be stated formulaically? I can find none. I invite anyone else to find one. This is all to simply show that the usual argument for why Doppler distortion exists is fundamentally wrong. It's wrong because it predicts it in this configuration and the above analysis says it won't happen. Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
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Randy, that equation is only defined for a static v.
It is? So if the train happens to speed up or down as it passes me, the Doppler shift ISN'T consistent with the train's instantaneous speed? Hello? knock knock knock Anybody home? Sudden profound insight! For trains, v isn't constant -- even when the train is traveling at a constant velocity! Its relative velocity varies, dropping as the train approaches the listener (reaching zero as the whistle passes opposite the listener), then increases as the train moves away. So the formula DOESN'T apply to train whistles? Isn't it amazing what we can learn when we give the math precedence and ignore what happens in the real world? This is my absolutely final posting on the topic. "Think, people, think!" -- Lex Luthor, "Superman -- the Motion Picture" |
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William Sommerwerck wrote: Randy, that equation is only defined for a static v. It is? So if the train happens to speed up or down as it passes me, the Doppler shift ISN'T consistent with the train's instantaneous speed? As I've often explained, that is because the low frequency component is almost negligably coupled to the air in comparison to the whistle and thus Randy's equation is a close approximation. In the case where they are equally coupled, the tube, no Doppler distortion occurs so long as we remain in the linear regime of the air and that equation does not at all describe what happens. Once again, at this point I do not deny the phenomenon. I don't know why you would think that if you've been following along. I just feel the actual reason for it should be understood because it gives considerably more insight into when and where it might be a problem and offers the strong possibility of a more quantitative formulation that can indicate the real degree of the phenomenon for real systems. The usual simple argument of a little, fast sinusoid riding a big slow one on a cone, while intutitively attractive, is wrong. This implies that the phenomenon occurs at the piston/air interface when in fact it evolves over space as the transfer function of frequency changes. In the audible range, there is probably little or no Doppler distortion in the very near field of real speakers because the low frequencies have not yet begun to decouple. I'm sure that to many, why the lightbulb goes on when you flip the switch is of no interest or value but I'm not one of them, and I presume that at least some others involved in audio aren't either. That's why I became an engineer/scientist in the first place. I doubt that anybody with the other perspective is still with this discussion anyway. Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
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In alt.music.home-studio,rec.audio.tech,rec.audio.pro, Bob Cain
wrote: Randy Yates wrote: This is in every physics book and dozens if not hundreds of web sites. To terminate this drivel, I'll type it in for you here. The perceived frequency, or Doppler frequency, fd, of a frequency f due to a relative velocity v between source and observer is given as fd = f*c/(c + v), Randy, that equation is only defined for a static v. So if you start changing v, the doppler effect stops until you leave v alone for a while? How does the doppler effect know to stop and start up again? Seriously (or you can answer the above question seriously if you like), do you have any reference for the equation being defined only for v being static? Bob ----- http://mindspring.com/~benbradley |
Linear Mode
