Why Does Sound Propagate?

[3v0]

So, only in the most indirect way does it represent a molecular distribution.

There may be a direct relationship. Understanding that gas pressure, volume, and heat are related we know that either an increase in heat or a decrease in the volume (compression) must be reason we see increased pressure.

If heat is the reason for the increased pressure we should try to confirm it.

If compression is the reason for the increased pressure we should try to confirm that too.

If heat is the source of the increased pressure the scatter graph is a reasonable representation of heat distribution as well as pressure.

But if compression is the source of the increased pressure then the scatter graph is a reasonable qualitative representation of both pressure and density.

In either case we have shown there are energy waves radiating from the speaker.

3v0

 

[crashsite]

Persistance of vision

There may be a direct relationship. Understanding that gas pressure, volume, and heat are related we know that either an increase in heat or a decrease in the volume (compression) must be reason we see increased pressure.

If heat is the reason for the increased pressure we should try to confirm it.

If compression is the reason for the increased pressure we should try to confirm that too.

If heat is the source of the increased pressure the scatter graph is a reasonable representation of heat distribution as well as pressure.

But if compression is the source of the increased pressure then the scatter graph is a reasonable qualitative representation of both pressure and density.

All that is macro thinking. How are having lots of molecules being compressed related to the concept of pressure. How does heating a lot of molecules affect their pressure. How does affecting the volume of a mass of molecules affect their pressure.

It's the big picture kind of stuff that lends itself well to mathematical modelling and, since that's the way they are dealt with in the practical world (you never actually deal with a single or just a few molecules), it's the typical way they are thought of. The problem is that "compression" and "pressure" and "heat" are not defined. What is happening between a piston and an air molecule to cause it to "compress"? What's the difference in how the molecules act or are arranged when they are at a state of more or less "pressure"? How does thermal energy affect the molecules to cause them to have more or less of that pressure? Indeed, what is that thermal energy in the first place?

One way to deal with the questions is to assume that the molecules will do what they do and you, as a person, only need to consider the result and deal with it on the scale you can see and measure. That's pretty much what I've done all my life.

But, then I ask a simple question..."Why does sound propagate?"...and suddenly, I discover that all that stuff I thought I knew just wasn't answering the question, I was forced to go to lower levels (that I hadn't really considered much before).

In either case we have shown there are energy waves radiating from the speaker.

Energy waves. Large masses of molecules exchanging potential and kinetic energy as they longitudinally (somehow) radiate away from the speaker at Mach 1.

Now, can we get to that, "somehow" part?

 

[3v0]

Now, can we get to that, "somehow" part?

I have tried numerous times to show that we need to understand and agree on the macro prior to looking at the molecular level. The macro level is the ONLY thing we have to verify that what we do or suggest at the molecular level.

We can move on if you are willing to accept that at least at the macro level science has it right.

3v0

 

[crashsite]

Mexican Standoff?

I have tried numerous times to show that we need to understand and agree on the macro prior to looking at the molecular level. The macro level is the ONLY thing we have to verify that what we do or suggest at the molecular level.

We can move on if you are willing to accept that at least at the macro level science has it right.

I'm not quite prepared to accept that the macro view explains enough to pronounce it "right". So, you may be asking, what's the hang up?

If the speaker acts like a piston to drive its energy into an enclosed volume (a box), in which you have a pressure transducer, and if you wait for the energy to even out within the box after each time you move the piston before making your measurment, then I agree with your macro view. There is a direct corrolation between the piston area and displacement and box size and pressure change.

But, if the piston is moving in free air and the effect that is coupled to the air is allowed to propagate, unfettered, from the piston...at a speed that's unrelated to the speed of the piston, it's not so cut-and-dried just what you'll be measuring or where. In fact, that's what this thread is all about.

It's a dynamic situation and before you even finish writing the first sentence about what you're dealing with, you're already faced with questions that can't be satisfactorily answered at the macro level.

So, yes, I agree that the macro view, when the parameters are well defined and allowed to stabilize, is the way science teaches it. But, I don't necessarily agree that the way science teaches sound (at least based on something like the way Wiki does it), is "right". Now, I'm not saying that I think it's wrong. Given some mental gyrations, you can massage the descriptions and definitions into your favored theory. However, I am saying that I believe it just doesn't address the issue in the right way so sound propagation makes proper sense on a conceptual basis.

We can nit pick this point until the cows come home so, I'm guessing that (since I so unreasonably refuse to toe the line of classical physics teaching) that there wont be any alternate description of how sound propagates forthcoming...

 

[3v0]

We should not expect the macro view to explain how. All I am asking is that we examine what we can at the macro level and take note of what we see. At the macro level some things like the speed of sound and how sound propagates may be inexplicable.

In spite of the fact that the macro view does not tell us how, it is a invariable yardstick. Any molecular view that we postulate MUST give rise to the same set of conditions we see at the macro view.

That is why it is so important that we agree on what is seen at the macro [EDIT: was molecular] level. We need to all use the same yardstick.

We can nit pick this point until the cows come home so, I'm guessing that (since I so unreasonably refuse to toe the line of classical physics teaching) that there wont be any alternate description of how sound propagates forthcoming...

From your perspective it may seem like nitpicking. I just want to keep any speculation in line with what can be observed.

3v0

 

[crashsite]

Brass Tacks

That is why it is so important that we agree on what is seen at the molecular level. We need to all use the same yardstick.
From your perspective it may seem like nitpicking. I just want to keep any speculation in line with what can be observed.

Did you mean to say, "macro level" here? But, either way, I agree.

On the macro level, I propose that what we see is movement of some disturber being coupled to the air. The air responds in two ways. One way is for a bulk amount of air to waft away from the point of disturbance. That would be akin to the puff of air one might make to move a spider web or blow out a candle. The other is a sonic wave front that speeds away from the point of disturbance at Mach 1.

There are probably other things that happen, too. Perhaps like the infared electromagnetic waves that result from the change in temperature from adibiatic effects and the gravitational variations created, etc. But, for purposes of this discussion, can we ignore them? In fact, can we also try to ignore the puff of air and concentrate only on the sonic effect? I'm not sure if the air puff will have sufficient impact that it needs to be considered or not...we'll have to let that chip fall where it may.

I further propose that, at the macro level, any measurement we make, modifies the conditions we are measuring. Because air is invisible to us, we cannot directly see what's happening and, even if we could, our persistance of vision would prevent us from accurately seeing most of it. So, any measurement requires us to insert some sort of impedance mismatch or artificial component into the medium that will turn the effect into a measurable quantity (ie: Like the sensation of sound from the ear drumm, a voltage level from a microphone diaphrahm or the addition of smoke or other agen to make the air visible, etc.). You may consider other ways to make meaningful measurements.

Can we agree that any other conclusions that we may draw about what's happening (at the macro level) can only be conjecture no matter how logically it may be derived. Can we also agree that any conclusions we may draw, at the molecular level, are also only conjecture and must remain so since there's no practical way to directly measure them?

 

[3v0]

There are probably other things that happen, too. Perhaps like the infared electromagnetic waves that result from the change in temperature from adibiatic effects and the gravitational variations created, etc. But, for purposes of this discussion, can we ignore them?

In general ignore what can not be seen or measured at the macro level.

I further propose that, at the macro level, any measurement we make, modifies the conditions we are measuring...

Can we agree that any other conclusions that we may draw about what's happening (at the macro level) can only be conjecture no matter how logically it may be derived. Can we also agree that any conclusions we may draw, at the molecular level, are also only conjecture and must remain so since there's no practical way to directly measure them?

I do not agree. The are working far above the quantum level.

With the right equipment it should be possible to measure air density and pressure. It is much easier to get qualitative information that allows one to see an effect but not obtain a highly accurate measurement.

Keep in mind that at the stage we are looking at what we see and not why it is there.

If you doubt ANY of the classical observations regarding sound at the macro level we need to deal with them one at a time. Because for each item you dismiss the yardstick becomes less accurate.

3v0

 

[crashsite]

Can't quite do it...

If you doubt ANY of the classical observations regarding sound at the macro level we need to deal with them one at a time. Because for each item you dismiss the yardstick becomes less accurate.

This is tricky. The Wiki article on sound says that the medium is oscillating. It even says that it's the exchange of potential and kinetic energy. I think we both agree that our speaker cone is oscillating. That is, it's moving back and forth.

But, is the air oscillating in the same way? The sound is moving through the air at Mach 1 and always outward from the disturber. If the air molecules are moving faster or slower but, still moving always outward, is the air "oscillating" or is it just changing speed?

Is there a wave in the air that is moving an appreciable distance forward and back, in sympathy with the speaker. Or even a wave that is moving an appreciable distance forward and back but, relative to the Mach 1 speed that the sound is traveling?

You can put your measuring device into the sound field and observe that it oscillates in sympathy with the way the speaker cone did but, does that mean that the air itself was oscillating or does it just mean that there was a conversion from whatever the air was doing to the oscillatory motion of your measuring device.

We can get a clue about this from the despised, Newton's Cradle toy. Do you see the intervening balls moving in sympathy with the ball you drop? You certainly see the end ball moving sypathetically. So, if the intervening balls don't noticably oscillate, or otherwise follow the motion of the fist ball, why would you believe that the intervening air, in a sound field, does?

These points (and others) are what I question about the way sound is classically described.

The problem with disagreeing with the classical definitions is that there's always an "out". There's nuances of menaing of terms. There's conditions under which the description can be made to agree. There are mathematical formulas that "prove" the classical defintions. There is the weight of the scientific commmunity saying that's the way it is so, that's what's accepted.

But, if the classical descriptions and definitions and math are so clear and unambiguous, how come there's so much confusion and misconception about how sound works even in an engineering community that rather prides itself on knowing how stuff works. There have been people, even in this thread, who have vowed that air density and pressure affect the speed of sound...even after posts that specify that it doesn't.

So, when you say that I must buy into every aspect the classical presentation, I have to say that it has to do a better job of explaining it to me before I do. I've generally been pretty happy with what I think has shaken out from this thread and I do believe it's helped to nail down some of the stuff that's not very clear in something like the Wiki descriptions (which I think we can both agree represent the "classical" view of sound).

 

[skyhawk]

Let me deal with this first.

But, if the classical descriptions and definitions and math are so clear and unambiguous, how come there's so much confusion and misconception about how sound works even in an engineering community that rather prides itself on knowing how stuff works. There have been people, even in this thread, who have vowed that air density and pressure affect the speed of sound...even after posts that specify that it doesn't.

It is simply because sound plays a very small role in a typical engineering or physics curriculum. For most persons in this category the only contact with the subject is part of an introductory physics sequence. In my case I had a three semester sequence: one semester of classical mechanics, one semester of E&M, and a catch all semester of oscillations, waves, optics, and thermal physics. The important thing about sound (as far as the course) was that it was a wave and underwent interference and diffraction. Off the top of my head I don't remember seeing an equation for the speed of sound. I suspect it was the same for most engineers here.

Except for physicists doing research in acoustics there is little interest in sound in the physics community. The interesting problems (in the opinion of physicists) were solved 150 years ago. The people doing work that touch on your interests are probably aeronautical engineers doing work in high speed low density flows, but that's just a guess. There are also people doing work in statistical mechanics of gases where careful measurements on the speed of sound are used to investigate the properties of the molecules themselves.

But, is the air oscillating in the same way? The sound is moving through the air at Mach 1 and always outward from the disturber. If the air molecules are moving faster or slower but, still moving always outward, is the air "oscillating" or is it just changing speed?

When one says that air is oscillating it is in the continuum mechanics sense and not in the molecular sense. Pick a point in space and a tiny volume around that point. Add the velocity vectors of all the molecules in that volume and divide by the number of molecules to get a mass velocity of the air. It is the mass velocity of the air that oscillates, i.e. changes as a periodic function of time. The motion of a single molecule will be a random zig zag path that will over time carry the molecule all over the room. The term for this is diffusion. I don't believe that I've seen diffusion mentioned in this discussion even though it is an important feature of the random motion of molecules in a gas. This is probably because the concept of the random motion of the molecules is misunderstood.

We can get a clue about this from the despised, Newton's Cradle toy. Do you see the intervening balls moving in sympathy with the ball you drop? You certainly see the end ball moving sypathetically. So, if the intervening balls don't noticably oscillate, or otherwise follow the motion of the fist ball, why would you believe that the intervening air, in a sound field, does?

Newton's Cradle is not a valid analog because it is limited to a few balls with motion subject to strong constraints. Sound involves the collective motion of large numbers of molecules where the motion of any one or small number of molecules is seemingly random.

 

[allcanadian]

@crashsite

But, why does a "packet" of those disturbances propagate away from the emitter, intact?

It is generally understood that sound is not the motion of any medium moving away from the source persay but pressure waves within the medium propagating away from the source. Sound is not something, it is a condition of something, this condition is air being compressed and expanded cyclically producing standing waves.

If the emitter is some sort of diaphragm that vibrates and moves the air back and forth just slightly, why doesn't the sound just stay there and become a more and more complex wveform with each cycle...with that cacaphony propagating,, by the interchange of energy between adjacent molecules, as basically a damped waveform as it loses energy with distance?

The diaphragm compresses the air in front of it in one cycle, this pressure wave starts moving away from the source(high pressure to low ambient). In the next cycle the diaphragm moves backwards rarifying(expanding) the air in front of it as the last cycle of compressed air continues onward. We now have waves of compressed and expanded air(the transfer medium) travelling away from the source, these pressure variations in air are what we percieve as sound. A common misconception is that the media moves forward like a fan pushing air but sound is pressure waves within the media, the pressure variations transfer energy.

In other words...why does the sound packet speed away from the emitter? What propels it?

Why does a rock dropped in a still pond produce waves which seem to travel away from the source? Both are examples of standing waves in which the losses or dampening effects are due to actual motion of the media and resistance to this motion(transformation to heat). The actual motion of energy relates to the condition of the media itself, high energy state seeks low energy state to find balance with ambient conditions. In this case we could say ambient conditions and dissipated wave energy surrounding the source propels the compression wave, high seeking low to balance itself.
AC

 

[3v0]

Both skyhawk and allcanadian made some good points.

So, when you say that I must buy into every aspect the classical presentation, I have to say that it has to do a better job of explaining it to me before I do.

Lets look at them one at a time.

This is tricky. The Wiki article on sound says that the medium is oscillating. It even says that it's the exchange of potential and kinetic energy. I think we both agree that our speaker cone is oscillating. That is, it's moving back and forth.

Indeed the air is oscillating. I just want you to see that the air is indeed oscillating between low and high pressure. Because when we finish with all the hand waving at the molecular level the air had better be oscillating. The molecular level must generate the conditions we see at the
macro level.

What would you accept as proof that the air is oscillating ?

3v0

 

[crashsite]

Fighting the status quo

The important thing about sound (as far as the course) was that it was a wave and underwent interference and diffraction.

One thing that has come up is the notion that sound is "isentropic" which was described that sound does not interact. That, for example, two sounds from different directions can cross and have no interaction. You hear the two sounds as two independent sounds, each coming from it's own source. Also that, once a sound wave front passes, the air goes back to the state it was in before the sound passed.

I take that to mean (perhaps incorrectly) that the sound only interacts at a point of impedance mismatch. For example, the beat note you hear when tuning a guitar by plucking two strings. Does that beat note exist in the air or does it only exist when a non-linear element, such as an ear is being acted on?

Except for physicists doing research in acoustics there is little interest in sound in the physics community.

That is sad and disappointing if true.

When one says that air is oscillating it is in the continuum mechanics sense and not in the molecular sense. Pick a point in space and a tiny volume around that point. Add the velocity vectors of all the molecules in that volume and divide by the number of molecules to get a mass velocity of the air. It is the mass velocity of the air that oscillates, i.e. changes as a periodic function of time. The motion of a single molecule will be a random zig zag path that will over time carry the molecule all over the room. The term for this is diffusion. I don't believe that I've seen diffusion mentioned in this discussion even though it is an important feature of the random motion of molecules in a gas. This is probably because the concept of the random motion of the molecules is misunderstood.

The motion of the single molecule may still be mostly zig zag but, it will have a bias that carries the sound. The question is how having a lot of molecules carrying the bias contributes to the local accumulation (or lack of accumulation) of the molecular density at a given space at a given time.

This is one of the problems of discussing (trying to discuss) this issue. In the interest of trying to nail down various concepts, one has to assume that other concepts that have been discussed will be remembered and taken into account. Unfortunately, it seems like, unless you bring in every detail that must simultaneously be considered, at each and every juncture, you get into a loop of trying to defend the same isures over and over again.

I've been trying to work my way through the (what I consider to be) distortions and half-truths of the macro definitions of sound propagation when I really believe that the action is happening at the micro level in the forst place.

Newton's Cradle is not a valid analog because it is limited to a few balls with motion subject to strong constraints. Sound involves the collective motion of large numbers of molecules where the motion of any one or small number of molecules is seemingly random.

If you must restrict you sample to just a few molecules to get an understanding of how they interact, I have to question if the Newton's Cradle toy might not still be a good analogy. Good both for demonstrating how the sound propagates through the molecules and especially how it acts at a point of impedance mismatch.

Of course, I can't say that it is a good analogy or not for sure because I don't have the physics background to say. But, it sure seems like it demonstrates how a slow speed event can propagate quickly through the medium and come out the other end with the pattern intact...just like sound does..

 

[crashsite]

Getting to Mach 1

Sound is not something, it is a condition of something, this condition is air being compressed and expanded cyclically producing standing waves.

As per my previous post (to user, Skyhawk), I question whether "standing waves" actually ever exist.

The diaphragm compresses the air in front of it in one cycle, this pressure wave starts moving away from the source(high pressure to low ambient). In the next cycle the diaphragm moves backwards rarifying(expanding) the air in front of it as the last cycle of compressed air continues onward. We now have waves of compressed and expanded air(the transfer medium) travelling away from the source, these pressure variations in air are what we percieve as sound. A common misconception is that the media moves forward like a fan pushing air but sound is pressure waves within the media, the pressure variations transfer energy.

What that dissertation failed to address is how or why the sound moves away from the speaker...much less why it does so at the rather fast speed of Mach 1.

Why does a rock dropped in a still pond produce waves which seem to travel away from the source? Both are examples of standing waves in which the losses or dampening effects are due to actual motion of the media and resistance to this motion

The "rock in the water" is a stricly macro event. I't's actually pretty easy to visualize. The rock locally displaces some water and that displacement (through the favored explanation of the exchange of poteneial and kinetic energy), moves away from the point of the displacement at a speed allwed by the properties of the water.

But, the "rock in the water" also produces another disturbance. A sonic effect that speeds away, unseen (and, unconsidered). Relative to this thread, those waves you see are moot. The sonic wave front is the item that needs to be considered and addressed. Actually, the thrust of this thread has concentrated on sound in air (which seems to be complex enough, without considering other media).

 

[crashsite]

Boxed in

Lets look at them one at a time.

Indeed the air is oscillating. I just want you to see that the air is indeed oscillating between low and high pressure. Because when we finish with all the hand waving at the molecular level the air had better be oscillating. The molecular level must generate the conditions we see at the
macro level.

What would you accept as proof that the air is oscillating ?

Let's say you are trying to explain how a car engine works but, you disallow disussing anything about the pistons or the connecting rods or the crankshaft or the camshaft or the valves or the spark plugs or the carburetor. You only allow yourself to consider that you see the end of the crankshaft turning and you must base your explanation on that.

I do believe there are multiple effects in the air that need to be considered and at some near-field proximity to the speaker, the air molecules are moving back and forth in sypathy with the speaker cone. But, I don't know if that's related to the sonic effects. This is similar to the rock-in-the-water. I accept that there is a displacement effect and water waves but, I don't believe that it's related to the sonic effect (it probably is in some what but, not in a direct way).

How those different effects show up in the air is the question here.

 

[skyhawk]

I take that to mean (perhaps incorrectly) that the sound only interacts at a point of impedance mismatch. For example, the beat note you hear when tuning a guitar by plucking two strings. Does that beat note exist in the air or does it only exist when a non-linear element, such as an ear is being acted on?

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

Superposition principle - Wikipedia, the free encyclopedia

The motion of the single molecule may still be mostly zig zag but, it will have a bias that carries the sound. The question is how having a lot of molecules carrying the bias contributes to the local accumulation (or lack of accumulation) of the molecular density at a given space at a given time.

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.

 

[3v0]

Lost in space

Crashsite:

I have been attempting to limit the set of solutions to that which does not go off the deep end.

Regarding the car. At this junction I am not trying to explain how it works. Rather I am trying to gather as much reliable information about it as possible. That way when people start waving their hands we do not end up with a rifle instead.

In short you have to understand what you are looking for prior to the search. You do not.

What would you accept as proof that the air is oscillating, compressed, etc ?

3v0

 

[3v0]

Everyone please

This is much like two people talking about an animal when one thinks it is a monkey the other a lizard.

Until crashsite is willing buy into the observable physical nature of sound regarding pressure and waves any attempt to proceed is doomed.

The duration of this thread points in that direction.

3v0

 

[j.friend]

@crashsite

It is generally understood that sound is not the motion of any medium moving away from the source persay but pressure waves within the medium propagating away from the source. Sound is not something, it is a condition of something, this condition is air being compressed and expanded cyclically producing standing waves.

Sorry mate but this id downright not true. A standing wave is produced due the interaction (superposition) or a wave and it's reflected co-part in the same medium of a transverse wave. For anything to be a standing wave the particles in the medium also have to be moving perpendicular to the transmission of energy.

In the terms of sound, this is most certainly not the case. Also standing waves have points of maximum and minimum movement (nodes and anti-nodes). If sound was to be propagated via a standing wave, there would logically have to be areas within the air with maximum variance of pressure and minimal variance of pressure. However it can be seen that the variance in pressure of the air during the transmission of sound is uniform (However it will dissipate with distance). there are no nodal or anti-nodal points

Why does a rock dropped in a still pond produce waves which seem to travel away from the source? Both are examples of standing waves in which the losses or dampening effects are due to actual motion of the media and resistance to this motion(transformation to heat). The actual motion of energy relates to the condition of the media itself, high energy state seeks low energy state to find balance with ambient conditions. In this case we could say ambient conditions and dissipated wave energy surrounding the source propels the compression wave, high seeking low to balance itself.
AC

Again these are not standing waves, they are transverse waves. These waves are not the same kind of wave that have been proposed to be responsible for the propagation of sound. The waves I am referring to are compressional or longitudinal waves, where the motion of the particles are aligned with the direction of propagation.

 

[skyhawk]

This is much like two people talking about an animal when one thinks it is a monkey the other a lizard.

Until crashsite is willing buy into the observable physical nature of sound regarding pressure and waves any attempt to proceed is doomed.

Agree 100%. What physical phenomenon is being propagated and what physical property is sensed or measured either by an ear, or microphone or other instrument?

 

[ronako555]

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