Why Does Sound Propagate?

crashsite said:

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.

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You post title "A sense of scale" indicates you may know what is going on here but posted nothing about it. That reinforces my thinking that you are not interested in a answer but in keeping this thread going forever.

If one were to examine a tiny cross section of air from the speaker outward and plot one dot for each molecule you would construct the scatter graph. The cross section for the plot show is small by choice. It enables us to see the difference in molecular density.

3v0

Yeah, it's dumbing down...

3v0 said:

You post title "A sense of scale" indicates you may know what is going on here but posted nothing about it. That reinforces my thinking that you are not interested in a answer but in keeping this thread going forever.

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There's an important piece missing from the typical, classic descriptions of sound propagation. It's the critical piece that ties heat into the picture.

One of the critical things that needed to be answered is why temperature is so important and how temperature change changes the speed of sound when all empirical logic says it should at least also be a pressure change.

3v0 said:

If one were to examine a tiny cross section of air from the speaker outward and plot one dot for each molecule you would construct the scatter graph. The cross section for the plot show is small by choice. It enables us to see the difference in molecular density.

Click to expand...

The scatter graph comes with two problems. The main one is that it comes with the random motion of the molecules subtracted out. Therefore, it gives the impression that the only motion involved is the compression and rarefication of the molecules. The second one is that the descrptions that come with the picture don't specify that you need to be down at the molecular level for it to make sense. In fact, they almost always present the scatter graph as part of the big picture of sound (and often reinforce the macro aspect with the Slinky toy).

I can't say that all of this thread could have been chopped by the introduction of the heat and random motion aspects of the molecules, with the sound energy being impressed on them; sort of like modulation on an RF carrier but, we could have gotten to this point a whole lot earlier if it had been.

On the other hand, if it had been presented that way, I wouldn't have had to do so much thinking about the implications of it and probably wouldn't have come up with some of the ancillary thoughts about it all that I have.

I'll reiterate. There are a lot of mis-conceptions and mis-information out there about sound propagation. I think this thread has helped to clear up at least a little bit of it. I know it has for me.

One thing I would like to see shake out of this is for the math geniuses (obviouly not me), to come up with some data, in forms that us math morons can make sense of it and use it. User, Skyhawk, for example, gave a tantalizing tidbit awhile back of a numeric number of air molecules in a defined space. That, coupled with things like molecular speed and interactions could make a chart or graph that puts some actual values on some of the conceptual things. In fact, there are a whole library that would be nice to have that somebody's already worked out the calculations for and present at the idiot's level that will allow people like me to "see" them..

crashsite said:

The scatter graph comes with two problems. The main one is that it comes with the random motion of the molecules subtracted out. Therefore, it gives the impression that the only motion involved is the compression and rarefication of the molecules.

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A scatter graph does not show the motion of molecules. ir shows the postion of molecules at a given time.

Crashsite said:

The scatter graph comes with two problems. The main one is that it comes with the random motion of the molecules subtracted out. Therefore, it gives the impression that the only motion involved is the compression and rarefication of the molecules. The second one is that the descrptions that come with the picture don't specify that you need to be down at the molecular level for it to make sense. In fact, they almost always present the scatter graph as part of the big picture of sound (and often reinforce the macro aspect with the Slinky toy).

Click to expand...

As j.friend indicated its a snapshot not a movie. The graph shows the period and the magnitude of the sound.

The compression and ramification is very important. Without it there would not be sound.

This is the big picture of sound. From a big picture view it makes sense. And we can only look under the hood to the molecular level once we understand the big picture. I illustrated this by using your heat idea to invoke anti-gravity.

We know that the energy compresses the air as it travels. Thus by looking at the scatter graph we can see how the sound energy is distributed.

Temperature is one of three properties illustrated by the Ideal Gas Law. The other two are pressure and volume.

When the band of energy arrives the air volume decreases, we can see that in the scatter graph. This decrease in volume will effect the heat and or the pressure. Unless our ears are infrared detectors there has to be an increase in pressure. I have yet to see anything that indicated how much heat increase there is.

As much as you would like to dismiss pressure it is the most important of the three. Without it sound would not be heard.

3v0

Random thoughts

j.friend said:

A scatter graph does not show the motion of molecules. ir shows the postion of molecules at a given time.

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If you take a true instantaneous snapshot of the molecules of a segment of air, whether some sound is passing through it or not, it will look like a homogeneous swath of gray. The random motion of the molecules will dominate and mask any of the sound variations that are there.

If the plot is not showing the molecules but, rather is showing a snapshot of air pressure, on a macro scale, is it showing anything useful regarding sound propagation?

Things just look different under a microscope

Emphasis mine:

3v0 said:

The compression and ramification is very important. Without it there would not be sound.

This is the big picture of sound. From a big picture view it makes sense. And we can only look under the hood to the molecular level once we understand the big picture. I illustrated this by using your heat idea to invoke anti-gravity.

We know that the energy compresses the air as it travels. Thus by looking at the scatter graph we can see how the sound energy is distributed.

Temperature is one of three properties illustrated by the Ideal Gas Law. The other two are pressure and volume.

When the band of energy arrives the air volume decreases, we can see that in the scatter graph. This decrease in volume will effect the heat and or the pressure. Unless our ears are infrared detectors there has to be an increase in pressure. I have yet to see anything that indicated how much heat increase there is.

As much as you would like to dismiss pressure it is the most important of the three. Without it sound would not be heard.

Click to expand...

I'm not trying to dismiss pressure. But, I'm trying to introduce the pressure in a manner that allows it to propagate at Mach 1.

To say that the speaker cone moves and that creates a band of pressure sounds good. To then say that the pressure pushes on other molecules and so the pressure extends out into the air space also sounds good. To say that you can go out some distance into the air space and see (directly measure) the pressure, sounds good. But, good as it sounds it doesn't address the topic of this thread. What is the mechanism for propelling the sound out into the air space at Mach 1.

For the scatter graph to make sense in the "big picture view" it needs to be put into context. Just sitting there, it could be a plot of air pressure change by mounting a speaker so it drives its energy into a box and you measure the pressure change in the box over time to make a plot. But, if you're measuring what's zipping past you at Mach 1, in the air, it's necessary to see the plot as some measure of distance. It's a snapshot of what the pressures are from a selected point A to point B in the air space. Of course, that still doesn't address the mechanism for how the pressures got there.

But, if you look at the problem not as an air pressure issue but, as what is happening at the molecular level, then you can get a sense of the mechanism for how you can get those pressures out where you expect them to be based on the speed of sound. But, examining it on the molecular level means that you need to think about what's happening differently, too. You need to ask just what represents that pressure on a molecule-by-molecule basis.

Making the examination at the molecular level doesn't mean that anything works any differently or that it negates measurements you might make at other scales. It's not a competition.

If looking at the big picture of sound answered the question of how it propagates, then this thread wouldn't even exist. But, it doesn't and thus the thread does exist.

I know you were being a bit facetious, but you brought up an interesting point. Are our ears infared detectors? Probably. Are the nerves in our bodies that tell us if something is hot or cold infared detectors? Sure, why not. Are our eyes electromagnetic wave detectors. Absolutely.

As regards yoiur earlier comment about keeping this thread going forever. Actually, I feel like the question has basically been answered. If it stopped right here, I would feel like it's done its job. But, there are a lot of loose ends and bits and pieces and ancillary items that are are fair game to discuss under the umbrella of this topic as well. When it will stop is something I suppose will happen naturally when enough people get bored with the thread and discontinue posting ideas and responses.

crashsite said:

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???

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I in no way intended to make it sound like the molecules where actually traveling a curved path. The vertical axis of the graph is potential energy. The marble rolling back and forth represents the transitioning from potential to kinetic energy and back (just as a marble does under the influence of a gravity potential inside a bowl). The actual force is radial (both attraction and repulsion, like a spring) and acts in all directions from the molecule (similar to gravity). At this point I would like to point out that I am using the Lennard–Jones potential as a simple molecular potential, and that the majority of molecular interactions will be more complex, including asymmetries and the possibility of multiple “troughs” (the majority of molecules have multiple atoms in them resulting in overlapping potentials that must be considered to get a true representation).

crashsite said:

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?

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I guess I’m not sure what you’re asking here, but will try and answer anyways. I’m introducing this principle to try and show you were the spring mass model comes from and how it shows what is happening (from the “classical” perspective). To fully understand sound propagation from this perspective and at this level you must start with simple harmonic motion, then build to multiple spring mass systems (this is where the majority of people stop), then into 2-d systems, then into 3-d systems, then on into nonlinear systems, and so on (again very complex concepts, that I probably don’t even fully understand) and this is just for a solid, liquids are even more complex and gasses more complex yet.

Elements

ljw10 said:

I’m introducing this principle to try and show you were the spring mass model comes from and how it shows what is happening (from the “classical” perspective). To fully understand sound propagation from this perspective and at this level you must start with simple harmonic motion, then build to multiple spring mass systems...

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Let me ask a dumb question. You said that to fully understand sound propagation you must start with simple harmonic motion and then build to multiple spring mass systems. But, if one jumps in with an assumption that the molecules are somehow colliding and knocking each other about in a Newtonian sort of way, is the spring mass model needed to visualize the action?

I don't object to getting some info about it but, since I've considered things further along, that I can mentally put it into the correct context.

I'm kind of getting the impression that the spring mass thing is more about how the molecules themselves interact as they collide, independent of anything else they might do or accomplish.

crashsite said:

If you take a true instantaneous snapshot of the molecules of a segment of air, whether some sound is passing through it or not, it will look like a homogeneous swath of gray. The random motion of the molecules will dominate and mask any of the sound variations that are there.

If the plot is not showing the molecules but, rather is showing a snapshot of air pressure, on a macro scale, is it showing anything useful regarding sound propagation?

Click to expand...

This is where the main difference lies in our way of thinking I do believe. If what we have agreed on to a certain degree, in particular that the propagation is due to the skewing of the direction of molecules, then there must be some change in density of the air.

In the case of air without sound travelling through it, the volume is a result of the repulsion between molecules due to their completely random and chaotic motion. As soon as you alter this, by skewing the direction of the motion of the molecules, you are decreasing the randomness of the system. This means that the molecules will have less collisions, on average in the direction perpendicular to the direction the sound is be propagated.

Looking at it with the idea that ljw10 introduced, the net energy of the air must be zero when taking into account all the collisions, or their will be movement of the general air mass. If you are skewing the motion of the molecules in one direction then there are more collisions that are going to happen in that direction. this means in other directions the number of collisions must decrease in order to maintain that overall zero energy of the air.

Assuming this to be correct, seeing as the actual velocity of the air molecules has not changed, the average force per unit area exerted by the molecules in the direction perpendicular to the direction the sound is being propagated at must be less. This means that molecules on average will be closer together, achieving the areas of higher density as shown on these scatter graphs.

If you want to look at it in another way yet again, think back to the idea that I posted a while ago:

You should be aware that the air is made up of molecules. Most of the characteristics we expect of air are a result of the fact that these particular molecules are very light and are in extremely rapid but disorganized motion. This motion spreads the molecules out evenly, so that any part of an enclosed space has just as many molecules as any other. If a little extra volume were to be suddenly added to the enclosed space (say by moving a piston into a box), the molecules nearest the new volume would move into the recently created void, and all the others would move a little farther apart to keep the distribution even.

Because the motion of the molecules is so disorganized, this filling of the void takes more time than you might think, and the redistribution of the rest of the air molecules in the room takes even longer. If the room were ten feet across, the whole process might take 1/100 of a second or so.

If the piston were to move out suddenly, the volume of the room would be reduced and the reverse process would take place, again taking a hundredth of a second until everything was settled down. No matter how far or how quickly the piston is moved, it always takes the same time for the molecules to even out.

In other words, the disturbance caused by the piston moves at a constant rate through the air. If you could make the disturbance visible somehow, you would see it spreading spherically from the piston, like an expanding balloon.

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If you look at this in terms of the scatter graph you would have to undoubtedly agree that the wavefront would have a different density different to the air ahead of it. With an actual sound source the density would also be different to the air behind it. This is what is happening in the propagation of sound.

I pointed out a few posts ago that a scatter graph for a small cross section of air from the speaker out would look much like the ones illustrated in books.

The energy travels through the air. The air is most compressed where energy is at its max.

Further more I maintain that it is possible to measure the air pressure differences as they go by. After all the microphone and ear are able to sense them.

3v0

Could be a revision in perception...

j.friend said:

This is where the main difference lies in our way of thinking I do believe. If what we have agreed on to a certain degree, in particular that the propagation is due to the skewing of the direction of molecules, then there must be some change in density of the air.

Click to expand...

We are agreed. But the implication of, "displacement" is that it's small compared to the random motion of the molecules due to heat (if it were large compared to the random motion, it would be considered that the random motion would be a displacment of the orderly motion of the sound).

As I mentioned to user, 3v0 in my last post, you don't change the physics by changing how you examine it. When thinking about this on the molecular level, can you think about that displacement as "pressure"? If you do, are you really just mentally transposing your thinking to the macro world while you think you're still in the molecular one?

j.friend said:

In the case of air without sound travelling through it, the volume is a result of the repulsion between molecules due to their completely random and chaotic motion. As soon as you alter this, by skewing the direction of the motion of the molecules, you are decreasing the randomness of the system. This means that the molecules will have less collisions, on average in the direction perpendicular to the direction the sound is be propagated.

Click to expand...

I hadn't considered the notion that, as you add the orderly motion of sound, that the system would have a 'decrease in its randomness'. I'm not sure what the implications of that are. I'll have to cogitate on that a bit.

j.friend said:

Looking at it with the idea that ljw10 introduced, the net energy of the air must be zero when taking into account all the collisions, or their will be movement of the general air mass. If you are skewing the motion of the molecules in one direction then there are more collisions that are going to happen in that direction. this means in other directions the number of collisions must decrease in order to maintain that overall zero energy of the air.

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I picked up on that, too.

One thing that comes up is the notion that the compression cycle of a sound wave propagates faster than the rarefaction cycle. In fact, for any direction of travel, the two must exactly cancel or there will be an overall movement of the air mass. I see it this way:

ooo ---> Direction of Propagation

If the dots represent a single molecule, the black dot is where the molecule would be due to random motion. The red dot, where it is under the influence of the disturber compressing and the blue dot, where it is during rarefaction. Just as the randoms must average over time, the reds and blues must also average over time for there to be no net movement of the air (over time).

But, this is making me reconsider how the air itself is acting as the sound is passing through. When envisioning every dot to be a red dot during the compression cycle of the disturber, that means the air is physically moving, stepping along faster than if it were in the random state with each molecular collision. When the disturber goes into the rarefaction cycle, all the dots are blues so they are stepping along slower.

I'm going to have to digest that for a bit before I comment on it further.

j.friend said:

Assuming this to be correct, seeing as the actual velocity of the air molecules has not changed, the average force per unit area exerted by the molecules in the direction perpendicular to the direction the sound is being propagated at must be less. This means that molecules on average will be closer together, achieving the areas of higher density as shown on these scatter graphs.

Click to expand...

This seems like exactly the sort of thing that needs to be thought of on the molecular level.

crashsite said:

We are agreed. But the implication of, "displacement" is that it's small compared to the random motion of the molecules due to heat (if it were large compared to the random motion, it would be considered that the random motion would be a displacment of the orderly motion of the sound).

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Whilst it may not be massive, the effect from each particle is compounded by the sheer number of air molecules. If each molecule is even only slightly skewed in one direction, the overall effect due to the number of the air molecules is comparitively large. if the effect were not large then our ears would have to be incredibly sensitive, moreso than they are to pick up on the miniscule variations of pressure.

crashsite said:

As I mentioned to user, 3v0 in my last post, you don't change the physics by changing how you examine it. When thinking about this on the molecular level, can you think about that displacement as "pressure"? If you do, are you really just mentally transposing your thinking to the macro world while you think you're still in the molecular one?

Click to expand...

I am not completely sure what you are referring to here. If it is the effect due to the skewing of the air molecules, i believe that it indeed can be transcribed into the macro world as pressure. I will reiterate that pressure in gases is dependent on the number and force of the collisions by air particles per unit area. If the displacement of molecules results in a greater number of collisions in one direction then it would effect an increase in pressure. This is how I see it at least.

crashsite said:

I hadn't considered the notion that, as you add the orderly motion of sound, that the system would have a 'decrease in its randomness'. I'm not sure what the implications of that are. I'll have to cogitate on that a bit.

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i believe it must. if there is a somewhat organisation tot he direction of the molecules there must be a subsequent decrease in the randomness in the system, otherwise sound would never propagated throughout a medium. However the randomness is restored as the number of collisions increases. Even if you introduce a level of order to the system the nature of the collisions and movement of the molecules would be such that as soon as driving force the affect would decrease.

this can be seen through volume at certain distances.

However this may also be seen as the skewing of direction being spread over a greater and greater area. (as the wavefront extends in a semi-circular nature).

crashsite said:

If the dots represent a single molecule, the black dot is where the molecule would be due to random motion. The red dot, where it is under the influence of the disturber compressing and the blue dot, where it is during rarefaction. Just as the randoms must average over time, the reds and blues must also average over time for there to be no net movement of the air (over time).

But, this is making me reconsider how the air itself is acting as the sound is passing through. When envisioning every dot to be a red dot during the compression cycle of the disturber, that means the air is physically moving, stepping along faster than if it were in the random state with each molecular collision. When the disturber goes into the rarefaction cycle, all the dots are blues so they are stepping along slower.

Click to expand...

This, I think, can be explained in terms as how the molecules are skewed. The way I have envisioned it is that the sound source does not add much if any velocity to the air but merely unifies the direction of them. This would mean that the actual velocity of the individual air particle does not change.

In the rarefaction cycle, the particles can on average just travel further in the opposite direction to the propagation of the sound before encountering a collision. This can be roughly also seen with the idea of the total energy being equal to 0. There are fewer collisions in the direction of the sound propagation therefore more in the perpendicular direction, meaning a greater force separating molecules. hence lower density.

The Macro View

3v0 said:

I pointed out a few posts ago that a scatter graph for a small cross section of air from the speaker out would look much like the ones illustrated in books.

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There's a big difference between "a small cross section of air" and a molecule-by-molecule basis. Even a cross section of a few microns is a macro view. The macro view looks across a lot of molecules and, depending on your means of measurement, can give a number of different pictures.

A molecule-by-molecule view pretty much forces you into focussing down to a single picture. That of how the molecules are displaced for different conditions.

3v0 said:

The energy travels through the air. The air is most compressed where energy is at its max.

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The concept of compressing the air or air pressure is a macro view. It's not invalid but, it doesn't get down to the root cause of what that pressure is.

3v0 said:

Further more I maintain that it is possible to measure the air pressure differences as they go by. After all the microphone and ear are able to sense them.

Click to expand...

All the points you are making require one thing. Time for whatever effect is happening to integrate; for the effect to build or abate over an appreciable amount of time compared to the time it takes for one air molecule to bump another. The ear drum and microphone diaphragm need a lot of time to respond. The scatter graph pictures encompass a lot of time.

When making the measurements you're talking about, how do you apply them to the way that sound propagates? I'm not saying that your measurements aren't accurate or valid...for what they show...I just don't think they apply to the process of how sound propagates.

But, I'm still open to a description of how something that can be measured by a microphone relates to how sound propagates.

Ham Strung

Let me zero in on this:

Quote:
Originally Posted by crashsite
As I mentioned to user, 3v0 in my last post, you don't change the physics by changing how you examine it. When thinking about this on the molecular level, can you think about that displacement as "pressure"? If you do, are you really just mentally transposing your thinking to the macro world while you think you're still in the molecular one?

j.friend said:

I am not completely sure what you are referring to here. If it is the effect due to the skewing of the air molecules, i believe that it indeed can be transcribed into the macro world as pressure. I will reiterate that pressure in gases is dependent on the number and force of the collisions by air particles per unit area. If the displacement of molecules results in a greater number of collisions in one direction then it would effect an increase in pressure. This is how I see it at least.

Click to expand...

Didn't we say the same thing? But, once you leave the molecular world to have your concept of "pressure", you can't really go back. So, what can you do besides bull ahead in the macro world and somehow hope that...well, I don't know what you can hope. What you can't hope for is to continue thinking about things on a molecule-by-molecule basis where you actually have a shot at describing the propagation of sound in at least a defensibly logical way.

One problem that I think plagues us is that we keep ourselves ham-strung trying to present just one facet at a time when what it really takes is to mentally keep several different factors simultaneously in mind. So, what happens is that at each step there's confusion about how that step is construed to fit into the big picture and lots of bickering about the ennui details. Maybe it's going to take a big effort to try to pull it all together.

You've tried to do that a few times so, you know that it's not an easy thing to do.

crashsite said:

Didn't we say the same thing? But, once you leave the molecular world to have your concept of "pressure", you can't really go back. So, what can you do besides bull ahead in the macro world and somehow hope that...well, I don't know what you can hope. What you can't hope for is to continue thinking about things on a molecule-by-molecule basis where you actually have a shot at describing the propagation of sound in at least a defensibly logical way.

Click to expand...

In a way, you can however look at pressure and say that we know what causes pressure in the micro world. So we can work backwards. We see that sound is essentially the change of pressure.

therefore we can work backwards from knowing that sound is changes in pressure and try to explain how the intermolecular interactions cause the change of pressure, which is indeed what i have been trying to do.

At this stage I would like to say that in the schooling system, especially in the sciences stream the concepts of basic things such as the propagation of sound is somewhat simplified in order for most people to be able to understand it. My teacher has said that once you get to second year chem at uni, they tell you that everything you learnt the previous year was wrong. They then proceed to tell you how it actually works.

back to the car analogy everyone knows that the engine works by the fuel burning. Not a great many understand that the fuel is undergoing harsh oxidation, and that the energy released and harnessed in this process is the result of the intermolecular bonds being broken. (this is the senior school understanding of whats happening, so probably something completely different when you get to 2nd year uni)

This can even e seen within your years at secondary school, with the understanding of even what atoms are composed of changing rather dramatically from electrons whizzing around in circles to electrons whizzing around anywhere within a certain area at a certain energy level.

crashsite said:

There's a big difference between "a small cross section of air" and a molecule-by-molecule basis.

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Not too long ago you were saying the scatter graph was cartoonish. Are you now saying the scatter graph is correct but does not apply ? If so you are wrong regarding both.

We have to look at and understand all we can at the macro level. When we start talking about the molecular level we must constrain the molecular model such that it generates the macro effects we have observed. The no pressure, no energy other then heat nonsense fails in that.

Perhaps you can pass off this brand of thinking in religious circles. When working in the scientific realm you need to have a firmer footing.

3v0

Taking the heat.

3v0 said:

Not too long ago you were saying the scatter graph was cartoonish. Are you now saying the scatter graph is correct but does not apply ? If so you are wrong regarding both.

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As a snapshot model of the air molecules, it's grotesque. As a representation of the pressures associated with the waveform, it may be accurate enough. And, if the intent is to show a macro view of the pressures associated with a waveform, it's good enough. If the topic is sound propagation, it's not directly showing anything useful. Inferences can be drawn from it but, I notice that you have been careful not to attempt to explain the mechanism of sound propagation using any part of the macro view (including the scatter graph). I would still be interested and am open to considering that appoach...with a good, cogent description.

3v0 said:

We have to look at and understand all we can at the macro level. When we start talking about the molecular level we must constrain the molecular model such that it generates the macro effects we have observed. The no pressure, no energy other then heat nonsense fails in that.

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Absolutely. The physics for the micro and macro level have to jibe. But, you also have to use the micro level to describe the things that require the micro level description (like sound propagation) and the macro level for things that work at that scale (like ear drums and microphones and speakers).

3v0 said:

Perhaps you can pass off this brand of thinking in religious circles. When working in the scientific realm you need to have a firmer footing.

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Air pressure is the result of integrating the molecular collisions (due to their movement) over time so it must be considered as part of the macro view. I don't see the voodoo or hocus poscus or religious experience in it.

"Heat" is tricky since it works in ways that are outside "normal" thinking. In a nutshell, "forces" (at least forces that are generated by molecular collisions, as has been discussed here) are related to the energy (movement) of the molecules and the energy of the molecules is determined by their heat. Air pressure is one of the forces that fits that catagory.

Again with the stupid labels.

3v0 said:

As a representation of the pressures associated with the waveform, it may be accurate enough.

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This is progress in that you now admit to the existence of pressure and waves associated with sound.

And, if the intent is to show a macro view of the pressures associated with a waveform, it's good enough.

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It does a good job of showing that.

If the topic is sound propagation, it's not directly showing anything useful.

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More correctly stated you can not see how it could be useful.

Inferences can be drawn from it but, I notice that you have been careful not to attempt to explain the mechanism of sound propagation using any part of the macro view (including the scatter graph).

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This is by design. I learned a few pages back that much of what I post is ignored.

To make any real progress it was necessary that you understand and accept the scatter graph as a representative distribution of molecules.


3v0

Scatter graphs

3v0 said:

To make any real progress it was necessary that you understand and accept the scatter graph as a representative distribution of molecules.

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Okay, I can see that we need to nail down just what the scatter graph is showing.

If you look at your slide (#2, What IS Sound), the scatter graph seems to be denoting pressure variations as corrolates to the associated waveform (in fact, it clearly says, "pressure"). So, only in the most indirect way does it represent a molecular distribution. Sort of like saying that a panoramic photograph of a beach represents the grains of sand.

On the macro scale, the scatter graph could represent a visualization of the pressure density by a simile of dot density.

But, when considered on the micro scale (and with the random motions of the molecules nulled out), it could be construed as being an instantaneous snapshot of the molecular distribution of a tiny segment of air.

To use the scatter graph, it's necessary for everyone to be using the same model. Is it showing the distribution of pressure or of molecules? You choose and we'll go along with your decision. If more than one type of scatter graph is to be used, you can specify the markers to look for to determine which one it is and, we'll go along with that, too.

What isn't working is nit picking about what scatter graphs show whe the subject is sound propagation.

The Outcasts

3v0 said:

This is by design. I learned a few pages back that much of what I post is ignored.

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Much of what we all post is ignored. Generally, what gets ignored is the parts that are agreed with but, a lot of it just gets lost along the way. If it's important enough, it comes up again later.

That's not a very good reason not to post your explanation of how the macro events of sound (air pressure, longitudinal waves, oscillation of the air, speakers and eardrrums, etc.) provide the mechanism for sound propagation. How do they work to propel the sound through the air at Mach 1?