Why Does Sound Propagate?

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Heat and Molecular Speed

A number of misconceptions and little time to deal with them.

The forces between molecules are not inverse square forces. That only applies to charged particles. The forces are a short range attractive force and a very short range repulsive force. This force is often modeled by a leonard-Jones 6-12 potential.

So, it would be fairly safe to generalize that when someone says that the molecules "collide" it's conceptually accurate? Much like thinking of pool balls knocking into each other when in fact they never do touch.

I have a problem with the "restoring force" thing as a way to explain the speed of sound. I can't make it jibe with the speed of sound remaining constant when there's a change in pressure (and, thus the molecules are physically closer or further apart).

I read an article that said that the attractive force is so weak, at the molecular level, that it can be ignored in most cases.

There is a wide distribution of molecular velocities for a given temperature. Take a look at the Maxwell-Boltman distribution.

There was a little animation posted by somebody and I can't seem to find it again. Anyway, the caption indicated that the simulation represented a gas under quite a bit of pressure and was slowed down like 2 trillion times. It was good to give some sense of scale (assuming it was indeed accurate) and there was definitely a distribution of speeds. But, that may have been an artifact of the animation since, in addition to knocking into each other, the molecules were also bouncing off the borders.

Empirically (there seems to be a lot of empirically sensible stuff on the subject of sound propagation that's just not true)...anyway, empirically, it would seem that a mass of some gas that is all exposed to the same heat source and has achieved at least near thermal equilibrium would have all the molecules moving at about the same speed. After all, when they collide, they don't exchange any energy, just direction so, I'm not sure why, if they are the same temperature, they aren't all at least very close to the same speed.

Now, something like air, that is a mix of gasses, will have heavier and lighter molecules impacting each other and so, I can see why there would be a significant variation in molecular speeds in air.

The speed of propagation is only independent of pressure for the range of pressures for which the ideal gas law is valid. I suspect (but haven't checked) that the ideal gas law is no longer valid for air at 3000 psi.

That was just a number I picked out of the air that's in common use and didn't sound crazy high like, 2 million psi. What's the pressure deep sea divers experience?

 

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I am going to cover just one point at a time.

Now, something like air, that is a mix of gasses, will have heavier and lighter molecules impacting each other and so, I can see why there would be a significant variation in molecular speeds in air.

Even in a gas with one molecule type there will be variations in speed.

Think about a starting point where all molecules have equal speed/energy but random directions of travel.

A glancing pass will transfer some energy. Depending on the geometry the degree of energy transfer will vary. There will be an enormous number of molecular encounters with an infinite number of possible geometries for the encounters. That is why the speed in not uniform.

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A glancing pass will transfer some energy. Depending on the geometry the degree of energy transfer will vary. There will be an enormous number of molecular encounters with an infinite number of possible geometries for the encounters. That is why the speed in not uniform.

If both molecules have the same amount of energy, from where and to where does the energy get transferred? Why don't they simply bounce off each other and each continue at the same speed but, in a different direction?

 

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If both molecules have the same amount of energy, from where and to where does the energy get transferred? Why don't they simply bounce off each other and each continue at the same speed but, in a different direction?

There are couple of things here.

A head on encounter of two molecules with identical speed would result in zero velocity. Need to look at Lennard?Jones potential - Wikipedia, the free encyclopedia to be sure this is true. Even if it is not the next this will cause unequal speeds.

If we give up the idea that all air molecules are alike (and they are not) encounters between unlike molecules will cause uneven speeds.

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Sticking Points

There are couple of things here.

A head on encounter of two molecules with identical speed would result in zero velocity.

I don't think you can just have the molecules collide and then have no velocity. I think this is where that restoring force that everyone keeps wanting to inject comes in. But, once it does the job of whatever it does when the molecules are in close proximity, I also think it pretty much falls out of the picture and the speed of the molecules takes over.

That is (and has been) pretty much the thrust of my theory about it.

If we give up the idea that all air molecules are alike (and they are not) encounters between unlike molecules will cause uneven speeds.

I have no quarrel with the notion that unlike molecules (as in air) lead to widely differeing speeds. I have more trouble with the concept that like molecules do, too.

The formula works out that the speed of the air molecules travel at about 1100 mph but, That must be an average but, I don't have any idea what the fastest or slowest speed is.

 

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Crazy Ideas

There is a wide distribution of molecular velocities for a given temperature. Take a look at the Maxwell-Boltzman distribution.

There must be a reason for it other than just saying that the Maxwell-Boltzman equations say so. It just seems like, in a homogeneous gas, that the velocities would tend to become more and more the same speed over time.

But, let me toss out a novel thought...just because. Usually, we (at least me) we think of gasses like nitrogen and oxygen as single molecules. But, if the show up as N2 or O2 molecules and depending on the relative orientations of such molecules as they collide, it does seem like there could be different levels of interaction and that could affect their speed.

I don't know if there's any validity to that line of reasoning but, just to clear the air and eliminate it, if nothing else, I submitted it.

 

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Adding Another Term to the Mix

I want to try to put this, "wave" business to bed once and for all.

I've made no secret of the fact that I do not like the whole notion of trying to explain sound propagation as some sort of "wave action" or "wave analysis' or "wave whateveryouwanttocallit".

But, I think a point of confustion comes in as to just what a "wave" is. Let me quote what the Wikipedia says about it as stated in their subsection, "Physics of Sound" under the "Sound" description article (empasis mine):

"Sound is transmitted through gases, plasma, and liquids as longitudinal waves, also called compression waves. Through solids, however, it can be transmitted as both longitudinal and transverse waves. Longitudinal sound waves are waves of alternating pressure deviations from the equilibrium pressure, causing local regions of compression and rarefaction, while transverse waves (in solids) are waves of alternating shear stress at right angle to the direction of propagation.

Matter in the medium is periodically displaced by a sound wave, and thus oscillates. The energy carried by the sound wave converts back and forth between the potential energy of the extra compression (in case of longitudinal waves) or lateral displacement strain (in case of transverse waves) of the matter and the kinetic energy of the oscillations of the medium."

I, myself, sort of got caught up in the "wave" thing when I opined that the sound waves are an artifact of the sound propagation process because there are compressions and rarefactions created out in space but, traveling at Mach 1 as a "traveling wave". What an absurd idea! Still, if you look up, "Sound" in the Wikipedia, the very first sentence is (emphasis mine):

"Sound is a travelling wave which is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations."

Even when we can (and we all have) observe what's really happening, on a macro scale that allows us to view it directly (with no magician's tricks or slight-of-hand), we refuse to acknowledge it. I've stated it and what's more, I wasn't the first in this thread to do so, that the model of what's happening lies in the executive string and ball toy.

With some reluctance, I'm going to throw another concept into this mix; discontinuity. I'm going to just make a following brief statement about it and then stop for now to give you a chance to incorporate it into your thinking.

RF guys will at least recognize the notion of discontinuity as being related to impedance mismatch. The notion that energy is coupled to the system and propagates through it unless and until it encounters a mismatch. That's what happens in the executive toy. The first ball couples energy to the system and that energy is propagated (by heat, as it turns out), through the intervening balls until it encounters a mismatch. The mismatch is the fact that the last ball doesn't couple its energy to another ball but, to the much lighter air.

Just as the intervening balls do not move, air molecules also do not move as sound propagates through them. The medium does not oscillate. What the air molecules do is continue to move, in a random manner, under the influence of heat but, with some directional bias imposed by the original sound source. It's only when the sound encounters a mismatch (such as a transition between air and an eardrum) that there is some oscillation taking place.

When you think in these terms it all makes a lot more sense.

 

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Question 1

If the air molecules do not move how can there be alternating bands of compressed and rarefied air ?

Question 2

If the answer lies in the executive string and ball toy you have equated molecular motion to the Newtonian physics where there is no restoring force. Have you discounted Lennard Jones potential ?

If you got this route you have to allow that two like molecules of like speed but inverse directions will collide and stop. But that is not the point so do not dwell on it.


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Basics

Question 1

If the air molecules do not move how can there be alternating bands of compressed and rarefied air ?

Question 2

If the answer lies in the executive string and ball toy you have equated molecular motion to the Newtonian physics where there is no restoring force. Have you discounted Lennard Jones potential ?

If you got this route you have to allow that two like molecules of like speed but inverse directions will collide and stop. But that is not the point so do not dwell on it.

Answer #1.

There are no bands of compressed or rarefied air. The molecules are moving vigorously but, they are in no way "oscillating" in any manner that's associated with any sounds.

You need to reject what the Wiki guys said and then "unlearn" what you learned in school science and physics about sound. Then, you can start anew. I am more and more convinced that schools teach the physics of sound wrong.

Answer #2.

I think you are correct when you say that this is not the point. Whatever the mechanism is by which the molecules carom off each other to set up the random motion is moot. It's also moot what the individual speeds of the molecules is or the distruibution of the speeds. What is important is that the molecules normally are random, that they have an average known speed and that energy from some disturber can impress a directional bias on that random motion.

Are you familiar with the general use and operation of of the TDR (Time Domain Reflectometer) test equipment that's commonly used for isolating faults in cabling and/or the digital MUX and DEMUX (multiplexer and demultiplexer) equipment commonly used in telecommunications systems? It's not critical but, I think they utilize concepts of operation that are useful in also thinking about this subject.

 

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Getting to the TRUE Basics

Let me interject something here. There are reasons why it's so hard to get a conceptual view of sound propagation. Of course, the first is the built-in bias from learning it wrong in the first place that needs to be overcome.

But, it's also complicated by the fact that you need to simultaneously consider several factors and then be able to meld them into a cohesive conceptual model.

• You need to think about what the molecules are doing based on their properties and the amount of heat they are receiving.
  
  
• Simultaneously, you need to consider the speeds of the molecules and the range of speeds (and realize that whatever the speed range, the important aspect is the average speed).
  
  
• Simultaneously, you need to consider how the molecules move randomly in the absence of a disturbing force (still air)
  
  
• Simultaneously, you need to consider how the action of a disturber couples to the molecules.
  
  
• Simultaneously, you need to consider how the disturbing force impresses itself onto the molecular motions while simultaneously considering that the effect is always propagating away from the point of disturbance at an average speed of, Mach 1.
  
  
• Simultaneously, you need to take into account the vector angles that the molecules are traveling relative to a direction of sound propagation while Simultaneously, considering how those vector values average to the speed of sound.
  
  
• Simultaneously, you need to consider what happens when an impedance mismatch is introduced into the path of the propagated sound.
  
  
• Simultaneously, you need to consider how changes of temperature and gas composition affect the speed of the molecules and thus the speed of sound.

The first set of "ground rules" I presented were more general in nature. These are more specific to the subject of sound propagation. There are refinements to the list of things that need to be considered but, I think these are the basic ones (keeping in mind that it's almost certain that other important ones have been missed).

 

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Answer #1.

There are no bands of compressed or rarefied air. The molecules are moving vigorously but, they are in no way "oscillating" in any manner that's associated with any sounds.

I only have time for a quick one.

If the air is neither compresses or rarefied how does a sound receptor like a eardrum or microphone detect sound ?

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Still Sparring...but, I think there's progress...

If air is neither compresses or rarefied how does a sound receptor like a eardrum or microphone detect sound ?

Here's where you need to get into the hair splitting. Do you recall my "Bill Clinton" analogy of what something means? Here, there's a question of what does, "wave" and "oscillate" mean. There's obviously something that's propagating at 768 mph.

The Wiki guy (and the physics world, as a whole) says it's a longitudinal wave and sometimes a traverse wave (in some solids) and that the medium oscillates. He even goes so far as to say that it's the interchange of potential and kinetic energy...blatant wave-like action.

Imagine a disturber, moving at subsonic speed (if it's supersonic, a whole different scenario exists), coupling some energy to an air molecule, impressing a directional bias on its otherwise random motion. Then, if that molecule bumps into a neighbor and the result is that the both molecules move off in different directions but, with both carrying the directional bias of the disturber, has a "wave" been set up or established?

It certainly hasn't in the sense that there's an oscillation. Just that there is a directional bias imparted to the molecules.

You're resisting this and, you're in good company. The Wiki guy would also resist this if he reads it and all the engineers and physicists, who were classically trained and have that strong "wave bias" also resist it.

But, you have to give up the wave thinking and concentrate on the molecule to molecule thinking. Think about how long it takes for one molecule to bump into the next one and how that relates to the speed of the molecules involved. Trying to think about how it all leads to different kinds of waves and oscillations is...well...just plain silly.

With each molecular collision, the molecules keep moving off in resulting different directions and carrying that bias of the disturber with them. For reasons that are a whole another discussion within the scope of the topic, the bias always moves away from the disturber (due to the speeds of the molecules, which in turn is determined by heat).

If you want to cosider the act of molecules bumping and zinging off in random directions, carrying a bias with them, as a "wave action" then, I guess you are free to do so. But, in my opinion (humble though it may be), I think it's the sort of thinking that will get you into trouble and inhibit you from ever figuring out how it really works.

I said that understanding the concept of sound propagation requires you to simultaneously keep many phenomena in mind and this is just a little sampling of why that kind of mental multi-tasking is needed.

You ask how the ear or a microphone is able to "pick up" or "detect" or "sympathetically move" or "generate a response to" or "etc. etc. etc." the "non compressed or rarefied" air molecules". I can't give you a simple or pat answer. I can only try to carefully and logically set up the different aspects of why and then try to tie them together to give the most logically derived and hopefully, most correct answer.

Right now, we're still sparring about the details of the pieces and you are resisting because, I believe, you still want to keep the classic descriptions about sound ad sound propagation you learned in schoool.

If you think about the ear drum or the microphone diaphragm physically vibrating in sympathy to the original disturber the way the end balls on the executive toy move and can separate that from the transmission of the disturbance through the intervening balls...or air molecules...then, you'll at least be on the right track of thinking about sound propagation.

 

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There's obviously something that's propagating at 768 mph.

We know it is not the air but rather the energy provided by the sound source. The sound energy moves through the air, the medium through which it travels.

Can we agree that a working definitions as the words are used in physics.

oscillate

To vary between alternate extremes, usually within a definable period of time.

wave is a little more complex

a. A disturbance traveling through a medium by which energy is transferred from one particle of the medium to another without causing any permanent displacement of the medium itself.
b. A graphic representation of the variation of such a disturbance with time.
c. A single cycle of such a disturbance.

Imagine a disturber, moving at subsonic speed (if it's supersonic, a whole different scenario exists), coupling some energy to an air molecule, impressing a directional bias on its otherwise random motion. Then, if that molecule bumps into a neighbor and the result is that the both molecules move off in different directions but, with both carrying the directional bias of the disturber, has a "wave" been set up or established?

It certainly hasn't in the sense that there's an oscillation. Just that there is a directional bias imparted to the molecules.

To look at a single disturbance takes it out of context and hides the fact that it is part of something larger. Looking at a single disturbance rather then a system of them is what has confused the issue.

The single disturbance (pulse) does not setup an oscillation. But it the disturber is vibrating (like a tuning fork) it will set up a series of disturbances (pulses) according to the frequency at which it oscillates.

If you want look at the movement of sound you it is better to look at a train of pulses. Given a pulse train we will see oscillation. While it is valid to study a single pulse you can not expect it to cause the same result as a pulse train.

But, you have to give up the wave thinking and concentrate on the molecule to molecule thinking

I again submit that the wave motion is a result of molecular movement.

Trying to think about how it all leads to different kinds of waves and oscillations is...well...just plain silly.

Why is it silly ? If we hook a microphone to a scope we can clearly see waves and oscillations. This indicates they must exist in some form.

If you think about the ear drum or the microphone diaphragm physically vibrating in sympathy to the original disturber the way the end balls on the executive toy move and can separate that from the transmission of the disturbance through the intervening balls...or air molecules...then, you'll at least be on the right track of thinking about sound propagation.

The executive toy idea needs to be trashed. It does not apply for the reasons previous laid out in an earlier post. You have to provide some space between the balls. Each travels a distance then encounters the next ball. The balls should not be hitting each other.

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Still Sparring

We know it is not the air but rather the energy provided by the sound source. The sound energy moves through the air, the medium through which it travels.

Can we agree that a working definitions as the words are used in physics.

Okay, we are definitely in big trouble, on a conceptual level, right from the very first sentence. I can see what you are saying and you are sort of coming at it from a slightly different perspective. I can't catagorically say that your approach is the wrong one but, I can see it leading to a "wrong conclusion".

That makes it really difficult to argue or defend my position; because I have to so carefully thread my way through the "right" and "wrong" bits and pieces of yours. Let me concentrate only on your first two concepts here and try to build out later.

If I'm reading your first two sentences correctly, you are saying that it's not the heat energy in the air that's propagating the sound disturbance but rather, that it's the energy of the sound source, itself that's moving through the air that we need to concentrate our thoughts on when thinking about how sound propagates.

On that topic, let me stop right there. Let's get that cleared up before we try to go further along that path.

oscillate wave is a little more complex To look at a single disturbance takes it out of context and hides the fact that it is part of something larger. Looking at a single disturbance rather then a system of them is what has confused the issue.

The single disturbance (pulse) does not setup an oscillation. But it the disturber is vibrating (like a tuning fork) it will set up a series of disturbances (pulses) according to the frequency at which it oscillates.

If you want look at the movement of sound you it is better to look at a train of pulses. Given a pulse train we will see oscillation. While it is valid to study a single pulse you can not expect it to cause the same result as a pulse train.

I can see how you are deriving your conclusion, I really can. And, because it says things that are not untrue, that this is going to be a tough nut to crack.

Again, we are in disagreement right from the first sentence. I think that's a good start that there is such a clear division of our thinking on such a basic level. It gives a good opportunity to try to sort it all out on this elementary level and then (hopefully) allow us to build out from there in agreement.

Again, let me paraphrase that you are saying that, to understand sound propagation, you need to consider how the sound acts over time (considering its oscillatory nature) rather than how it acts (in a near-instantaneous manner) on a single molecule (or over only a small number of molecular interactions).

Did I characterize that correctly?

I again submit that the wave motion is a result of molecular movement.

A book could be written on that sentence. But, we don't really have the luxury of writing it as a book so, we need to figure out how to boil it down to a few basic concepts and then apply those concepts to sound propagation. And, perhaps just as importantly, to disassociate those concepts from things that, on the surface may appear to be important to the subject of sound propagation but, really aren't.

Ah, I almost forgot. The answer to the question of whether we can accept the, "working definitions as the words are used in physics" is a resounding, NO.

 

[3v0]

Again, let me paraphrase that you are saying that, to understand sound propagation, you need to consider how the sound acts over time (considering its oscillatory nature) rather than how it acts (in a near-instantaneous manner) on a single molecule (or over only a small number of molecular interactions).

Did I characterize that correctly?

Yes

Quote:
Originally Posted by 3v0
I again submit that the wave motion is a result of molecular movement.

A book could be written on that sentence. But, we don't really have the luxury of writing it as a book so, we need to figure out how to boil it down to a few basic concepts and then apply those concepts to sound propagation. And, perhaps just as importantly, to disassociate those concepts from things that, on the surface may appear to be important to the subject of sound propagation but, really aren't.

We disagree on what is important. If you want to write it off as unimportant you need justification.

Ah, I almost forgot. The answer to the question of whether we can accept the, "working definitions as the words are used in physics" is a resounding, NO.

You have not responded to my statement

The executive toy idea needs to be trashed.

Does this mean you agree or are still thinking about it?

Without a working definition of oscillate and wave we can not proceed.

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Defining Moments

We disagree on what is important. If you want to write it off as unimportant you need justification.

absolutely.

You have not responded to my statement (regarding trashing the executive toy)

Does this mean you agree or are still thinking about it?

I don't believe the toy will work unless all the balls are in contact with each other when the toy is at rest. I need to verify that it is constructed as such before I answer but, I'm very sure that's the configuration of it.

Without a working definition of oscillate and wave we can not proceed.

Fair enough.

Oscillation

I would define an oscillation as something that is freely moving back and forth by the interchange of potential and kinetic energy (by deflection and restoring force). Of course, that is a "pure" definition.

If we think of something like a relaxation oscillator (sawtooth) or a multivibrator (square wave), we call them oscillators and the waveform, "oscillations". But, are they really oscillations?

One could argue that it's oscillation because a stress is being built up on a capacitor and then relaxed by periodically discharging the capacitor. That type of oscillator also produces a pulsed output if you are looking at the discharge cycle of the capacitor. The multivibrator is really only a specialized version of the relaxation oscillator.

But, let's say we set an item, such as a hammer, on a table top and then, either at random times or on some schedule, grab it and move it from one side of the table to the other. Is the hammer oscillating or is it merely being set on one side of the table or the other?

What if the hammer is being moved around on the table top in a random manner but with a bias that tends to gradually move it further and further a specific direction with each move? Is there still any way that you could consider it to be oscillating?

Does the definition change if the hammer is replaced with a pool ball? What about if additional pool balls are placed on the table and all are set in motion, with some striking the ball in question in some manner? And, so on.

We encounter a similar challenges in the world of sound. When is something oscillating? When is it merely moving around? When is it mostly just moving around but, with a directional bias? When do you have to think about what's happening as a mass and when do you need to think about it on a unit-by-unit basis?

To think of something like a mass of air oscillating, you need to think of the mass of air actually moving or undulating in some manner. Sometimes, such as when you drop a rock in a pool of water, you can actually, physically see the undulations and, by floating a cork in the water, you can even see that the apparent traveling of the expanding waves is really just an illusion.

A major point of disagreem between us is that I believe that sound propagation can only be understood on a molecule-by-molecule basis and you are convinced that it can only be understood by analyzing ...well...at this point I can only say, not on a molecule-by-molecule basis because I'm not exactly sure of your difinitions of oscillation.

Wave

I would define a "wave" as a periodic displacement from some point of equilibrium in a medium. A good example is the water wave that is set up from dropping in a rock.

But, does a wave need to be periodic? The leading edge of the wing of a supersonic aircraft is creating an impulse of pressure. It's called a, "shock wave" but, it certainly has no periodicity to it. It's just there.

Calling the shock wave a shock wave, in my opinion is part of the problem of why people think about sound propagation all wrong and I wish somebody had come up with a more unique term for it.

When someone uses the term, "longitudinal wave" they are most certainly speaking of a periodic wave. When they then clarify it by saying that it's the interchange of potential and kinetic energy there's no getting around that they are speaking of a periodic wave. In that context, I also think about it as a wave.

But, I don't think about impressing a directional bias on a random event as being a wave action.

Conclusion

Do these difinitions and thoughts about, oscillations and waves supply sufficient information that we can agree to disagree about them with a solid enough foundation to respectively argue our points about sound propagation?

 

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I don't believe the toy will work unless all the balls are in contact with each other when the toy is at rest. I need to verify that it is constructed as such before I answer but, I'm very sure that's the configuration of it.

The toy does have the balls touching at rest which is not a useful model of gas molecules. To make it a better but still not great model you need to move them apart a bit.

oscillate Quote:
To vary between alternate extremes, usually within a definable period of time.

Notice that this is a description of change. It does not even try to say what moves or why. It is a description of motion like UP DOWN AHEAD BACK.

Oscillators are devices that produce oscillation in a medium. A tuning fork is a relevant example. The medium be it mechanical, electromagnetic, or spiritual... does not matter. If it causes a change that is oscillatory we have oscillations.

While it is possible to use a restoring force it is not required. We can create oscillating motion by other means.

But, does a wave need to be periodic? The leading edge of the wing of a supersonic aircraft is creating an impulse of pressure. It's called a, "shock wave" but, it certainly has no periodicity to it. It's just there.

Recall the following

wave is a little more complex Quote:
a. A disturbance traveling through a medium by which energy is transferred from one particle of the medium to another without causing any permanent displacement of the medium itself.
b. A graphic representation of the variation of such a disturbance with time.
c. A single cycle of such a disturbance.

No where does it say that a wave needs to be a series of oscillations.

But, I don't think about impressing a directional bias on a random event as being a wave action.

You impress a bias not upon a random event but on a multitude of random events. A speaker cone displaces a huge number of air molecules. The ones next to the cone physically move the displacement of the cone. So much for molecules not moving.

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The Toy

The toy does have the balls touching at rest which is not a useful model of gas molecules. To make it a better but still not great model you need to move them apart a bit.

This post will concentrate solely on the executive toy.

The molecules in the steel of the balls are not touching nor are the molecules at the interface between the balls.

As the collision effect propagates through the balls, I'm sure there's some discontinuity seen at the interface between each ball. But, it's minimized by the (almost) physical contact. In any case, the discontinuity is not enough to keep the toy from working well.

If you put even a small gap between the balls, each time the collision effect gets to the edge of a ball, the ball will see the discontinuity and will move (just as the end ball normally does). That eats up energy and so the end ball will only move as far as the first ball, minus the friction losses, minus the energy lost moving each ball. If the sum of the gaps is equal to half the distance that the first ball travels, the end ball should move somewhat less than half the distance of the first ball.

If there's a gap in the balls, not only will they move as the collision effect passes through, but they will also rebound (the pendulum restoring effect) and will swing back and collide with the ball toward the source. That sets up a reflection.

It gets even more exciting (and graphic) when you consider the case of a transmission line between a radio transmitter and the antenna. An impedance discontinuity (mismatch) acts just like the balls to cause energy to be reflected back toward the transmitter output circuit. Energy, thus "lost" does two bad things. First, it doesn't get radiated and second, it gets dissipated in the transmitter output (which is usually already dealing with power dissipation issues already).

In the case of the toy, working as it is properly designed to do, almost all the energy of the first ball reaches the end ball which swings outward. Then what does the toy do?

The toy is an excellent example of what's happening in the toy itself, in a transmission line and in the air (or other medium) through which sound passes.

 

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Molecular Motion and Oscillation

This post will concentrate solely on the "motion" and oscillation.

Quote:
Originally Posted by 3v0
oscillate Quote:
To vary between alternate extremes, usually within a definable period of time.

Notice that this is a description of change. It does not even try to say what moves or why. It is a description of motion like UP DOWN AHEAD BACK.

Oscillators are devices that produce oscillation in a medium. A tuning fork is a relevant example. The medium be it mechanical, electromagnetic, or spiritual... does not matter. If it causes a change that is oscillatory we have oscillations.

While it is possible to use a restoring force it is not required. We can create oscillating motion by other means.

You are hell-bent on inserting waves into the issue of sound propagation. Changing the wording doesn't change the intent.

If you want to include any change of any kind in your definition of what contitutes a "wave" then, so be it. Sound propagation is a wave phenomena. But, what I've really just done is add another layer of cow dung that has to be shoveled through to get back to just what kind of waves and what the characteristics of the waves and what the nature of the waves is before we can get back to talking about sound propagation again.

There's plenty of wave phenomena that needs to be sorted out without looking for more. For example, there's the oscillatory nature of an air disturber such as a speaker cone. As you correctly pointed out, there is some physical air movement at the interface of the cone. There are wave effects, in addition to sound propagation effects in disturbing air.

Consider this: Cool dude casually leans back and blows a perect smoke ring from his expensive, imported cigarette. Pretty girls gasps in delight and amazement. Cool dude gets lucky.

That smoke ring is a graphic example of how a volume of air, with a specific shape can waft through anoother volume of air. There must be a lot of pretty complex physics going on there but, it's outside the scope of the discussion of sound propagation to consider here. Likewise, there's a lot to be said and learned about waves. It's just not a good fit when discussing sound propagation...and it kind of disheartens me when everyone tries so hard to pound that sqare peg into that obviously round hole.

Eventually, I believe that you will conclude (as I have) that sound propagation is a) the result of molecule to molecule collisions, b) that there is a direct relationship between the speed of the molecules and the speed of sound, c) that sound is propagated by heat, d) that sound propagation must be considered on an instant-by-instant basis (down to the time it takes for one molecule to collide with another), e) there's only molecular motion as random motion with a bias and NO oscillatory aspect to it.

You can think of constructing waves fropm the aggregation of "pulses" or "time segments" or "fraculations of the nebular continuum" and, I'm not saying that it's invalid to do so. It's just that it's an exercise that has nothing to do with sound propagation.

 

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This must be disposed of first.

This post will concentrate solely on the executive toy.

The molecules in the steel of the balls are not touching nor are the molecules at the interface between the balls.

This is the problem with your reasoning. The steel balls in the model represent molecules and they are touching by virtue of the scale. Even if you move then a wee bit apart they are still many magnitudes closer then molecules in air.

You can not use this flawed model. If you wanted to model air molecules with these balls you need to place them quite far apart. Even then you still have the problem with the wrong kind of forces.

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