Why Does Sound Propagate?

Scattered.

3v0 said:

Either air is effected by sound as show by the scatter graph on the bottom and density graph on top or it is not.

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I included the answer to this in the post just previous to this one (to j_friend). If you'd already read through it, double check as I appended it after receiving your post.

You'll see no periodic increase or decrease in the number of molecules (like you see in the pressure distrubution diagrams (scattergraphs)...often presented even in this thread). It's always the same number of molecules. It's just that they have a slight directional bias from their normal random motion that varies in accordance with the movement of the disturber. (I have to add this disclaimer for the fastideous ones...yes, there are pressure variations in the near field much like there's a pressure change when you blow a breath of air but, here I'm thinking of sound propagating a 'substantial' distance.)

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This is a huge problem. It may not be noticeable or seem important to you, j.friend, or 3v0, but to any flow modeller this sticks out like a sore thumb. It is a fatal flaw. It does not conserve mass. It is impossible to have a variation in velocity without a variation in density.

I agree skyhawk. The pressure waves exist and it is possible to measure them.

Scattergraph of Scatterbrain (sorry, couldn't resist the pun)

skyhawk said:

This is a huge problem. It may not be noticeable or seem important to you, j.friend, or 3v0, but to any flow modeller this sticks out like a sore thumb. It is a fatal flaw. It does not conserve mass. It is impossible to have a variation in velocity without a variation in density.

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I agree with you and user, 3v0. I have a huge problem with it, too. Let me redact my comment about the typical scattergraphs that have been presented to represent sound propagation to say what I really intended to say about them.

It's obviously impractical to make one of those pictures to any reasonable scale. Any attempt looks like a uniform gray swath across the picture. So, a cartoonishly garish exaggeration is employed (sort of a Bizzaro version of what the real chart looks like).

In the cartoon chart, reality is secondary to showing an effect. The pressure variations are a major component (essentially the only component) and the random motion is not shown at all. Worse, there is usually a sine wave associated with the scattergraph shoing how the peaks and troughs corrolate to the variations in molecular density. I've got more to say about this later but, for now...

Nowhere in either the picture or the associated text is there anything to put all this into some sort of real-world context.

Is it any wonder that impressionable young people come out of science and physics classes with a distorted sense of how sound propagates? All they've ever been exposed to is this kind of stuff. They get it in school, they get it on Wikipedia, they get it in 3v0's slide show, they get it from web page links people put into this thread and on and on. It's little wonder that people are convinced that the air mass oscillates. The data "proves" it.

But, the data's wrong, isn't it? In actuality, even on a greatly expanded scale, the scattergraph, at best, will show an essentially uniform distribution of molecules. Because the miniscule time of interaction between the air molecules and the disturber, the amount the disturber moves at each interaction is small even compared to the intermolecular distances of the air molecules (logically derived). So, at best, even if the chart resolution gets right down to the molecular scale, you'd only see a displacement of the molecules due to the sound, not a pronounced compressing and rarefying action (logically derived) like it's shown on the scattergraph.

Okay, back to user, 3v0's Power Point presentation. Right on slide 2 (What IS Sound), there's the ever-present scattergraph. But, look at it. No, seriously, go look at it and then come back and continue to my next paragraph.

https://www.physics.ucsd.edu/~tmurphy/phys8/lectures/10_sound.ppt









Hey, no peeking! Go look.

























If you look closely at the picture and corrolate it with the sine wave, you'll notice that the compression concentration is the positive peak and the rarefaction is the negative peak. And, of course, you're thinking, "Yeah, okay. So?".

During both the positive and negative peaks of the associated sine wave, there is zero (or near zero) change (disturber at minimum velocity) and at the zero crossing, there's maximum change. Therefore, when there's maximum change of the disturber, there's maximum displacement of the molecules (maximum bias gets added).

When the disturber is traveling in the positive direction (per the convention of the sine wave), the molecules get displaced one direction (let's say, to the right) and when the disturber is traveling in the negative direction, they get displaced to the left. That is the pattern that is propelled through the air by heat.

Like I said in my recent answer to j_friend, what gets propagated is not the waveform. It's a pattern that can be integrated into the waveform (at the time the pattern reaches a point of impedance mismatch and re-integrates back into a motion that mimics the original disturbance).

The author of the slide show (obviously a product of the educational system), is thinking about the gross volume of air and how it's all pressure of the air mass and waves and wave analysis and that's what his picture proves. Too bad it has nothing to do with sound propagation.

During both the positive and negative peaks of the associated sine wave, there is zero (or near zero) change (disturber at minimum velocity) and at the zero crossing, there's maximum change. Therefore, when there's maximum change of the disturber, there's maximum displacement of the molecules (maximum bias gets added).

When the disturber is traveling in the positive direction (per the convention of the sine wave), the molecules get displaced one direction (let's say, to the right) and when the disturber is traveling in the negative direction, they get displaced to the left. That is the pattern that is propelled through the air by heat.

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I do not see the problem with this. As you say at the minimums of the sine waveform there zero changes in velocity and hence zero force applied to the molecules. This means there is minimal change in pressure at this time, which I believe is aptly represented by the scatter graph. the time of maximum velocity of the piston, and therefore the maximum force imparted on the air there is the greatest change in pressure/distribution of the molecules. I honestly cannot see the point that you are making.

This is a huge problem. It may not be noticeable or seem important to you, j.friend, or 3v0, but to any flow modeller this sticks out like a sore thumb. It is a fatal flaw. It does not conserve mass. It is impossible to have a variation in velocity without a variation in density.

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Indeed this is true. Any time that you have alignment of the velocities, or the direction skewed as it may be, the molecules will logically form a more dense area. If you have a box of matches pointing in any direction they will have much a lesser density than if you align them. This may be a crude example but it does illustrate it.

Okay, now your logic. When you get to your step 3, you talk about the disturbing force "aligning" the molecules. This gets a little tricky but, I don't like the idea of thinking about them being "aligned". A better term might be, that the molecules are slightly skewed, from their entirely random motion, in the same direction as the disturber moves. and, when I say slightly, I mean slightly as in how far the disturber moves in picoseconds.

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I was simply trying to express what you have said in earlier threads. In essence the molecules are aligned to a degree, as their motion has been skewed in one direction. This in turn causes the 'waves' of pressure.

You'll see no periodic increase or decrease in the number of molecules (like you see in the pressure distrubution diagrams (scattergraphs)...often presented even in this thread). It's always the same number of molecules. It's just that they have a slight directional bias from their normal random motion that varies in accordance with the movement of the disturber.

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:
Originally Posted by skyhawk
This is a huge problem. It may not be noticeable or seem important to you, j.friend, or 3v0, but to any flow modeller this sticks out like a sore thumb. It is a fatal flaw. It does not conserve mass. It is impossible to have a variation in velocity without a variation in density.

Click to expand...

I agree with you and user, 3v0. I have a huge problem with it, too. Let me redact my comment about the typical scattergraphs that have been presented to represent sound propagation to say what I really intended to say about them.

Click to expand...

Is it just me but I'm a little confused as to what you do believe. Either you agree that there is a change in density, AKA a change in number of molecules, or you don't.



I Would also like to know what is your definition of a wave. And could you please give an example of something that is propagated by waves?

j.friend did a good job so I will only add this.

Like I said in my recent answer to j_friend, what gets propagated is not the waveform. It's a pattern that can be integrated into the waveform (at the time the pattern reaches a point of impedance mismatch and re-integrates back into a motion that mimics the original disturbance).

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It is possible that you are using a different definition of integration.

It is a compression wave. The graph at the top shows the degree of compression. It is not expected to look like the compression wave.

Discretum vs. Continuum

Emphasis mine:

j.friend said:

I do not see the problem with this. As you say at the minimums of the sine waveform there zero changes in velocity and hence zero force applied to the molecules. This means there is minimal change in pressure at this time, which I believe is aptly represented by the scatter graph. the time of maximum velocity of the piston, and therefore the maximum force imparted on the air there is the greatest change in pressure/distribution of the molecules. I honestly cannot see the point that you are making.

Click to expand...

I'm going to zero in on just this one point in this post.

You look at the sine wave and the scattergraph picture and see that the greatest change of pressure corresponds to the time that there is the greatest speed of the disturber (near the zero crossing). And, when analyzing a waveform, that's valid.

When analyzing the molecule-by-molecule interaction between the air and the disturber, you need to conceptually use a different frame of reference. Rather than seeing the variation of the pressure follow the variation of the disturber, you need to think about what's happening at each collision between the two. To see that it's not a pressure gradient that's being created but, a discrete series of steps.

A pretty good analogy is PCM coding of an audio signal. Each sample is unique unto itself. Over time, the samples can be integrated in a D/A converter back to sound but, in the digital format, you need to deal with the samples individually. To understand sound propagation, I think you need to have a similar mindset. While the sound is propagating through the medium (air assumed), it does so as a train of molecular collisions, carrying the bias of the disturber along.

You're probably too young to remember the, "bucket brigade" delay line that enjoyed a brief period of popularity in the '70s and early '80s but, it's an even closer analogy to sound propagation. It was an analog delay line. It consisted of a series of sample and hold circuits. You put your analog signal into the front end which was clocked through the S&H stages and analog came out the other end...delayed.

Trying to think of the sound propagating as a wave just gets you into trouble. Hint: I've asked for someone to tell me how a longitudinal wave manages to propagate sound at Mach 1. So far nobody's even tried (smart of them, in my opinion).

But, if you ask how heat can propagate sound at Mach 1 (after some 35 pages of this thread), I think I can give a pretty credible answer. I still have no idea how a longitudinal wave could do it.

Integrating and Compressing

3v0 said:

It is possible that you are using a different definition of integration.

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I know that the term "integration" has a different mathematical definition than you might use when thinking about how an R/C circuit can integrate a series of pulses to a steadily increasing (or decreasing) voltage level across the capacitor. I'm using the term more like the R/C circuit where the biases from the molecules sum, over time, to construct the waveform at an interface like an ear drum.

3v0 said:

It is a compression wave. The graph at the top shows the degree of compression. It is not expected to look like the compression wave.

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What does a compression wave (as depicted in the slide show) have to do with the propagation of sound?

More Defining Moments

j.friend said:

Is it just me but I'm a little confused as to what you do believe. Either you agree that there is a change in density, AKA a change in number of molecules, or you don't.

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I do question whether there is enough displacement of the molecules, due to impressing sound energy on them, to consider that there's more or less of them into a defined space. It's sort of like having a confined volume of a gas and then heating it up. There is a change but, the number of molecules and the space they occupy remains the same. The molecules just act differently within the space. I know that's pretty conceptual and open to a lot of interpretation.

j.friend said:

I Would also like to know what is your definition of a wave. And could you please give an example of something that is propagated by waves?

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I kind of like the usual definition of the cyclic interchange of potential and kinetic energy.

Something actually "propagated" by a wave? I don't know. Pretty ripples on a pond? The famous cork-in-the-water experiment shows that nothing else is actually propagating.

crashsite said:

When analyzing the molecule-by-molecule interaction between the air and the disturber, you need to conceptually use a different frame of reference. Rather than seeing the variation of the pressure follow the variation of the disturber, you need to think about what's happening at each collision between the two. To see that it's not a pressure gradient that's being created but, a discrete series of steps.


Trying to think of the sound propagating as a wave just gets you into trouble. Hint: I've asked for someone to tell me how a longitudinal wave manages to propagate sound at Mach 1. So far nobody's even tried (smart of them, in my opinion).

But, if you ask how heat can propagate sound at Mach 1 (after some 35 pages of this thread), I think I can give a pretty credible answer. I still have no idea how a longitudinal wave could do it.

Click to expand...

Now as to how sound propagates at mach 1 due to compression waves. I think i might take the stupid option and give it a go.

Rather simply put, the thermal energy interactions that you have quite correctly identified as the method that the sound is propagated is the way in which the sound is propagated. Sounds pretty similar to your side of the story doesn't it.

Lets just make the rash assumption that the skewing of the random movement of air molecules results in a wave of higher pressure and alternative waves of lower pressure. i think we have so far managed to agree on this.

A compression wave is just that. A wave like propagation of energy such that there are waves of pressure. I believe that your heat energy is the mechanism for the compression wave. In simple terms it is one and the same. The compression wave is a result of the kinetic interactions of the molecules due to their thermal energy.

I love my analogies so I'll give you another one. You ask anyone how a car works, what gives it the power to make it move. The most common answer by far will be that it is the motor that allows the car to move. However the motor depends on the fuel. Without the fuel there will be no driving for the unfortunate driver.

The same, i believe can be said for the propagation of sound. Compression waves is what drives sound. However if you don't have the thermal energy of the molecules the sound cannot propagate.

crashsite said:

I do question whether there is enough displacement of the molecules, due to impressing sound energy on them, to consider that there's more or less of them into a defined space. It's sort of like having a confined volume of a gas and then heating it up. There is a change but, the number of molecules and the space they occupy remains the same. The molecules just act differently within the space. I know that's pretty conceptual and open to a lot of interpretation.

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I think the is the main issue at the moment. if you agree on this I think you would agree with the theory presented by others for the propagation of sound. Whilst the example you have given is true, to an extent, it is not indicative of the conditions upon which the propagation of sound takes place under.

Firstly as you have said many many times, you are not heating up the molecules within this space, therefore this analogy is incorrect.

Secondly, If we are to accept your idea about the skewing of the direction of molecules, which incidentally I agree with, you must also accept the implications. If there is a greater proportion of molecules moving in a given direction, there is a lesser pressure on all other directions due to fewer collisions in these directions.

This means the molecules will be closer together and therefore more dense

crashsite said:

I kind of like the usual definition of the cyclic interchange of potential and kinetic energy.

Something actually "propagated" by a wave? I don't know. Pretty ripples on a pond? The famous cork-in-the-water experiment shows that nothing else is actually propagating.

Click to expand...

this is a good example of a transverse wave....

Without knowing what you can not understand how or why

crashsite said:

There is a change but, the number of molecules and the space they occupy remains the same.

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This is false.

As the first molecule encounters the speaker face, the cone is traveling forward and will do so for a time based on the frequency of the sound (min to max forward speaker travel). This imparts additional energy to the air molecules in front of the speaker over a period of time.

A band (not a point or line) of energy moves through the air at the speed of sound compressing the air as it travels through it.

3v0

How it works...

3v0 said:

As the first molecule encounters the speaker face, the cone is traveling forward and will do so for a time based on the frequency of the sound (min to max forward speaker travel). This imparts additional energy to the air molecules in front of the speaker over a period of time.

A band (not a point or line) of energy moves through the air at the speed of sound compressing the air as it travels through it.

Click to expand...

It's said that the devil is in the details.

On a theoretical level, I don't think we disagree that energy is added to the air by the speaker. Likewise, if we were discussing snails and one of us were to invoke relativistic effects associated with the snail's movement, we couldn't discount it (at least on theoretical grounds).

You seem to be advancing the notion that, as the speaker cone moves forward the air compresses more and more (and, presumably, as it moves back, rarifies mroe and more). You seem to be thinking of the effect as something that happens over a period of time (per your notation about the frequency of the signal driving the speaker). Then, somehow, that compression propagates at Mach 1. But, you stop short of explaining just how that compression "moves through the air at the speed of sound compressing the air as it travels through it".

Am I reading what you are saying correctly???

Now, I'm not saying that I disagree with that on principle but, I really would like to read your explanation of how the sound propagates in your scenario.

crashsite said:

You seem to be advancing the notion that, as the speaker cone moves forward the air compresses more and more

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Lets look at one full stroke of the speaker cone from back to front. It distributes (adds directed energy) to a band or layer of air. That energy moves away from the speaker at Mach 1.

if the speaker is vibrating at 50Hz, the out stroke is about 1/100th of a second. If we could watch the energy pulse as it traveled the end of it would be 1/100 of a second after the leading edge. The width would be 1/100 *(speed of sound).

The speaker may only move a fraction of an inch but the energy pulse it made is much wider.

3v0

Nit Picking

j.friend said:

Firstly as you have said many many times, you are not heating up the molecules within this space, therefore this analogy is incorrect.

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That example was not intended to be analogous. I was meant to say that you can have more than one explanation for a phenomenum. In that case, you could think of the displacement of the molecules, under the influence of a sound disturber, as being more or less pressurized or you could think of them as simply remaining in the same proximity but, acting differently.

How about this. Your car is in the garage. The sun heats the earth and the concrete of the garage floor expands a little, moving the car. Can you think of that as "moving the car"? Or, if a micro-tremor in the Earth's crust jiggles the concrete a bit? At what point do you consider the movement to be incidental and ignored in the context of larger movements?

j.friend said:

Secondly, If we are to accept your idea about the skewing of the direction of molecules, which incidentally I agree with, you must also accept the implications. If there is a greater proportion of molecules moving in a given direction, there is a lesser pressure on all other directions due to fewer collisions in these directions.

This means the molecules will be closer together and therefore more dense.

Click to expand...

Not necessarily. If the random movement of the molecules is large compared to the deflection caused by the sound energy, there will be a slight overall bias for them to be more or less dense, per the sound energy but, a lot of the time, the affected molecules could be in the unexpected state of density.

crashsite said:

It's said that the devil is in the details.

On a theoretical level, I don't think we disagree that energy is added to the air by the speaker. Likewise, if we were discussing snails and one of us were to invoke relativistic effects associated with the snail's movement, we couldn't discount it (at least on theoretical grounds).

You seem to be advancing the notion that, as the speaker cone moves forward the air compresses more and more (and, presumably, as it moves back, rarifies mroe and more). You seem to be thinking of the effect as something that happens over a period of time (per your notation about the frequency of the signal driving the speaker). Then, somehow, that compression propagates at Mach 1. But, you stop short of explaining just how that compression "moves through the air at the speed of sound compressing the air as it travels through it".

Am I reading what you are saying correctly???

Now, I'm not saying that I disagree with that on principle but, I really would like to read your explanation of how the sound propagates in your scenario.

Click to expand...

Ok lets look at it in the way of the effectiveness of the speaker to skew the molecules motion. As the cone moves through the air it skews the molecules motion in the direction of the propagation of the sound. I think I can fairly safely say that the proportion of molecules skewed in the direction of the propagation will increase proportionally with the time that the cone is moving out (or in).

The key concept at this point is that the effect of the speaker on the band of air particles directly in front of it is for our purposes uniform for all the molecules. This means that your example of the molecule-molecule interaction spreads out in a band uniformly in a band. I believe that we have also established that the skewing of th particles in the direction of the sound results in a higher pressure. This means that the band of molecules moving out under the molecule-molecule interactions previously discussed effectively creates an band of pressure.

This band of pressure moves at the speed at which the molecule-molecule interactions occur, meaning that it propagates at the speed of sound.

We have also established that the cone of the speaker will have a times of maximum and minimum movement. The compression of the band of air is dependent on the speed of the cone (which affects the force imparted). The maximum 'skewing' of the molecules happens when the speaker cone is moving the fastest. whilst the minimum skewing of molecules direction will occur at the times of minimal motion. This effect on the relative compression of the air means that it will change over time in direct proportion to the instantaneous velocity of the cone.

does this sorta make sense or am i completely wrong?

I’ve been following this thread from the start (I, like crashsite, have been thoroughly enjoying the mental exercise of thinking about sound in depth and questioning my understanding of the subject), and have been trying to resist joining this forum just to add my 2 cents, but could resist no longer.

I would like to try to give crashsite what I believe he is looking for and that is the conceptual view of sound propagation at the molecular level from the “classical” physics perspective (at least how I understand it). To do this I must start by examining intermolecular interactions, and this starts with the Lennard–Jones potential (a simple enough assumption that has been experimentally derived). Note that this is only looking at molecular energy (aka heat), a type of energy that skyhawk failed to mention in his write up on energy a few pages back (see below for a side bar about energy). If we fix a molecule in space and place a second molecule next to it at absolute zero the molecule is sitting at the bottom of the “trough” in the curve, and as energy is added to the molecule it moves back and forth along the curve to the level in which it has energy (think: a marble rolling up and down the curve). If it has energy greater than zero it is a “free” molecule (aka gas). If you add a third molecule, the molecules would arrange themselves such that they move around the corners of a triangle (assuming energy less then zero). Adding a fourth turns it into a pyramid. This 3-d vibrational system is already a fairly complex system (As more molecules are added the complexity goes up extremely quickly because even in a gas ALL molecules are constantly influencing ALL other molecules, not only those “colliding”). Now, for the system to be “liquid” it must be in an energy state such that during these movements the average molecule may pass between two others but not above an energy level of zero relative to all other molecules, otherwise that molecule would become free. From this it is easy to see were the spring mass model comes from, at least for solids. For liquids it is a little more difficult because the springs may “slip” past each other, and for a gas it is even more difficult because the intermolecular forces are very non-linear and primarily driven by repulsive forces and dependant on the repulsive forces of other molecules or some other external force, such as gravity for restoration (very different from the typical spring). This non-linearity and the shear number of molecules involved (not the assumptions made to get here) is the reason for the complexity of the math involved in solving this problem. Now if you disturb a molecule in this system with an added force (sound) the disturbance travels through the system per these intermolecular forces (read as: independent of input force, except for the molecules directly interacting with the source) and can be shown to do so with some very complicated math (which I have not done, nor do I believe I could do if I wanted to, but you can see the book referenced several pages ago if you feel the need to see this done). The complexity of this problem is one reason the linear spring mass system (aka slinky) is used for demonstrational purposes and class room calculations, another is the ease of its observation and correlation with calculations (the math involved for the simplest of true materials is over the head of the majority of even physics/math majors heads). As a side note you can also derive vapor pressure, molecular packing and other molecular concepts from this as well (Note how all of these concepts fit nicely together into a single hypothesis, as is the goal of most of physics). I do admit that because none of this can be observed directly it still is only a hypothesis, a hypothesis with hundreds of years of experimental data supporting it, but a hypothesis none the less. (In the scientific method there is no “prove hypothesis” only draw conclusion and retest)

To directly answer the original question “why does sound propagate?” simply F=ma. Here F is intermolecular forces (per Lennard–Jones potential, which is material dependant), m is the mass of the molecules and a is the resulting acceleration. From this you can easily see how the speed of sound is independent of input force.

Now my thoughts on a few of the issues brought up throughout this thread:

As far as energy is concerned skyhawk is on the right path, energy is just something we define so that we can conveniently relate how different concepts interact. Note that different types of energy have been added as our understanding of different things have developed, but the idea correlates very well to observed data, and the concept of conservation of energy is yet to be proved invalid (it probably never will, because new types of “energy” can always be added).

As to the dependence of the speed of sound on temperature but not pressure, let’s look at F=ma. Pressure is defined as force per area, from this we can see that the F is only dependant on the pressure of the gas and the “collision” cross sectional area (how head on the molecules collide, which must average to zero over enough collisions otherwise there would be a net acceleration of the air). So increasing the temperature but leaving the pressure the same leaves F the same, but decreases the number of molecules for a given distance. Therefore each molecule is accelerated at the same rate, but fewer molecules are required to be accelerated for sound to travel a given distance. Conversely, increasing the pressure but leaving the temperature the same increases the F but it also increases the number of molecules in that given distance. Therefore, each molecule is accelerating faster, but the sound must accelerate more molecules to travel a given distance, resulting in little to no change in the observed speed of sound.

Can sound travel in a medium at absolute zero temperature, NO, but only because sound is molecular energy and by definition temperature is a measure of molecular energy in the system. Therefore the simple act of producing the sound (adding energy) means that the medium is no longer at absolute zero.

The MIT steel ball on a column of air does a very good job of demonstrating the “springiness” of air, nothing more. And yes, it is a steel ball. Don’t underestimate the influence of pressure. A few psi over a few square inches gets to be a very large force very quickly.

Newton’s Cradle and the pool ball analogy are both just spring mass systems as is everything else (See: bulk modulus, young’s modulus, etc. Even setting a feather on a steel table deforms the table somewhat.) In Newton’s Cradle the kinetic energy of the initial ball goes into a compression wave that travels through the center balls at the speed of sound in steel (~6000m/s), and this compression wave throws the last ball from its initial rest. Under a high speed camera and magnification I believe you would actually be able to observe this, but I was unable to find such a video.

The pool ball is a more complex system because it is both non-linear and discontinuous (F=0 until the balls touch then it increases rapidly until the balls begin to move apart), but it is still a spring mass system. It is true that this may be the best analogy for sound moving in a gas, however it is no where near a good representation of sound moving in a solid or liquid, and even in a gas there is molecular influences on other molecules when they are not “colliding.”

Standing sound waves are common (see wind instruments, breaking a glass with sound, etc.)

Crashsite,
First, I don’t see how you can dismiss wave analysis so easily. We know that the speaker/sound source of any kind imparts energy into the medium and this energy travels at mach 1 through the medium. Anywhere along the path of this energy we can observe the increase in energy then a return to (almost) ambient. This is, I believe, by definition a “wave”, an energy wave, but a wave none the less (This is independent of molecular movement assumed, if any at all).

Second, I don’t think you will convince anyone (at least not someone who has sufficiently been taught the wave/spring-mass model of sound propagation) by the methods you are attempting. First you must fully understand classical thinking before you can argue against it and second because you are going against classical thinking, it is on you to do one of two things 1) show how classical thinking fails to meet observed results or better yet 2) plainly state your own hypothesis and show how it better matches observed results. I have yet to see either of these conditions met, but if you do you will have a much easier time convincing people. However, I believe, I can disprove your hypothesis (“The thermal energy of the air is providing the power to propagate the sound") simply by looking at water. The speed of sound in water is ~1500m/s but the speed of sound in water vapor at 134ºC is only ~500m/s (The Nature of Sound), and I think you would agree that molecules in water vapor have much more thermal energy than those in water. If you limit your hypothesis to gasses, you are probably very close, but then your hypothesis varies from classical thinking by very little at that point as well.

Lastly, I’m having a hard time seeing how your vector biasing does not lead to molecular oscillation and on into the wave analysis. A velocity vector bias in one direction leads to bulk movement of air (and a local pressure increase, or else sound travels at infinite velocity), this movement of air forms a reduction in number of molecules (vacuum) in front of the speaker, and this vacuum “sucks” air back into this area, returning air to its original location. And as this bulk movement (local pressure increase and trailing vacuum) moves away from the source a wave is observed.

There are many issues here so I probably have missed several, but that’s enough rambling for now.

It is key to know that the moving energy band creates the pressure changes in the air.

Even at the face of the speaker cone the energy transfer happens first.

This allows us to first look at the energy movement and then compression.

A cold douche of reality...

You said a lot there. Maybe we can eat this pie a bite at a time (or, alternately, just go and buy an apple pie at the bakery and accept that it was made somehow).

ljw10 said:

I would like to try to give crashsite what I believe he is looking for and that is the conceptual view of sound propagation at the molecular level from the “classical” physics perspective (at least how I understand it). To do this I must start by examining intermolecular interactions, and this starts with the Lennard–Jones potential (a simple enough assumption that has been experimentally derived). Note that this is only looking at molecular energy (aka heat), a type of energy that skyhawk failed to mention in his write up on energy a few pages back (see below for a side bar about energy). If we fix a molecule in space and place a second molecule next to it at absolute zero the molecule is sitting at the bottom of the “trough” in the curve, and as energy is added to the molecule it moves back and forth along the curve to the level in which it has energy (think: a marble rolling up and down the curve). If it has energy greater than zero it is a “free” molecule (aka gas). .

Click to expand...

To someone trying to think of sound propagation in fairly mechanical terms (ie: molecules bumping each other along), this is a very esoteric and a step removed concept. I suspect that even my visualization of a marble in a bowl is probably already wrong as a mechanical view of what the molecule is doing (moving along some mathematically derived energy curve as opposed to physically moving in an arc'ed path)...or maybe it is a literal description???

Okay, I accept that there is some preliminary information that needs to be ploughed through to get to the actual impressing of the sound onto the molecules. Perhaps a preamble of why you are presenting a preliminary concept that explains at least why it fits into the big picture would help...or perhaps, confuse?

A valid question you may ask is, "if this has to go back to Physics 101, is it my place to try to teach this guy physics 101? Especially, if I need to do it with one hand tied behind my back of not being able to use math the way I learned it." You've had some 38 pages of my ramblings and logic (in this thread alone) to give you guidance.

I guess the upshot is that, it's likely unrealistic for me to go the classical physics/mathematicsl route. That leads to the question of, "is there even a way which the topic can be covered that imparts at least a useful, conceptual explanation of what's happening?". Actually, that I feel that I've learned quite a bit from going through this excersize tells me that there is. Of course, I may simply be deluding myself in my ignorance.

A sense of scale

3v0 said:

It is key to know that the moving energy band creates the pressure changes in the air.

Even at the face of the speaker cone the energy transfer happens first.

This allows us to first look at the energy movement and then compression.

Click to expand...

When you get past the cartoonishly distorted depictions of the longitudinal waves and frequencies and pressures at the macro level and apply the principles to tiny variations, I think we really are pretty much on the same page on this.