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Why Does Sound Propagate?

Discussion in 'Mathematics and Physics' started by crashsite, Dec 30, 2008.

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  1. crashsite

    crashsite Banned

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    A Question of Relevance

    I would agree that the energy to propagate a wave comes from the source of the disturbance. Dropping a rock in the water provides the energy to drive the surface ripples (waves). The sound that gets propagated is another matter altogether.

    That's true when the molecules are the same mass or temperature or some equivalent combination of mass/temperature. It's also a key as to how tiny, low-powered disturbers can propagate so much sound. They don't have to do much work because the medium is supplying the energy.

    That's a whole 'nother topic. But, it doesn't necessarily take all that much energy to tear a molecule apart...if all you're doing is stripping off electrons to form ions or to modify molecular compounds. Combing your hair with a plastic comb can do it. Petting your cat can do it. Putting a cake in the oven can do it.

    Sound propagation (at nominal levels) doesn't even supply enough energy to ionize or otherwise change the nature of the medium, much less try to split or fuse the nucleus.

    Do you know how long the Brontosaurus exhibits in museums and pictures in science books had the wrong head because learned scientists got it wrong and, after becoming learned professors, taught it wrong? Let's just say it was a long time.

    People pay good money for entertainment and here you are complaining about getting it for free.
     
  2. Sceadwian

    Sceadwian Banned

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    I'm sorry, but at no point in any way shape or form in the last several posts that you've made have you provided any reason or information to make that statement make any sense at all. The only way I can make any sense is that you're trying to differentiate the information contained in the sound from the pressure wave itself which you can't do because one does not exist without the other. If there is no sound then there is no pressure wave, if there is no presure wave than there can be no sound. This is concretly proven with a simple fact. Less any weird radiation effects or other quantum oddities a sound/pressure wave will stop dead as a doornail once it encounters a vacum, of course assuming the energy in the wave isn't high enough to cause the boundry molecules to actually travel to another molecular space. Generally the entropy is maintained by causing the molecules at the vacant space to spread out so the energy can be disipated, or depending on the environment (such as in a rigid solid) will be reflected back from the vacuum junction.
     
  3. crashsite

    crashsite Banned

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    I think we might be getting just a little warmer...

    Yes.

    I think part of the problem is that there's a distorted sense of scale as to what's happening. I can understand this because I've succumbed to it and have had to fight it, myself.

    Sound is pretty mechanical and we technical types think of it in terms of time and space. Sound travels at 1100 feet per second so, a 1 khz sound must have a wavelength of about 1.1 feet. We can develop experiments to back that up with direct measurements (for example, by creating reflections in a space and then measuring the pressures at different points in the environment as standing waves).

    But, are we really measuring what we think we're measuring? I guess it's pretty obvious that I've come to the conclusion that it's not.

    If we technical types are going to rely on our sensibility of following the laws of physics, then we can't shirk when the laws seem to get shaky. We can't just shrug when a cricket is asked to move hundreds or thousands of tons of air. We can't deny the role of the energy in the medium.

    Sound will not travel through a vacuum. That can be shown experimenatally and it makes sense. If sound travels by molecular collisions, if there's no molecules, there's no collisions and no sound.

    But, when there are molecules, then you need to consider them as they are and at the scale at which they work. And, that scale is not 1.1 feet.

    That scale is one that I can't tell you because I don't know enough of the details to say. But, I do know enough to know that molecules work at a scale where, if they are traveling at 1100 mph, they interact in a picosecond time scale. Therefore, I believe that's the time scale that you need to consider when considering sound propagation. I've been hoping someone here would calculate some real numbers for me but...no takers (yet?).

    You talk about the interface between a vacuum and a solid. Of course, it's a solid. A gas or a liquid would be sucked (technically pushed) into the vacuum. But, it's a valid point and one that you should explore more throroughly: What IS happening at that interface between the solid and the vacuum?

    Once you figure that out you'll also better understand the role the impedance mismatch plays in converting molecular displacements to pressures and how that plays in converting sound into mechanical vibrations. An impedance mismatch is what you have at the junction between the solid and the vacuum...or the solid and the air...or the solid and any other medium that has a different characterisitc...or any two mediums that have different characteristics.

    In seeing what you write, I really do think that you've got a good chance to work through this...if you can but only shed the albatross of pressure waves.
     
  4. dave

    Dave New Member

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  5. Sceadwian

    Sceadwian Banned

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    We are measuring exactly what we think we're measuring, you have provide not so much as one letter of text to prove otherwise, NOTHING AT ALL. For the third or fourt time now I will repeat, the air DOES NOT MOVE IT OSCILLATES. HUGE difference. It is basically a harmonic effect, when the conditions are right huge power can be transferred through the medium. Simple example, a low power tone of the proper frequency can shatter glass. There is nothing to calculate on a smaller scale because sound is a macro process it can't be looked at under such a fine scale because the complexity is too high.

    There is nothing else to figure out I've stated it all clearly, such information is available on sites such as wikipedia or many other web sites independantly. You're continuing with every post you make to prove you have absolutely no point to make, no information to share and are trying to do nothing more than look intelligent when you in fact haven't the foggiest clue what you're talking about.
     
  6. crashsite

    crashsite Banned

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    Getting colder again...

    Okay, I admit it. I was wrong. We're not getting warmer about "getting it".

    If air is "oscillating", it's moving. The Wikipedia is clear that there's physical movement (interchange of kinetic and potential energy). You can continue to deny that oscillating thousands (even millions) of tons of air takes a lot of energy but, that doesn't change the facts or the physics of it. Trying to make it seem like only the tiny bits of air that each person in a crowd is hearing are oscillating when we know that everyone in the entire air mass can hear the sound makes no sense.

    The whole "harmonic" thing is kind of out in left field. Yeah, it's something that needs to be considered when thinking about resonances but, it's a moot point here. What needs to be thought of here is just exactly what's moving that glass in the first place. Is it a pressure wave as you think or is it molecular displacement encountering an impedance mismatch as I think? I know of no inherent resonance or harmonic tendency in air (or other medium) that would relate to frequencies in the audio range...but, I'm open to any that might be presented.

    The fact that I can answer specific questions about the physics of it that you, Feynman and the Wiki author can't, should be a clue...but, the wave addiction can be so strong as to cloud reason.

    So, I guess at this point we can agree to disagree and hope that each will come around to the other's thinking. I do hope that you will at least continue to process this as a background mental task and, perhaps at some future time you, too will have your "aha" moment about it.
     
    Last edited: Oct 18, 2009
  7. crashsite

    crashsite Banned

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    The Power Amplifier

    Let's consider the job the power amplifier in a sound system plays.

    The job of the power amplifier is to provide the power to the speaker voice coil (typically) and to provide a low impedance electrical path between the speaker and the amplifier output to ensure that the speaker is forced to follow the driving signal. That takes power and the amplifier output power is generally rated in Watts.

    We're pretty spoiled these days and tend to turn up our noses at lower power amplifiers. But, it wasn't all that long ago that 5 Watts was considered to be a high fidelity amplifier and, frankly, it still is.

    But, regardless of the amplifier's output power rating, what is that power actually doing? In short, it's moving air, overcoming mechanical resistances in the speaker and producing heat. What it's not doing is propagating sound.

    It takes energy to move the speaker's cone. Part of that energy is used to overcome the resistance and inertia of the speaker itself. It also takes energy to pump air; to push that air out of the way as the speaker cone vibrates. Of course woofers take more power than the midrange speakers and tweeters. They are larger and have larger excursions and thus move more air.

    But, once the amplifier has provided the power to move that speaker cone, the energy in the air then interacts with the cone to propagate the sound. It takes no energy from the amplifier to propagate the sound. I've already explained in detail how that works (with color-coded molecules) so, I wont repeate it here.

    Let me note that this post is not intended to try to convince those of you who disagree with my analysis of sound propagation. It merely adds some data and another dimension to the discussion.
     
  8. Pommie

    Pommie Well-Known Member Most Helpful Member

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    Mr crashsite,

    You really should take a lecture or two in physics. Your lack of basic understanding is astounding.

    Mike.
     
  9. crashsite

    crashsite Banned

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    A voice in the Wilderness?

    Welllll...as a person who sounds like they've taken a lecture or two in physics, you have me at a disadvantage. You are able to quote and cite others while I'm stuck trying to make it come out right.

    How does a disturber that's moving a few miles per hour (often slower), manage to propagate sound at some 770 mph by the process of air molecules pushing on other molecules?

    Feynman tells us that you can't consider it on a molecule-by-molecule basis (I claim it must be considered on a molecule-by-molecule basis). He says that you must consider, instead, tiny but, apparently arbitrarily sized packets that contain enough molecules to be able to define a pressure and then calculate how fast that pressure is able to equalize between it and another, adjacent, tiny but, apparently arbitrarily sized packet that has enough molecules to be able to define the adjacent pressure.

    So, anyway. That's what's in Feynman's lecture on the physics of sound. Does that sound about right to you...as a person who's up on what's being lectured in physics?

    Feynman also tells us that we don't need to consider either the source or the receiver when discussing sound propagation. It's good enough to consider only the way it travels through the medium. Conversely, I say that it's imperative that both the source and the receiver must be dealt with and it's the actual propagation of the sound that's less important.

    As someone up on what's being taught in physics, does that sound right to you that neither the source or the receiver need to be considered in order to understand sound and sound propagation? That only the way the "pressure" moves through the medium is of importance.

    Feynman simplifies the path that the sound takes as a direct one between the source and the receiver. He does this (he says) to simplify the math. But, as a person up on physics lectures, do you believe that there's only one path of sound propagation that needs to be considered in order to understand it or, do you believe (as I do) that there are a 'zillion' paths all simultaneously conveying the sound as it moves outward from the source in all directions, and they must all be simultaneously considered (ultimately having to oscillate thousands or millions of tons of air...as some here reject happens at all).

    Feynman says that we must consider the sound as a pressure wave that moves, as a longitudinal (sometimes traverse) wave through the medium. Do you, as a person familiar with modern physics lectures, find it as as odd or as interesting as I do that, after Feynman spends the whole lecture on sound talking about pressure waves, he wraps it up by equating the speed of sound to the velocity of the molecules (as I say it works all along)? Or, that once he finally acknowledges the role molecule velocity plays just ends the lecture rather than explaining how or why?

    Do you consider these kinds of things or do you just shrug and say, "I believe what the teacher told me and I'm not going to waste my valuable time rethinking it."? Im reminded of that olde timey gospel song, Give Me That Old-Time Religion? It was good enough for grandpa, it's good enough for me.

    Or, you could explain those things so they DO come out right...or...just sigh heavily and pronounce me too stupid to "get it" even if you tried...
     
    Last edited: Oct 18, 2009
  10. Pommie

    Pommie Well-Known Member Most Helpful Member

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    I love Richard Feynman, he is my hero, I even posted links to his lectures clicky.

    The thing I really love about him is his ability to explain things in terms that are understood by most people. You on the other hand just spout nonsense that is understood by no one but yourself.

    Shame on you for even mentioning such an icon in your inane ramblings.

    Mike.
     
  11. crashsite

    crashsite Banned

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    Really...It's not a personal attack on anyone...

    So, are you saying that I mis-characterized either what Feynman said or meant in his lecture on, Sound. The Wave Equation? I've posted the link to the lecture but, I'll be happy to make quotes if that's what it takes.

    I've said it before, in this thread. I like Feynman and I appreciate his style, both of writing and of trying to sort of figure things out as he goes along. But, that's not enough...it also has to come out right and I truly do think he missed the mark on sound and sound propagation.

    Whether I happen to be right or wrong is a moot point. What is important is that, if I have a viewpoint, that I publish it...you know, put it out there for others to read and accept it or reject it. I'm pretty impressed with the power of the press. Let me relate a small anecdote:

    About 15 or so years ago I was working for a division of Loral. They had bought out Ford Aerospace and the division I was working for (which services the US Air Force Satellite Control Network down in Sunnyvale, California). When I would end a phone call, I'd say, "bye bye" (just a habit). But, people nearby were hearing it as, "buh bye". A small point. But, they made kind a joke of it and started saying it, too. If you go to a Burger King, as you are leaving, see if there's a small sign on the door that says, "Buh Bye". If there is, I can't say that it came from me...through the grapevine...but, I have to wonder. I'd be curious if it made it to Aussieland.

    We'll have to defer to time to see if any of this thread becomes the de facto standard for sound propagation. Keep an eye out.

    BTW: I couldn't help but notice that your reply had nothing but hurt feelings and provided no answers to any of the technical questions I posed.
     
    Last edited: Oct 18, 2009
  12. Sceadwian

    Sceadwian Banned

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    Sorry, but again you're clearly showing that you have no idea what you're talking about, it's not me denying anything, it's established scientific theory. You also apparently have no idea how sensitive human hearing and it's dynamic range range, and how low the energy involved in sound transfer really is. Our hearing is logrythmic, and it takes an increase in power of 100% for us to even NOTICE that the volume has changed.

    Again showing that you have no idea what you're talking about. Harmonic resonance in air cavities is a MAJOR player in just about every form of sound transfer, especially in solids, and in human environments. A basic example is that during certain morning conditions on a glass smooth lake when the weather is right you can actually hear sounds from miles away further than you could normally because their base frequency is resonant with a boundary layer that forms just over the surface of the lake due to the difference in the bulk air above the lake and the water temperature.

    I don't even know where that came from I'm not sure what it means but if you think you're one up on any of them you're sorely mistaken. Your ego is a detriment to this conversation.

    No absolutely not, you are wrong and asking questions which have no meaning and can not possibly advance knowledge or understanding of the subject in any way shape or form.
     
  13. Sceadwian

    Sceadwian Banned

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    As to you power amplifier post again you'd trying to separate cause and effect, which CAN'T BE DONE, they're interdependent, without a cause there is no effect, without the effect there can't have been a cause, so you're talking yourself into a brick wall right there trying to seperate the two as individual components.
     
  14. Sceadwian

    Sceadwian Banned

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    You have not given a scientific basis for that rational, so it is irrational.

    Not only, but it's the primary consideration.

    Doesn't matter what you think, all paths CAN'T be simultaneously considered, it's a statistical impossibility, you can only look at them all as a whole, not individually, you said so yourself in an earlier post.

    I at no point said millions of tons of air didn't oscillate, in fact I specifically said they did, you're deliberately misrepresenting my words in a post now, so you've completely lost all credibility as far as I see it (and I'm pretty sure the majority of other users here see it the same way)

    The energy required to move a million tons of air 1mm in one direction is incredibly HUGE. The energy required to propagate a pressure wave through a million ton mass of air by oscillating it at an agreeable frequency is very SMALL. Your thought that the mass of air itself somehow produces energy in the process to allow the sound to propagate is completly unfounded, and unprovable, because if that were the case in a semi closed system sound propagation would result in the gradual cooling of a block of matter that sound was being propagated through, if you try to test this, and you are welcome to, what you will find is that the block of matter will in fact heat up due to the slight frictional loss from the induced pressure wave.
     
  15. Sceadwian

    Sceadwian Banned

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    Weather you're right or wrong is VERY important. You again have provided no testable conditions that would contradict existing theory, so at this juncture, anything you write is a waste of time. This is how the scientific process works, go against that and you shouldn't even be posting here, you should post on a free energy forum where such free baseless free thinking is welcomed. Imagination is good, but not if it's not grounded in the real world. You're just blowing smoke. Every post in this thread has so far universally rejected pretty much everything you've said, believe me, the people here that respond are going to be far kinder than if you post something of that nature in a true scientific forum.

    You think... You have provided not one single mathematical model that is different from established theory.

    Your ego is truly stunning... even suggesting that you may have been the originator of a language diversion like that is a simple self patting on the back for some other perceived lack of ability.

    You posted not one technical question, not one mathematical formula, only distorted words with no rational cohesive explanation. My feelings are not hurt by any posts you've made. If you see vehemence in my post it is from the simple fact that you are, while maybe not consciously aware of it are deliberately contradicting a scientific theory with no ration reason for it other than your own ego search for proving yourself among a group of others, because there is absolutly not scientific merrit in it.

    I'll be generous. I'll give you 50 words to work with, we'll go with established theory being a given as you can look that up anywhere. Posit a theory, and then a single currently testable experiment that can prove your theory to be correct and different from established theory.
     
    Last edited: Oct 18, 2009
  16. crashsite

    crashsite Banned

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    Accurate Results

    First things first. The "hurt feelings" comment was not for you. It was for user, Pommie per his indignation about me perportedly denigrating his hero, Mr. Feynman.

    I don't see vehemence in your posts. I see someone who buys into the subject as it's taught in the schools; as pressure and waves.

    One needs to be careful. Sometimes what you measure is what you set out to measure. If you set out to measure pressure and waves, then you'll likely measure pressure and waves. That seems to be pretty universal for all sorts of things. For example, if Rush Limbaugh sets out to characterize President Obama as a liberal pinko, he's going to come up with a guy who's a liberal pinko.

    I'm saying that just because you measure pressure and waves doesn't necessarily mean that there are pressures and waves to measure. It may be that your measuring instruments are creating pressures and waves as part of the measuring process.

    Let's say you hypothesize that a person walking across a town just magically appears from the corner of one block to the corner of the next. To test your hypothesis, you set up cameras at the corner of each block and as the perons crosses the street at each corner you take a picture. At the end of the test, you look at the pictures and see that the person sequentially appears in each picture but, there's no sign that there was any intervening activity between the times that the pictures were taken. Conclusion: Your hypothesis is correct. The person magically appears at each corner.

    If you generate a sound and then go to some receiver (such as an eardrum or microphone) and discover that there's a pressure, can you automatically assume that there is a pressure that travels all the way from the source to the receiver? You would say, "yes". I would say, "no". In fact, I would say that there's no pressure except at the receiver. There's none at the source and there's none in the intervening space.

    You keep saying that I've provided nothing to back that up. In fact, I've explained exactly how it's done (at the bottom of page 41, with color-coded molecules, yet!). At least it explains how the sound gets encoded onto the molecules in the medium and propagated. How they are decoded at the receiver has also been explained but, not as simply.

    We would both have to conclude that there's something going on between the source and the receiver. You'd say, it's a pressure wave. I'd say, it's a statistical molecular displacement. So, how does one prove who's right? You need to set up tests that don't color the result. Because of the nature of sound and matter, that really does mean doing a whole battery of tests to weed out the stuff that doesn't add up and accentuates that which does.

    Regarding your "lake effect", I'll just say that you've described something very common in both nature and technology. It's a conduit effect where the sound gets trapped and channeled very much like light in an optical fiber. That same effect is not uncommon with RF (especially at VHF and UHF frequencies) where the radio wave gets channeled over much longer distances than the normal line-of-sight normally associated with those signals. It's different than the so-called skip which uses the ionosphere.

    More to come.
     
  17. Tesla23

    Tesla23 Member

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    This thread reminds me of this, but I'll respond anyway, although you have answered your own question by noting that you disagree with Feynman on some elementary physics. There is so much confusion on this thread that I think it would be good to have some physics.

    Crashsite. you are right about the velocity of the gas molecules being the key to the behaviour of a gas. Google 'kinetic theory of gases' and you will find a very well worked out, and consistent theory. Key to this is that the gas molecules have no interaction except for elastic collisions - this means that energy is conserved in the collision.

    This is where the theory (and facts) deviate from your proposals. Key to this theory is that the number of molecules is very large, in excess of 10^20 in any typical container. To understand what happens in this situation you need a theory that can track a large number of molecules, this is what statistical mechanics does. You haven't presented any theory apart from a picture of a handful of molecules (color coded as you point out!). The kinetic theory of gases models a large number of molecules, all moving in random directions, and can deduce some physical properties from this. It turns out that the molecules don't all travel at the same speed, but they spread out over a range of speeds, this is called the Maxwell-Boltzman (M-B) distribution. I can't believe that you haven't come across this, you can derive it with relatively simple mathematics as a consequence of entropy considerations and the conservation of momentum and energy in the molecular collisions. Your assumption of all the molecules travelling at the same speed and is not what is experimentally observed (yes the M-B distribution can be measured) and is not consistent with basic and accepted physical laws (conservation of momentum and energy).

    In air at normal conditions, the mean free path of a molecule is measured in tens of nanometers and the collision frequency in GHz. So when you are talking about disturbances occurring at audio frequencies, air does approximate a continuous medium. When the speaker cone moves forward it does impart a forward velocity bias to those molecules that impact the cone as it is moving, but this is quickly spread amongst all the molecules involved in collisions. The forward momentum imparted to the molecules is, however, preserved (a simple consequence of Newton's third law), which is one of the things that makes the molecular view consistent with the macro view that a volume of air is pushed forward and gains some momentum. This seems to be the issue that you are trying to bring out - but it is already there.

    Now you seem to be either totally confused or paranoid about waves. When we are talking about wave propagation in air, we are almost never talking about sine waves! We are talking about disturbances (like your cricket's leg). When physicists talk about waves in air they mean that for small disturbances the air obeys the wave equation. Basically this is because if you push on a bit of air, it moves it's neighbour a bit and it tends to push back but also pushes on the next bit of air - so the disturbance propagates. This is similar to a displacement in a guitar string (1-D), a drum membrane (2-D), disturbances in solids (3-D). Why physicists talk about waves is not to form a conspiracy but because they see that when you displace a volume of the medium, the interaction with the neighbours conforms to the mathematical form of a wave equation. Physicists don't like solving the same problem more than once, so when they see an equation they have seen before, they use the solutions they have made before. Like engineers, if the simplest solutions can be found using sine waves, they are likely to use those, but they know that in practice any real solution will be a sum of sine waves, just like in engineering.

    The acoustic wave equation is
    where
    is the speed of sound. You can look up all the details. I know that you don't like maths, but it is a pity that you can't see the beauty of an equation like this, the solutions maintain their shape and move in space - waves. Are you disputing the fact that small pressure disturbances in air behave in accord with the wave equation?

    Now the solutions to the wave equation show that what happens is that the cricket's leg moves a small amount of air locally, and this disturbance ripples out, just like ripples on a pond. The energy from the leg is spread over ever increasing area, so gets feinter. You only collect a very small amount of this energy on your ear (in fact your ear has the external part to collect as much of the disturbance as possible to feed into the eardrum). Your ear is very sensitive and can detect very small pressure changes. You seem to be suggesting that the cricket moves all the air, I assume meaning that the pressure fluctuations at your ear would be the same as at the cricket, I can assure you that if the cricket was kicking your eardrum directly you would know how little energy you usually get.

    To your central contention that seems to be that there is some conspiracy that 'hides' the fact that the speed of air molecules is the 'same' as the speed of sound. Firstly this is not true as the molecules have a wide range of speeds according to the M-B distribution, but secondly, why is it so surprising that the speed of sound is comparable to the speed of molecules - after all they are what are conveying the disturbance! This is a well known fact pointed out in most elementary physics courses on kinetic theory.

    Just something else to ponder, from the kinetic theory and the wave equation it is easy to show that the speed of sound is equal to 68% of the rms speed for diatomic molecules and to 75% of the rms speed if the molecules are monatomic. So for two containers, one full of say Argon, the other full of nitrogen, at the same temperature they will have identical molecular speed distributions, however the speed of sound will be 9% faster in the Argon!
     
    Last edited: Oct 19, 2009
  18. smanches

    smanches New Member

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    So this is philosophy to you?

    You really need to study and learn the difference between fact, speculation, and opinion. Fact (or proof) is that which can only be based on other facts to fully explain it. Speculation is that which has some factual basis, but not all the variables are known yet, so we improvise. Opinion is that which cannot be proven by fact no matter which way you look at it.

    Feynman can simplify things because he knows the facts behind it. Just because you won't prove (see definition above) your theories does not mean that nobody else will or has.
     
  19. smanches

    smanches New Member

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    If you create a void in a volume of air, how fast does the air rush in to fill that void?
     
  20. crashsite

    crashsite Banned

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    Finally some common sense...up to the "waves" anyway...

    Well, you said quite a lot there and, I agree with all of it right down til you got to the "wave" part. So, rather than reiterate it all again, I'll just make a synopsis of each of your paragraphs and then more fully discuss the waves.

    If you think I'm going to disagree with that, you're nuts...

    ...and, not just gas molecules. Any molecules whether they are moving around or merely vibrating in place.

    Of course there are a lot of molecules and you can only consider them on a statisticaql basis. That's why I've been repeatedly saying that you have to deal with them on a statistical basis and no-one was agreeing. They were all still trying to deal with it as pressure waves. I even went so far as to try to equate the process with the development of actuary tables for the insurance industry and...still no believers.

    So, I put it down to the level of three (count 'em 3) color coded molecules to show the basic action and still no-one "got it". I am considering contacting the Fisher-Price Company to see if maybe they've developed a crib toy that might show the concept.

    Yes! That's why I've been pounding on the notion that you have to think of the process of sound propagation at the picosecond time scale.a Trying to deal with it on the macro, wave level has you pushing air around...not propagating sound.

    That does a really good job of describing how you get ripples on a pond and ripples of air in an air mass when you push on or displace them. It does nothing to explain how the sonic shock wave takes off at Mach 1. The problem with invoking the wave equation is that the topic of this thread is not, "How Do Ripples Propagate?".

    Feynman did make a valiant effort to try to equate the wave equation with the speed of sound (with his tiny but arbitrarily sized air pressure packets, adjacent to other tiny but arbitrarily sized air pressure packets and a bunch of math to try to eventually come up with an approximation of the "right" answer that the speed of sound is actually related to the velocity of the molecules). Then, he neglected to say how or why the speed of sound is related to the speed of the molecules.

    That's what I'm trying to do in this thread.

    Not at all. So long as you're referring to pressure disturbances and not sound propagation.

    If you're talking about the air a few millimeters from the cricket, that's probably true. Out the 300 feet, in my example and some almost 2000 tons of air removed from the cricket I'd absolutely disagree with you.

    (Emphasis mine) No, no, no, no, no! I am suggesting that the cricket moves none of the air (beyond the air that's in the immediate proximity of the leg and thus is affected by the ripple effect). Any of the air that's involved in propagating the sound of the cricket is only displacing the position of the molecules (as you so correctly pointed out earlier) and, that is being done by the thermal energy contained in the air. The cricket is completely passive when it comes to propagating that chirp.

    The fact that the molecules are still almost exclusively in random motion and changing positions (and direction) every few picoseconds precludes them from bunching up and making "pressure cells" or "pressure waves" (or whatever you might want to call them). But, that positional bias does get carried from molecular collision to molecular collision and, because the molecules are traveling at an average speed (taking every effect into account) of Mach 1, that positional bias also gets propagated at that speed.

    Whether you show that graphically with three molecules or statistically with 10^20 molecules, the process is the same.

    I don't think it's a conspiracy any more than I think it was a conspiracy that people used to think the world was flat. After all, it sure looks flat and, by golly, those ripples sure do look like they might well be a solution to any sort of propagation, huh?

    I can't comment on that because I have no knowledge or experience with it. But, thanks for the info...I'll tuck it away in my memory banks.

    * * * * * * * * * * * * * * * * * * * * * * * * *

    The first part of this is how the sound gets encoded onto the molecules by molecular displacements. The second part is how it propagates by those displacements. The third part is how those displacements are decoded (turned into the pressure that vibrates the ear drum or whatever). If we can get through this part, I'll explain that third part. But, unless or until you accept the molecular displacement model, the third part will make no sense to you either.
     
    Last edited: Oct 19, 2009
  21. crashsite

    crashsite Banned

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    Good Question

    Let me jump in and give you a quickie answer on this.

    A lot slower than Mach 1.
     
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