Why Does Sound Propagate?

[3v0]

Why can't we compare electromagnetic theory of propogation with this? won't this provide an answer?

As long as Crashsite thinks monkey is a lizard there is no point in theorizing how sound propagates.

Read the previous 500 posts and you will understand why.

3v0

 

[crashsite]

The Freeway

What physical phenomenon is being propagated and what physical property is sensed or measured either by an ear, or microphone or other instrument?

Okay! I consider this progress. What indeed is the physical phenomenon being propagated and what physical property is being sensed or measured? It's also important to ask if they are the same thing. I would argue that they absoltely are not.

If we look at the Los Angeles freeway system, we can come up with traffic patterns. Rush hours, midday, evening, weekend, etc. But, whatever we come up with is not real. It's merely a statistical representation of what's really happening. What's really happening is that there are cars and trucks and motorcycles out there on the blacktop/concrete occupying some space on an instant-by-instant basis. If we aggregate and integrate that instant-by-instant data, we can come up with traffic patterns.

In the case of sound, you said that all the interesting stuff was done 150 years ago. I assume you meant the experimental measurements and deriving the formulas and equations and constants to be able to write it into the physics books for people to learn and use. I have no desire to go back and repeat any or all of that work. I accept that the experiments and mathematics does what it says it does.

But, the reason it works is because of the instant-by-instant actions and reactions and positioning of the molecules. To understand how it works requires that you go examine what's happening down there. That's why I resist getting caught up in the macro view. I accept that, if you put a microphone out you can integrate the effects of zillions of molecules into an electrical signal and with the ear, hear a symphony. But, are you measuring what's really happening or are you just integrating a lot of cars into some sort of traffic pattern?

I ask how the macro view explains how the sound at the speaker zips away at Mach 1 and get no takers. That tells me something. I ask how looking at the instant-by-instant molecular view does it and I see a definite mechanism for explaining it. For some reason I can't quite fathom, I seem to be a minority of one on this viewpoint...

I'm especially surprised that you are not championing the molecular view since you seem to be a proponent of the control volume. Now, as I understand it, you think of your control volume, not as a different way to view a phenomenon but, rather as a way to see the same thing as the big view but just in a smaller, more easily definable space. But, if the control volume is tiny enough........

 

[crashsite]

One small step for a molecule

It is impossible to determine a bias following the motion of a single molecule. There must be a statistically large enough sample in order to have a bias, i.e. mass velocity. A spacially varying mass velocity must lead to changes in density.

I'm not sure that's true. If you're walking through the snow at a steady stride, you leave a regular pattern of footprints. If one step is a millimeter longer or shorter, you've introduced a bias. If you walk another 20 miles, continuing at your steady stride, you still retain that 1 mm bias.

In sound, given a piston of small enough area and a pulse of sufficiently short duration, you (at least theoretically) could affect the motion of one molecule. That molecule would propagate exactly the same as a zillion of them, carrying that bias from molecular collision to collision as the pulse propagates.

 

[crashsite]

Math!

Beats exist because of superposition, a property of linear systems.

Superposition principle - Wikipedia, the free encyclopedia

A common measure for audio amplifier linearity is to input two different frequencies (typically 300 Hz and 3 kHz) and measure the amount of energy coming out at 2.7 and 3.3 kHz. That's usually published as THD. In a perfectly linear amplifier, the energy is zero. Both frequencies are simultaneously present but, there's no beat note.

But, of course, here comes the "snarky" comment about what happens after the mathematical modeling comes into the picture....

We come to the notion of, "isentropic" and, once again we have a situation where multiple sounds and sound sources can exist in the same space, seemingly not interacting...while apparently having them interacting becuase it's a "linear system".

 

[crashsite]

Trying to pre=emptively eliminate a nag

There's something I want to try to get out of the way because it may very well rear its ugly head later, otherwise.

There were a couple of effects I wasnted to null out as not being important enough to consider for sound propagation. One was the adibiatic heating and cooling of the air due to compression and rarification and the other was gravitational effects.

I'd like to try to add another. If one puts a resistor in a volume of air and passes a current through it, the resistor will dissipate some power and, will impart some of that heat to the air. It will do so two ways. Air in direct contact with the resistor will heat by conduction. Air that's more remode will be heated by thermal radiation.

A speaker or other disturber will also add some energy to the air but, generally it will be small. I propose to ignore it for all except the most powerful sounds. the conduction of heat will be slow compared to the speed sound propagates and the radient heat will be slight and most wont be absorbed by the air near the speaker, anyway.

 

[crashsite]

Missing Link

As long as Crashsite thinks monkey is a lizard there is no point in theorizing how sound propagates.

You sound like a guy who never had a pet monkey lizard as a kid.

I was thinking about thinking about sound on the macro level and I can see how easily it can interfere with the kind of thinking that needs to be done to try to get a grip on the basic mechanism of it.

I think there's a point at which the macro view descriptions and definitions start getting in the way of understnding the basic mechanism of it and then it's time to go to the molecules.

There has been an elusive piece that I think I have figured out. But, it's pretty conceptually involved so I want to work on making it as clear as I can before I post it. It has to do with the way the molecules themselves use their heat energy to propel the sound.

 

[crashsite]

Vapor Pressure

I want to try to eliminate yet another possible snag from the thread. It has to do with the air molecules at the interface of something like the surface of a speaker cone.

Anyone familiar with high vacuum practice is aware of, "vapor pressure". A simplified description is that air pressure presses a thin layer of air molecules to the surface of something. The usual problem with that is that it interferes with the deposition of materials onto a surface. In our case, there's no direct implication except to say that the randomly moving air molecules never actually impinge on the surface but rather against the layer of molecules adhereing to the surface.

I propose that we ignore that layer of molecules and assume, for descriptive purposes, that the air molecules are directly contacting the surface of something like a speaker cone.

Just trying to pick a nit before it becomes an arguing point later...

 

[Boron]

Sound Propagation Simulation

Hi all,

I see this is a rather heated topic so I thought I might share a simulation demonstrating the propagation of sound on the molecular level.

I think its pretty cool because you can see how compression of the molecules causes longitudinal waves of pressure while the molecules themselves remain roughly in the same region of space, which is the sound we hear.

Hope it helps.

PhET Wave Interference - Electric field, Interference, Diffraction, Double Slit
- run the simulation
- click the sound tab at the top left
- click particles on the right

 

[3v0]

I was thinking about thinking about sound on the macro level and I can see how easily it can interfere with the kind of thinking that needs to be done to try to get a grip on the basic mechanism of it.

Reality has a way of biting one in the rear. But if you ignore it they foreclose on your house and the wife leaves with the kids. Not always a bad thing but then you get to pay alimony and child support for kids you do not get to see.

I have been asking for a single restriction on the thinking on his issue. When we are done, the theory we suggest must results in sound as understood at the macro level. To do that we must understand what that is.

3v0

 

[ljw10]

A speaker or other disturber will also add some energy to the air but, generally it will be small. I propose to ignore it for all except the most powerful sounds. the conduction of heat will be slow compared to the speed sound propagates and the radient heat will be slight and most wont be absorbed by the air near the speaker, anyway.

I think you misunderstand thermal energy (I believe this is what you mean when you say “heat”, if not the speaker may be cooled so that it imparts zero or even negative heat into the air, and you may ignore the rest of this post). I said it in my original post and I will say it again: Sound is thermal energy (molecular energy relative to each other), therefore you cannot ignore the energy imparted by the speaker (otherwise there would be no sound). The problem most people have with this is that thermal energy is typically thought of as random, while sound is thought of as very directional. But if we look at sound long enough (through enough reflections/echoes or over enough distance) it appears to be random (theoretically the sound is still there our ears/microphone just aren’t capable of distinguishing it from any other molecular movement). Two molecules at the bottom of the Lennard–Jones potential relative to each other have zero thermal energy, even if together they are traveling hundreds of miles per hour. Temperature is just a measure of the average molecular energy (thermal energy) in a given system (and this includes sound). Conduction and radiant heat transfer require a temperature gradient and will always move thermal energy from hot to cold. Neither of these is required of sound, however sound will always increase the temperature of the medium it is in (note the need to differentiate between thermal energy, heat, and temperature).

 

[crashsite]

Focus

I have been asking for a single restriction on the thinking on his issue. When we are done, the theory we suggest must results in sound as understood at the macro level. To do that we must understand what that is.

There are a lot of things to consider when it comes to sound and sound propagation. For purposes of this thread, I'm trying to resolve the mechanism for how sound radiates away from some source at Mach 1. I'm happy enough (for now) to just think about how it happens in air.

How it difracts and refracts and reflects and dissipates and difuses, etc. are important but, not for this thread of trying to figure out "Why does sound propagate".

So, the question that needs to be answered, on the macro level, is just how in the heck does sound manage to zip away from that proverbial speaker cone at 1100 feet per second when the cone itself may only be moving at an aggregate rate of a few inches per second.

Once the basic mechanism of how that's done is defined, at the molecular level, there'll be time enough for aggregating and integrating and assembling that data over lots and lots of molecules and lots and lots of time to come up with pressure gradients and traveling longitudinal waves and scatter graphs and potential and kinetic energy exchanges, etc. But, right now, none of that stuff is doing diddly to explain how it happens.

 

[crashsite]

Elementary my dear, Watson...

Yes, I said to FULLY understand sound propagation, and I stand by that comment, admitting that I do not FULLY understand it (by fully I mean completely describe the continuous motion of individual particles and the forces between them).

I have to believe that a few concepts can be picked out of the complex world of physics and presented at a fairly elementary and mechanical (read that, "non-mathematical) manner. Otherwise it would make no sense to even try to discuss sound propagation except with the likes of Einstein and Hawking.

But, I have to be realistic that any elementary descriptions of what's happening will be open to lots of (well justified) criticism by people who are more astute in physics than myself. Still, I have to hope that my child-like models will give sufficient conceptual information to allow the other math morons, out there in the world, at least a reasonably defensible model on which to base their understanding of sound propagation.

I believe that the molecular view is the correct one to gain that conceptual understanding.

As a side note, you keep using instant-by-instant, however to me this implies discrete time stepping and I believe you actually mean continuously. This is kind of a math term so I’m sorry, but I believe it much more accurately describes what you mean and what is going on.

I absolutely mean, "instant-by-instant" (regardless of mathematical definitions of the terminology). Instant, as defined by the time it takes for a molecule that's been bumped by another molecule to then bump another one. That seems like it's typically only a few picoseconds (which to me is pretty instant-like).

Conceptually that should be sufficient (for a gas), however I believe this is what the majority of the information presented here is trying to do and you are dismissing all of it. I would also like to mention that the spring mass model is very much a Newtonian system.

It just seems like the, spring mass model has more to do with the way the molecules themselves interact with each rather than how the molecules interact to move the sound along.

But, I'm open to why it's also germain to the actual sound propagation process.

I’m not sure whether your arguing against standing waves in general or just in sound/pressure....

Waves, in general are a phenomenon that happen over time. If one were to take an instantaneous snapshot of the positions of air molecules and examine it, would one see a wave? If the answer is "yes" then waves and standing waves can exist. If the answer is, "no" then the wave, or any wave structure, can only be thought of as the result of what happens to the molecules over time.

As I've said many times, I think this is the primary problem with the way schools teach sound and sound propagation. They start out with waves and then when it comes time for sound to propagate at 1100 feet per second...there's no reaons given except that, magically, somehow, it does.

I ask someone to tell me how longitudinal waves in the air manage to zip away from something like a speaker at 1100 feet per second. How can they? That's just not the right venue to try to do it.

I think you misunderstand thermal energy (I believe this is what you mean when you say “heat”, if not the speaker may be cooled so that it imparts zero or even negative heat into the air, and you may ignore the rest of this post). I said it in my original post and I will say it again: Sound is thermal energy (molecular energy relative to each other), therefore you cannot ignore the energy imparted by the speaker (otherwise there would be no sound).

I'm sure I do! So, I restrict myself to the very mechanical model of heat making molecules move and more heat making them move faster. I allow that different materials hold their molecules differently and so the motion of the molecules can be different in different materials.

I accept that heat transfer by conduction is from the energy exchanged as molecules, with different energies, bump each other. But, I have a virtually non-existent understanding of how radient heat is either absorbed by or radiated by a molecule.

I also assume that, when the subject is sound propagation, there is at least a tacit assumption that all the molelcules are about the same temperature and so they merely change directions when colliding rather than transfering energy to/from each other.

 

[crashsite]

This sounds very familiar...

Hi all,

I see this is a rather heated topic so I thought I might share a simulation demonstrating the propagation of sound on the molecular level.

I think its pretty cool because you can see how compression of the molecules causes longitudinal waves of pressure while the molecules themselves remain roughly in the same region of space, which is the sound we hear.

Hope it helps.

PhET Wave Interference - Electric field, Interference, Diffraction, Double Slit
- run the simulation
- click the sound tab at the top left
- click particles on the right

Sorry. Your link did not go to the simulation. It went to a general page where people can seem to find this sort of thing.

But, from your description of longitudinal waves, I doubt that it shows the molecular level. More likely it shows some sort of animated "scatter graph" where the dots represent some pressure gradient as conceptually displayed by a concentration of "molecules".

Here's the kicker. Using that simulation, explain how what's shown translates into the sound moving away from the point of disturbance at Mach 1. And how, if the overall pressure changes, that Mach 1 remains the same speed.

 

[crashsite]

The Answer

I've tried to make the simplest graphic for this explanation so, it might take some extra words.

Let's look at the case for the rarefaction cycle of some sort of piston first. The piston is represented by the heavy black line. The initial direction of motion of the molecules (red, green and blue dots) are shown by the light arrows.

**broken link removed**

The top half shows the surface of the piston at rest. Three air molecules are heading straight for it, each traveling a nominal 1100 mph. Of course it would be rare for three molecules to be in this condition. More likely they would be coming in at various angles. But, we just happened to catch them like this. The red molecule will strike the surface of the stationary piston and rebound off it, where it will be struck by the green molecule. The green molecule will rebound off the red one where it will then be struck by the blue molecule.

In the lower half of the picture, the same sequence of events will happen but, the piston is moving away from the oncoming molecules and is displaced slightly in the rarefaction cycle. The red molecule will stricke the piston later than it would have. It will still rebound and be struck by the green molecule but, also later. That delays the time that the green molecule will be struck by the blue one. And, that delay will continue to be felt, moving away from the piston. That is the "bias" I've been talking about. The change in position between where a molecule will be when there's no sonic disturbance and when there is.

The next picture shows what happens when the piston is moving in the compression cycle. The top half again shows the same static piston situation as above (and provides a reference to see the direction of piston displacement).

**broken link removed**

During the compression cycle, the piston is moving toward the oncoming molecules. The red molecule will strike the surface of the piston and rebound from it but, will do so slightly earler. That causes the green molecule to strike the red molecule earlier and the blue molecule to strike the green one earlier. That effect also continues moving away from the piston.

Of course, there are 'zillions' of molecules all doing this basic action near simultaneously so, there's actually a mass of molecules propagating the effect of the moving piston but, always on a molecule-by-molecule basis and always outward, away from the piston.

That is the mechanism by which sound propagates. The reason it propagates (in air under standard conditions) at about 770 mph is directly related to the fact that the molecules themselves are traveling at about 1100 mph (as per my earlier graphic...as corrected, mathematically, by user, Skyhawk).

I've been asked what answer will satisfy me regarding this question. I can now give an answer. I think this answers it to my satisfaction. That's not to say that all the questions I have about it are satisfactorily answered but, the basic question of, "Why does sound propagate?" has been.

All the stuff about longitudinal waves and pressure gradients and material density and the medium oscillating and the all the math and the equations, etc., have their place and are important but, I believe that only a molecular view such as this actually shows the mechanism of how sound propagation happens.

One of you math types might want to figure out and give us some sort of scale for the size and speed of this action. Just based on data from some of the links that have been presented in this thread, I've surmised that the molecular actions are taking place in the picosecond time scale (but, it's just a semi-educated guess). I would be interested to see some sort of chart, graph or graphic that shows this information in a textual or visual sort of way.

User, Skyhawk gave a numerical value for the number of molecules per unit space awhile back and other links have provided the 1100 mph value. From that data it should be fairly easy to make a chart or graph.

 

[crashsite]

No smoke screen

If the piston is very small in area, relatively few molecules, of an air mass, will be involved in the sound propagation process. As the piston becomes larger, more molecules are affected.

It's beyond my math abilites but, it seems like someone who's capable and interested in statistical analysis could figure out at least the number or percentage of molecules involved in a short burst of sound like a "click", for a given piston size and be able to chart or graph how the number of molecules involved in the sound propagation compares to the total number of molecules as the click radiates away from the piston (for standard and perhaps other conditions). Perhaps it's even a simple enough problem that a little program could be written to solve it?

Thinking about how sound may selectively travel through only some molecules of a mass of air by considering it on a molecule-by-molecule basis might also shed light on something like how a smoke ring can waft through a mass of air while maintaining it's rather complex toroidal shape for a significant amount of time (several seconds), before it difuses and dissipates. After all, since we know that smoke rings can exist (by direct observation), there must be some mechanism that can explain it. In fact, silly me...I'll bet there's a conceptual, non-mathematical way to do it.

 

[crashsite]

Implications of molecular displacements

When thinking about sound propagation on a molecule-by-molecule basis, there are actually a number of things to consider that, quite frankly, I'd never bothered to consider or analyze but, have sort of wondered about. Here's one:

We generally think about woofers as being the "powerhouses" of the audio world. After all, they give us the booming, bone-rattling bass notes and beats, right?

But, when it comes to sound propagation, that slow-moving speaker cone actually displaces the molecules very slightly as compared to a fast-moving tweeter cone and especially as compared to something like an even faster moving ultrasonic transducer.

I don't know if you've ever dipped your finger into an ultrasonic device like a lens cleaner or mister. I have and it can "sting". Now, it's fairly easy to see why. Even though there's not much movement of the transducer, there is a lot of displacment of the individual molecules in the water and in your finger.

Where the woofer needs to be large and have a large excursion to produce its power (integrated over long time periods), the tweeters produce their power more by molecular displacement. And, the ultrasonic devices do that even more dramatically.

Still, there's no direct corrolation between the frequency and molecular displacement. It's determined by the amount the piston moves from one molecular collision with it and the next. That is affected by both frequency of the sound and the amount of the excursion (which translates to being the speed of the piston).

That information needs to be considered when thinking about how the different sounds propagate. Think about listening to a train off in the distance. What sounds do you hear? Basically the mid-range sounds of the whistle or horn seem to propagate best.

Sure, the massive, low frequency sounds are powerful up close but, how efficiently do those small molecular displacements propagate before they are diffused away to nothing by the medium?

The high frequenies get eaten up by phase shifts as the sound travels through the air. Even though they have a large molecular displacement, the wavelength is so short than even small things like temperature variations and air currents can shift the phase of the waves significantly. It's interesting to consider that those large molecular displacements must still exist in the air mass but, they have been "re-jumbled" so that they are no longer associated in a so well defined way with the longitudinal waves (that everyone is so anxious to talk about) so the wave gets attenuated.

Or, think about hearing a band or concert being held at an open auditorium, such as you might find at a county fair but, from a distance of several blocks away. What range of sounds predominates?

I'm sure most of you will have your own views on all this and they may well differ from mine.

 

[crashsite]

Another Implication of Molecular Displacement

Okay, this gets a little more "theoretical". But, I'll try to present it in a logical, defensible manner.

If neither the frequency or the amplitude of a sound is directly dependent on the molecular displacement (both are dependent on a combination of the two) then, really only the rate at which the displacements change, over time, determines the frequency. That's true whether it's cyclically over time, as for a continuous wave, or just a slope for some segment of a waveform.

A reduced molecular displacement but, changing at the same rate will retain the same frequency but with a reduced amplitude.

We know that the amplitude of a sound naturally falls off as a square of the distance (just as light does) so, there must be some mechanism that makes it do so. Analyzing the molecular displacement of a sound provides an explanation for that phenomenon.

In the initial example a couple of posts back (with the red, green and blue molecules), the sound would propagate at about 1100 mph because of that unlikely alignment of the nmolecules. But, when the molecules are moving at all sorts of crazy angles, the rate will average to the speed of sound in air of about 770 mph.

But, all those angles have another legacy. As the sonic wave expands away from the piston, the displacements of the molecules start "dividing up" as the molecules collide obliquely and with grazing collisions, with molecules further and further from the piston receiving less and less energy, resulting in less and less displacement.

This is where my poor math skills fail me. While it's fairly easy to visualize the rate at which the displacements of the molecules to follow that square law, it's something I can not mathematically defend.

Even if the sound is trapped and channeled through some sort of conduit it will then naturally expand once freed. Sort of like light, trapped in a fiber optic strand does.

 

[crashsite]

More Editorializing

In light of the explicit molecular action of sound propagation, in the last few posts, I'm going to revisit one of my sore spots regarding how sound is taught in schools. I hope that this editorial will be better received now than it has been.

Again, I'll go back to the "standard" of the Wikipedia (augmented by the fact that there's been very little challenge to it and none directly regardarding the article's lack of even acknowleding the molecular role in sound propagation).

The first sentence of the Wiki article says:

"Sound is a travelling wave which is an oscillation of pressure transmitted through a solid, liquid, or gas..."

The article also goes on to say:

"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."

Can it be any wonder why there is such a strong bias for thinking of sound in terms of "oscillation" and "simple harmonic motion" and "waves"?

If the Wiki article had stopped with the first sentence, one might argue that "oscillation" isn't really, "oscillation" (at least not in the sense that the air is "oscillating" rather than simply continuing to follow the motion of some sort of disturber as it propagates). But, when the article goes on to explicitly state that the air is oscillating in terms of the exchange of potential and kinetic energy, there can be no doubt that the author is envisioning the air to be "oscillating".

And, because that's the way it's taught in science class, that's the way it generally seems to be understood and that bias has come through loud and clear in this thread.

Which brings me to the Lennard-Jones thing. What does it have to do with sound propagation? It was brought up twice...by two different users. Here's the first sentence of the Wiki article on that topic:

"The Lennard-Jones potential (also referred to as the L-J potential, 6-12 potential or, less commonly, 12-6 potential) is a mathematically simple model that describes the interaction between a pair of neutral atoms or molecules."

To have a highly detailed, complete understanding of sound propagation and the mechanics of all the factors involved for all the motions and interactions of the molecules, at some point, this information is going to be needed. But, when the question regards the basic mechanism by which sound propagates, this information is more a distraction than a help.

I don't know. Maybe I'm wrong. Maybe one of you guys can explain how understanding the Lennard-Jones Potential relates to the basic mechanics of how sound propagates away from a speaker cone at some 1100 feet per second.

 

[crashsite]

The Problems with Medium Oscillation

The Wikipedia article is very specific about the medium "oscillating". I think the problem of trying to disagree with that position is that you instantly get embroiled in arguments that just don't have anything to do with sound propagation.

I think that saying that blowing a puff of air isn't the same as sound propagation would get little argument. I could say the same about the act fo breathing in and out. There's certainly an "oscillation" of a mass of air moving in and out of the lungs when breathing. But, is that oscillation, sound propagation? Can you go out to a distance of ten meters and see the air mass oscillating in a fairly crisp sympathy with a person's breathing? If you have ten people in the area, all breathing and you go off ten meters and measure the air mass, can you determine the direction and characteristics of the breathing of each of the ten individuals? Is sound propagation involved in the air oscillations of breathing?

If I suggest that you have the same ten people breathing and speaking and ask if you can determine who and where they are by listening to them speak, what would your answer be? Is sound propagation involved in the determination? Is oscillation of the air mass involved?

In this post, I want to address that second question.

When a person speaks,his or her vocal cords vibrate and that vibration gets coupled to the adjacent air molecules. The vocal cords move so the air molecules have to move, too. As those molecules move, they continue to move other molecules, in a ripple effect. That ripple happens because of the interchange of kinetic and potential energy of the air molecules (classic deflection and restoring forces and very much akin to the way the ripples spread across the surface of a pond...but in 3 dimensions). That effect continues outward all the way to the receptor (an ear drum located ten meters away), causing it to vibrate in sympathy with the speaker's vocal cords. Of course it takes some time for the effect to travel from the speaker to the listener. But, since that ripple travels at Mach 1, it doesn't take long for the sound to reach the listener

Isn't that essentially what the Wiki article says happens? The sound is generated and, through oscillations of the medium, propagates to some receptor where the sound is "heard".

For sound to propagate that way, the vocal cords are actually moving the ear drum but, through an intermediary substance. It's sort of like pushing a spider with a stick. Your hand is still pushing the spider but, not actually directly touching the icky thing.

That analysis comes with a multitude of problems. Here's a couple of the biggies. 1) The ripple on a pond does not travel at the speed of sound in water nor does the ripple in air travel at the speed of sound in air. 2) The vocal cords simply do not have the power to oscillate the required mass of air from the voice box to an ear drum ten meters away, much less to tens or hundreds of thousands of ear drums in a crowd. 3) Having the air mass oscillating does not explain the precision with which you can detect the direction specific sounds are coming from in a cacaphony of sounds or to be able to detect and identify individual sounds in that cacaphony.

No. The air mass does not oscillate. So, there must be some other mechanism at work.

I believe that the power to move sound does not come from the sound generator. It comes from the heat energy in the medium itself. More specifically, through the thermal movements of the molecules in the medium.

Rather than thinking of the air (or other medium) oscillating, I believe you need to think about it as a series of discrete molecular displacements and, only when those displacements are integrated over time, to consider them to form the measurable, hear-able sounds. Those molecular interactions occur in the picosecond time frame so they do move fast enough to account for sound propagation at Mach 1 (in fact, they are the reason for it).

And, yes! I am going to editorialize.

I just don't understand the reluctance for people to just sit down and think a little bit about what must be happening based on what is known to be happening. I'm particularly disappointed in the way schools teach this subject...at least the way it was taught to me in science class and, apparently, the way the teacher taught it to the Wiki author.

I don't think it's any secret that I don't have all the answers but, even a dope like me can see that there are problems with the status quo and I do feel pretty confident that at least the gist of my analysis is at least sort of accurate.

 

[crashsite]

The Semantics of Oscillation

When you get into the question about the medium oscillating, unfortunately, you also get into a question of semantics. Just what does "oscillation" mean and when and how does one determine an "oscillating medium".

One might ask that even if you are considering sound propagation as it relates to discrete molecular interactions and displacements, when you integrate that over sufficient time to see wave phenomena develop, can that be construed as an oscillation of the medium?

There is certainly a natural oscillatory nature to something like a speaker cone. In fact measures must be taken to null or dampen or filter out that natural tendency. The cone is deflected by an audio signal and then naturally returns to some neutral position. In order to maintain good fidelity, the driving amplifier needs to maintain a low (electrical) impedance so the position of the speaker cone tracks the current through the voice coil. There's also a natural resonance of the speaker cone that is generally nulled out either by cabinet design or electrical filters (or both).

Close to the speaker, the cone works like an air pump. Air, adjacent to the speaker cone is moved. In that respect the speaker acts a lot like the diaphragm under our lungs. But, is that moving/oscillating air related to sound propagation. My gut feeling is "no". It's there, it's real but, it just doesn't have a tendency to zip away from the speaker at Mach 1.

We have to differentiate between the act of moving air and propagating a sonic wavefront.

It seems to be an unpopular analogy but, the Newton's Cradle toy does show how two different pehnomena can exist in the same device. You can see the slow event of the balls dropping on one end and popping up on the other and the fast event of the effect of the balls traveling through a string of seemingly motionaless balls. Yet, somehow, even with such a powerfully visual example, there seems to be a lot of resistance to the notion that a similar action could be taking place in the air (or liquid or solid or other gas or plasma).

That brings us (me?) back to the question of whether the air is oscillating. It's certainly doing something just as the intermediate balls in the toy are doing something. So, what do we do? 1) Do we get into the semantics that even though something isn't moving but is still conveying some sort of energy that it constitutes, "oscillation"? 2) Do we get into the semantics that merely having that energy following the motion of some disturber constitutes, "oscillation"? 3) Do we simply assume that because the sound generator and the sound receiver are vibrating in sypathy with each other that there's "oscillation" going on in the medium between them, regardless of the actual mechanics of how it's happening?