Why Does Sound Propagate?

"I want to get away from waves and think in terms of instantaneous actions."

Examination of the instantneous actions will show they are compression waves.

"That there is a traveling or standing wave or pressure gradients is an artifact of the process."

You can make a strong argument that sound is the artifact (byproduct) and the wave is a direct result.

3v0

3v0 said:

"I want to get away from waves and think in terms of instantaneous actions."

Examination of the instantneous actions will show they are compression waves.

"That there is a traveling or standing wave or pressure gradients is an artifact of the process."

You can make a strong argument that sound is the artifact (byproduct) and the wave is a direct result.

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It's true that if you string enough instants together, you'll come up with a continuum. And that continuum will be a wave structure. But, does coming up with that contiuum answer the question of how sound propagates? For example, if the question is, "how does a person walk", an eventual answer could be, "He or she will go on a hike". The problem is that it doesn't answer the question of how a person walks. It doesn't address the root questions of balance and muscle action and coordination and bending of the joints, etc. etc..

The same is true when you equate the generation of sound waves with the process of sound propagation. Yes, at some point in the flow of things you end up with waves. They just don't answer the question of how sound propagates.....or, maybe they do?

So, let me ask the question of, "how does examining compression waves explain how the sonic wavefront zips away from the source of the sound at Mach 1"?

This issue was covered in the previous thread.

You are aware he is a audiophile, Right?
Current physics and known basis of operation does not fully fit into an audiophiles realm of sound perception.
Their hearing goes from 0 - cosmic ray frequencies and sound travels at anywhere from 0 to Mach 100 by their standards of evaluation.

Logic, math and science be dammed! That wooden button on the amplifier remote control does make a difference just as putting aluminum foil on the cats tail helps bring out the crisp multi MHz highs all the while dampening that static electricity charge fluffy imparts into the CD's laser beam light reflective clarity quotient.

crashsite said:

It sure seems like that should be half true. Empirically, it seems like less air density would give a slower speed of sound but, a slower speed of sound would give shorter waves for a given frequency.

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Yes I got that the wrong way round.

At lower pressures, the speed of sound reduces, meaning the wavelength at a given frequency gets longer. The lower the pressure the greater the distance between the molecules.

If the distance between the molecules is greater, this means that one molecule moving will have less influence over neighbouring molecules meaning that it will not conduct sound so well.

Shorter wavelengths and larger distances also mean that attenuation will be greater at higher frequencies.

If the signal is 1 KHz, that same half inch of air movement will only affect the signal by a few degrees and thus there will be less nulling action and less attenuation.

But, here we're talking about the waves and waveforms and not the process of the propagation of them. There's a time to analyze waves and a time to analyze something else.

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You've forgotten that the very act of compressing and decompressing the air as the sound wave propagates also looses energy.

If the amount of energy released over a single wavelength for a given amplitude were constant, then the attenuation would be the same over a given number of wavelengths. This means that a 1" 10kz wave will be attenuated the same over 10" as a 10" 1kHz wave over 100".

Unfortunately (as you've said) it's not that simple, there are many non-linear effects.

Back in the mud pit

3v0 said:

This issue was covered in the previous thread.

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No, it wasn't. It has, however, been neatly sidestepped more than a few times by "wave analysts".

Now that the question has gotten away from the "vacuum issue" and back into the root questions of sound propagation again, I don't object to vectoring it back to this thread again. This is where it should be.

But, see. I'm in a little bit of a quandary here and maybe you can give some guidance. I had posted the question regarding sound propagation as pressures near a vacuum as an effort to get an answer to just a small chunk of the physics. It's obvious that asking the big question about sound propagation right off the bat didn't yeild andy semblence of a cohesive, usable answer. I thought that trying to ask a bit at a time and then trying to stitch the results together might work better. The "vacuum" post seems to have proven that approach wrong because the immediate response was right back to waves again.

It would be pointless to try to backtrack to the bits-n-pieces tactic in this thread. I already feel like I'm forced to return to square one with almost every post in this thread already.

If the desire is to ask for the bits and pieces, what format would you suggest might be effective? A format that would be most likely to get some simple, straight answers.

Questions like: How do the molecules act and interact in a volume of air at different temperatures? What characteristics of the air make the molecules act and interact that way? What characteristics of the air molecules makes the sonic wavefront travel at a given speed for a given tmeperature, independent of the pressure? How do the air molecules moved, relative to their normal, random movement when a disturbance passes through them? How can air molecules go immediately back to their random state after a disturbance has passed through them while retaining no memory of the disturbance?

And, a whole host of other pertinent questions of the basic nature of the medium. And, that's only, air and only considering subsonic disturbances and only cosidering nominal pressures and temperatures.

I keep trying to vector to these questionn but, the strong bias for wave analysis, in this community, keeps getting in the way.

So, anyway, no, this issue has NOT been covered.

Hero999 said:

Yes I got that the wrong way round.

At lower pressures, the speed of sound reduces, meaning the wavelength at a given frequency gets longer. The lower the pressure the greater the distance between the molecules.

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You said the exact same thing again as you'd said before. And, the basic premise that the speed of sound is related to pressure is still completely incorrect.

Hero999 said:

If the distance between the molecules is greater, this means that one molecule moving will have less influence over neighbouring molecules meaning that it will not conduct sound so well.

Shorter wavelengths and larger distances also mean that attenuation will be greater at higher frequencies.

You've forgotten that the very act of compressing and decompressing the air as the sound wave propagates also looses energy.

If the amount of energy released over a single wavelength for a given amplitude were constant, then the attenuation would be the same over a given number of wavelengths. This means that a 1" 10kz wave will be attenuated the same over 10" as a 10" 1kHz wave over 100".

Unfortunately (as you've said) it's not that simple, there are many non-linear effects.

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I don't know that I agree with very much of that. Yes, there are losses (friction, and other, losses) but, I don't know that they affect any particular frequency more or less than others. I do know that the phasing thing is a major contributor to the high frequency rolloff.

Remember that you do have more power to begin with in higher frequenies (think of the need for pink noise filters vs. white noise ones).

Tall Tales

tcmtech said:

You are aware he is a audiophile, Right?
Current physics and known basis of operation does not fully fit into an audiophiles realm of sound perception.
Their hearing goes from 0 - cosmic ray frequencies and sound travels at anywhere from 0 to Mach 100 by their standards of evaluation.

Logic, math and science be dammed! That wooden button on the amplifier remote control does make a difference just as putting aluminum foil on the cats tail helps bring out the crisp multi MHz highs all the while dampening that static electricity charge fluffy imparts into the CD's laser beam light reflective clarity quotient.

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Dog-gonnit, boy! I've told you at least 10 million times not to exaggerate!!!

crashsite said:

You said the exact same thing again as you'd said before. And, the basic premise that the speed of sound is related to pressure is still completely incorrect.

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I don't know what's wrong with me.

I meant to say that the wavelength for a given frequency gets shorter so my reasoning is all wrong.

I think you may have exhausted the resources here. Try talking to these people.

Physics Help and Math Help - Physics Forums
Scientific physics forums for professionals and students to discuss serious issues and topics regarding physics in theory, practice, study and reality.

Sojourner in a Strange Land

3v0 said:

I think you may have exhausted the resources here. Try talking to these people.

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Well, it's discouraging and disappointing that, even in a global pool of engineers, I fear you are probably right. Once ya get away from the formulas and equations, I guess it concptually must fall apart for the math types.

But, I'm glad I was able to at least be able to make a "test run" at it here to find out what some of the pitfalls of trying to glean some basic info in which even the guy who wrote the Wiki article on the subject seems to falter.

It's still something I want to understand so, I'll try to regroup and try your suggested forum. Wish me luck...

The primary collection of engineers here are electronics by majority. It sort of gives you a rather limited resource to work with from the start.

It sort of like asking the Case IH tractor engineers how to describe how a vehicle works. Not that they dont have any idea but its no their ideal area of working expertise!

Granted I would like a vehicle with the 20k - 30k hour running life expectancy and fuel efficiency that tractors have!

Your odd pondering did make me think more about sound than I probably ever have before and the conclusion I came up with is, I just use it and it works, thats good enough for me!

Odds are....

tcmtech said:

Your odd pondering did make me think more about sound than I probably ever have before and the conclusion I came up with is, I just use it and it works, thats good enough for me!

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Hmmmmm...I thought my pondering was pretty linear and straightforward. Silly me.

But, hopefully, if nothing else all this may have had some entertainment value and possibly may have kept some of the denizens here off the streets at night.

Thanks for your collective inputs.

"If you have to ask for help on an IQ test, your really not getting the point." -- tcmtech

Or, perhaps you are?

The reason it can look like Jasonbe is because there is such a strong bias for "wave analysis" when the issue is "sound propagation" and I'm not willing to accept some crappy answer about waves, schmoozed over with mathematical machinations, to gloss over what's really happening.

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The fact is that if one starts from the basic laws of physics, namely conservation of mass, Newton's second law of motion, and conservation of energy and applies them to a fluid described by the ideal gas law then it follows that small pressure disturbances propagate like waves. It is an experimental fact that at sufficiently high temperatures and low pressures the pressure-volume-temperature relationship for air is well described by the ideal gas law. So where does the chain of reasoning breakdown?

The point worth pondering is why air that is composed of individual molecules can be described as a fluid. In particular, why does the concept of pressure work within the interior of a volume of gas? How does a pressure gradient within the volume of air cause the air to accelerate or decelerate?

Swan Song?

skyhawk said:

The fact is that if one starts from the basic laws of physics, namely conservation of mass, Newton's second law of motion, and conservation of energy and applies them to a fluid described by the ideal gas law then it follows that small pressure disturbances propagate like waves. It is an experimental fact that at sufficiently high temperatures and low pressures the pressure-volume-temperature relationship for air is well described by the ideal gas law. So where does the chain of reasoning breakdown?

The point worth pondering is why air that is composed of individual molecules can be described as a fluid. In particular, why does the concept of pressure work within the interior of a volume of gas? How does a pressure gradient within the volume of air cause the air to accelerate or decelerate?

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Maybe it's absurd of me to believe that the physics of sound propagation can be brought down to the language of the common man and still be accurate and make sense. Of cours, when I say, "accurate", I don't mean numerically accurate to some number of decimal places but, rather "conceptually" accurate.

In your couple of short paragraphs, you bring up some of the things that need to be addressed in order to understand the concept of it. The question is if they can be conceptualized verbally rather than mathematically. I have to believe they can or I wouldn't be bothering to try.

Even with my limited powers, I can see problems with the notion of explaining sound propagation with longitudinal waves. Or SHM. Or traveling waves. Or waves on any macro level. In fact, when I ask the direct question of how those waves account for a subsonic disturbance suddenly accelerating to Mach 1 and propagating at that speed...I get zero response. And, I expect zero response becuase I don't think that's where the action is.

I did go over to the physics forum suggested by user, 3v0 and found an active thread about the speed of molecules. 'While I do believe that is one of the things that needs to be resolved to understand sound propagation, it appears to me that they are also bumbling around with it (but, of course, copiously throwing out their pet equations to support their individual views).

I think that's the key. How to break things down in an orderly and logical way so that each little concept is defined and explained and then integrated into the larger picture until the question is answered. Not unlike working a jigsaw puzzle. But, along the way, there are many pitfalls. The most grievous being that the first thing that happens is people dumping their "wave analysis" puzzle pieces into your box and then trying to convince you to assemble the puzzle with mostly wrong pieces.

About now, you're probably thinking..."hmmmmm, if he believes that, why doesn't he just take a physics class and let them take him through things properly?". First, they would take the math approach so, I'd be lost. Second, the wave analysts are the ones that got that classical physics education. Third, I hate homework.

In a forum, such as this, I'm not sure it's even possible to take the things you brought up (and many others) and discuss them in a conceptual, rather than mathematical, way and to be able to maintain enough control of the thread of each to keep it on topic and moving forward.

I do believe there are people out there who have that rare gift of being able to understand the mathematecal intracacies of physics while also being able to conceptualize and simplify them and then present them at a level that does make sense to the common guy.

I would like to point out that matter is primarily composed of space. In reality there is no solid matter but that in newton stars or such.

If you keep digging deeper where do you think this will end up ?

3v0

3v0 said:

If you keep digging deeper where do you think this will end up ?

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Well, you know, I've given that some thought and I actually have a pretty real answer.

Once I realized that sound is propelled by heat, defining the nature of heat and how it accomplishes that feat seems to be the next logical step. But, by doing that exploration "right", I believe there will be answers to a lot of other things forthcoming from it.

This is extremely simple-minded but, it seems likely that "the" universe (the one we exist in) may well consist of nothing except, nothing (space), energy (in the form of matter, molecular vibreation and electromagnetic radiation...all of which eventually end up as matter) and gravity. At least when getting down to the atomic level. I'm sure things get really hinky at lower levels.

Sound propagation seems to be conceptually explainable by the transfer of energy by the vibration of the molecules from one to another. But, there are some paradoxes to it that need to be resolved by figuring out how a medium that is transmitting sound acts.

So, to answer your question: I see it as going into a discussion of how air (and then other materials) work, at the molecular level. Then I see it extending to how the heat propels the sound and then how the heat itself works and that will pretty much end it.

Perhaps it's naive' but, I'm not convinced that there isn't a fairly straightforward explanation of how heat works. At least on the conceptual level.

For example, I had a glimmer of a flash of something that I believe must have been an epiphany for Einstein. That's pretty bold of me, huh? Of course, I'm no Einstein so, I got a muddled concept of it where it must have been a flash of inspiration for Einstein since it led to his equivalence of matter and energy.

Briefly, If atoms and molecules can transfer heat by collisions, there is a way to basically apply F=MA to describe the energy. But, if the molecules are vibrating at gigahertz rates (microwave frequencies), they must also be miniature radio transmitters, radiating some of the energy away as electromagnetic waves (radient heat). But, the amount of radient heat can be measured by the heating effect it has on other atoms and molecules. Because there is a defineable amount of heat in the radiated energy that can be related back to F=MA, and since radient energy is traveling at the speed of light, there must be a relationship between mass/energy and the speed of light.

For me, the glimmer stops there. For Einstein, I believe it led to e=mc^2. Of course, I could be completely wrong in my assumptions but, something led Einstein to that formula.

The more I get into this the more convinced I am that a science book needs to be written that spells out these concepts, unencumbered by mathematical contrivances.

There are 3 forces. Gravity is one of them. The other two are the weak and strong forces.

May the Force be with you.

3v0 said:

There are 3 forces. Gravity is one of them. The other two are the weak and strong forces.

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I'm digging back into my recollections of PBS science shows but, I seem to remember that there were 4 forces being touted. The 4th being electrical and the strong and weak being nuclear and then gravity. Of course that assumes that my recollection from a few years ago is still good enough.

But, are those forces actually germain to discussing energy and heat? Especially as it relates to something really practical and measurable like sound propagation?

crashsite said:

But, are those forces actually germain to discussing energy and heat? Especially as it relates to something really practical and measurable like sound propagation?

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I'd normally say no, but from the way you've been pushing this topic, probably.