Why Does Sound Propagate?

[Palladyne]

From the wording of your initial post, I got the impression that you thought that the waves always existed and just needed to be tapped into.

Not the waves but the frequencies themselves exist. How they travel is another issue.

If the question is: Is there some mechanism by which sound waves on one side of a vacuum can somehow appear on the other side. Well, who knows? Maybe there's some as yet undiscovered linkage between sonic and electromagnetic energy that would make it possible...or some completely undiscovered phenomena.

Yes, I believe so. If radio transmissions (frequencies) can be received by a mars lander, then sound (frequencies) can theoretically be propagated through the same medium. ---p

 

[notauser]

Nope sorry.. The waves respect the same equations but their medium is different.. Sound needs molecules to propagate, EM energy (light spectrum) propagates through space itself..
The only mechanism that allows things to be where they shouldn't, is quantum mechanics.. What you're talking about is called tunnelling.. Molecules themselves have a pretty damn small deBroglie wavelength though, so potential for that is beyond limited..

 

[crashsite]

Level of Understanding

And do you understand the intricacies of how any of those actually work? Have you worked with RL/RLC circuits?

That's not easy to answer. The glib answer is, "yes". I'm familiar with all those networks and have used and designed circuits using them quite a lot. Do I understand the "intricacies" of them? Well, certainly on an electronic technician's and basic engineer's level, again I'd say, "yes". On a physicist's or mathematician's level, probably, "no".

Further, I tend to think of them in a more "mechanical" way. Capacitors charging through resistances and the resulting dynamics of the voltage drops across the components and the interchange of energy between the inductor and capacitor in an LC tank circuit, etc.

I have a sense of the values of the components for different frequenies and periods, mostly from experience and not so much from having done a lot of mathematical modelling (although, certainly the design work I've done has involved applying math to some degree...if only to plug numbers into the formulas).

 

[crashsite]

Can we be certain.....?

Nope sorry.. The waves respect the same equations but their medium is different..

I'm reminded that the scientific community pretty much rejected Dr. Ohm's crazy, overly simplistic notions of the relationships between voltage, current and resistance when they were first advanced....

 

[notauser]

You're going backwards really fast if you think physics just hasn't found a way to send sound through space.. Space has pretty much no molecules..

 

[crashsite]

You're going backwards really fast if you think physics just hasn't found a way to send sound through space.. Space has pretty much no molecules..

I'm just saying that science just may not have caught up with science fiction...yet. For example, it may not be completely out of reason to define "sound" as the sensation of sound in our brains rather than as the mechanical machinations of molecules and, perhaps the electrical auditory nerve impulses can be transmitted through a vacuum...even via ESP...to distant recipients in a way that allows them to "hear" the sounds.

So, I'm just asking if it's reasonable to reject any notion or theory, as being impossilbe or beyond the realm of science, just because it seems totally crackpot?

But, while leaving the door open, it's certainly reasonable to concentrate on known physical laws to explain things (and, if they build and branch out into other, unexpected realms, then you deal with them as the come up).

Meanwhile, even though it makes good reading for the science fiction buffs, science fiction is still that...fiction.

I do NOT want to get into the psycho-cybernetics of sound propagation here. Would rather stick with the known and accepted.

 

[notauser]

Nope, that would be electrical nerve impulses, or ESP propagating through a vacuum and stimulating your auditory region.. Sound is propagted by molecular motion.. We aren't wrong on that because we defined it.. Even if there was a way to use say EM energy to oscillate the molecules adjacent to the ear from a distance, it would only be a technological breakthrough applying EM and sound principles.. On the otherhand if we find something entirely new like sub-space on star-trek, then it will get its own distinction like EM/Sound..

 

[crashsite]

Balls!

Nope, that would be electrical nerve impulses, or ESP propagating through a vacuum and stimulating your auditory region.. Sound is propagted by molecular motion.. We aren't wrong on that because we defined it..

Ah, it's so easy to deal with science when it's science fiction and any crackpot theory is as valid as any other (much like religion...and, NO, not to open that can of worms). But, science fiction aside:

There is still the mechanics of propagating sound. There is obviously some real and tangible mechanism that puts an air disturbance, essentially intact, approximately 110 feet away from its source a tenth of a second later. If that mechanism is related to SHM, then fine. I can accept that...if I know how SHM...or resonance...or restoring forces...or fluid dynamics...or aerodynamics...or longitudinal wave theory...or any or all work in conjunction with each other or with other forces and actions as yet unmentioned.

For example, in the balls on strings executive toy, I can see a number of actions. I see a penulum action. I see energy being transferred. I can see how one ball bounces one ball at the end and two balls bounce two balls and so on (whch I see but, do NOT understand the mechanism of). What's still not clear, even in that mechanism is the delay (which seems pretty nearly instantaneous to the human eye and brain) of getting the action of the first ball propagated through the intervening balls to the last one. I can't be sure but, I suspect that the mechanism of that may relate to the propagation of sound.

BTW: Well, into the 19th century (possibly even into the very early 20th), the atom was defined as "pudding-like" with the electrons floating around in it like raisins so, merely defining something may not be the final word in how it works...

 

[crashsite]

Frequency?

Sound propogates depending on frequency and sorroundings of the environment.

There are certainly environmental factors that affect the way sound propagates (the medium, temperature, etc.) but, I'm not sure that frequency affects the mechanism of sound propagation.

For example, if sound travels at 1100 feet per second in the air, a 100 Hz signal and a 10,000 Hz signal will both travel outward from the source at 1100 feet per second.

 

[notauser]

He doesn't know what he's talking about.. He's just making statements that are too vague/incomplete to declare right or wrong, but I'm leaning towards wrong..

 

[crashsite]

Would someone please tap that grammophone tone arm?

He doesn't know what he's talking about.. He's just making statements that are too vague/incomplete to declare right or wrong, but I'm leaning towards wrong..

Let me try coming in on this at yet another direction. You say that sound waves are lonitudinal. Of course that makes sense whether you think about an air disturbance (sticking with air) is cyclical or not. I suppose the notion that there are compressions and rarefactions of the air molecules could lead one to assume some sort of traverse action (if only as a way to put more or less molecules into the same space) but, it's pretty easy to visualize the disturbance changing as it moves outward from the source in a longitudinal manner.

Unfortunately, that visualization does nothing to explain why the sound is moving outward from the source in the first place (much less, why at 1100 feet per second). In the most empirical example, it would make sense that something like a speaker cone would simply move air back an forth as it pushes and pulls on it. Sort of like the action you see when you alternately blow and suck on a straw and watch the bubbles simply move up and down in the staw without any particular proclivity to propagate anywhere else.

I know I sound like a stuck record but, I keep coming back to the same question: What mechanism makes the air disturbance propagate and the ancilary question of why, at the speed it does?

I get that the air molecules colide with each other and, in that manner, impart energy and that one could visualize the sound propagating outward by that mechanism during the compressions but, it doesn't address why the sound also travels outward from the source (with similar characteristics) during the rarefactions. It also doesn't do a good job of explaining why there's a "copy" of the original disturbance out there in a space at a distance dictated by the speed of sound and a time of transit.

 

[chconnor]

can't resist.

Wow, I'm nervous to post in this thread, given the crazy ground covered, but I find that I can't help myself, for a couple reasons:

- chaos attracts me, and this thread is nutso

- I strongly agree with crashsite (if I may put some words in his mouth) that there is a dangerous tradition among intellectuals (big and small, but maybe more often among the small ones) to equate mathematical modeling with understanding; i.e. "I understand the manipulation of an abstract grammar, and have a clear picture in my head of that intangible symbol-machine, and I can use it to make accurate predictions about the physical world; the nature of that world in front of my tangible eyes presents no psychic discomfort to me because I have a comfortable internal metaphor for it, even though that metaphor is essentially nothing more than an equation that happens to fit the data set". There must be a classic name for the difference in philosophical school between those people and others who want something more to hold on to. I'm not talking here about spiritualism, that's obviously a different subject, and I'm not implying that crashsite demands that any property of the universe fit inside his brain or else he eyes it suspiciously. I'm suggesting that maybe some of the string theorists and particle physicists get a little caught up in the math and the empirical correlation of their data. Since there is only a handful of people on the planet that understand that math, I'm obviously speculating, but even my high school physics compatriots fell into the trap... Seems like the world of electronics and acoustics are especially rife with this: "but why does 3-phase 240V blah blah reduce the current in blah blah inductance blah blah?" Answer: "well, simply apply Ohm's law like so..." It's circular logic: they're saying "phenomenon X happens because of equation Y that was developed based on the fact that phenomenon X happens".

It's taken 12 pages so far and crashsite is still unsatisfied. Far more informed minds than mine have attempted answers, but I had to throw in my hat!

(*Let me say that I in no way accuse intellectual people across the board of that stuff, and obviously in some cases equations not only predict results but function as powerful -- and obviously useful -- metaphors for the physical world.)

Here's my own head-model of sound waves, for whatever it's worth. Some or all of this may be misguided. I'm no expert, I just love an explanation challenge:

With air, I imagine a space filled with particles that are repelled from each other (and by virtue of those repelling forces, effectively "attracted" to voids).

These molecules are "jiggling" very rapidly: they are moving, and the kinetic energy they possess (temperature, I'm told) has them traveling in one direction a very short time only to bounce off the field of a neighbor. This motion, which I'll call "random" at the risk of falling off an existential cliff, doesn't result in displacements or shifts on any kind of large aggregate scale. I think in the "crowded room of people" metaphor (not a great metaphor), this is everyone shuffling around in place, but not walking anywhere. If everyone just so happened to shuffle in the same direction at the exact same time, that'd be a different story, but it's a statistical wash, and no such thing happens.

I hold that the dynamics of the repelling and void-seeking are responsible for the particulars (putting aside who-knows-what myriad other sophisticated forces at play), and that this is a knowable system by non-mathematicians (like me, and crashsite). Now, this seems to be the crucial point for crashsite: clearly, our repellent molecules are going to shift when perturbed, but why would they shift the way they do, at the speed they do, etc.

I liked the "long rod" image brought up before. Let me modify it:

Imagine a series of foot-long rulers laid end-to-end for hundreds of feet. There is a one inch gap between each. Pushing the first ruler at a steady rate means that the "gap closing" will move at a much faster rate along the line of rulers than the original push does. (Move three inches, and the "gap closing" is already happening ~3 feet away, a factor of ~12 "faster"). This shows nicely how the propagation rate doesn't relate in a simple way to the perturbation's rate. Of course, these rulers were massless and frictionless.

Now, abstracting a bit, instead of there being a hard edge to the rulers' interactions, their edges have a magnetic repulsion to each other. The same effect can be seen, but no contact is needed.

However, these rulers also have mass, and therefore inertia, and they take time to move away (ignore friction in this universe). You can see that after pushing the first ruler an inch and then holding it there, time is taken for the subsequent rulers to move to their preferred distance. They are still moving one at a time, but now it's kind of blurred: the first ruler is pushed maybe half an inch forward and just held there. As the first "push" ruler moves, the second starts to move away, and the third starts to move as well, before the second has totally settled into its final position, etc.

The interplay of these dynamic forces is crucial: how strong the "spring" repulsion is, how far out its effect reaches (e.g. can a given ruler "feel" just the neighboring ruler, or two rulers away as well?), how much inertia the ruler has, how fast the initial push was, and so forth.

But it's clear that in this line of rulers, if one was to push the first and watch this "wave" propagating down the ruler chain, and one wanted to then take that wave back, one is lost: that wave is out of reach of the first ruler. It can only generate new waves that will follow the original.

Why does the "wave" propagate? It's simply the system of rulers trying to rebalance itself. Or, more microcosmically, it's the result of localized imbalances of forces: they propagate directionally, in this case, because that first ruler has set a new edge they must react to. The first ruler may move back in the future (like a speaker cone returning to rest or beginning a rarefaction cycle), but the propagation has already happened.

If one pushes the first ruler, waits for the second ruler to start to move, but then releases the first ruler before the second one has finished moving, the first ruler starts to move backwards a bit because of the mutually-repelling force from the second ruler, which has not yet achieved its preferred distance from the first, even though it's on the move. So if one wants to wait until there is no danger of the first ruler moving backwards, how long must one hold the first ruler in place? Even with an infinite line of rulers, one must wait forever, actually, since the backwards force is a limit approaching zero. (Back on Earth, of course, there is a noise floor of jiggling, there is damping, and all kinds of things happening that mean you don't have to wait forever.)

Note also that jiggling a given ruler a tiny bit side-to-side, very fast, (e.g. less than a millimeter, at 5000 Hz), has almost no effect on adjacent rulers, because they are too sluggish to react in an appreciable way (though they do of course react).

Note also that after pushing the first ruler and holding it, the second ruler may accelerate on it's way to the third ruler, and the gathered speed may actually result in the second ruler "bouncing" back from the third ruler, depending on the springs and how they function over distance, etc. As a result, the character of the "push" of the first ruler (sharp? slow? happening in gradual steps?) may not be perfectly translated into the subsequent rulers, depending on all these factors, and the balance of masses and springs and so forth may result in "bouncy" behavior amongst the rulers instead of a clean "wave" going down the line. It all depends on the parameters of the rulers and the nature of the perturbation.

If these rulers also have an attracting ability when they get too far from each other, you can see the analogous effects when creating a rarefaction with the first ruler.

The continual interplay of these forces is obviously "complex", and hopefully it's clear that it can result in behavior that is difficult to wrap one's head around, or at least to predict casually. For example: rulers at opposite ends of the chain are moved, and two waves come towards each other. Visualizing how they pass through each other without affecting each other is kind of a head-trip, but it works out if you play it out.

Thinking of this cascading effect moving down the chain is a human-applied metaphor: is it a "wave", "moving through a medium"? Sure, whatever, Human! It's stuff reacting to stuff. So when those two waves in the ruler chain come at each other, are they "passing through each other without affecting each other" or are they "bouncing off each other" or are they trading some energy, dancing the Charleston, etc? We can look at it how we like, but it's stuff reacting to stuff.

The first ruler may move at a certain rate, but the second ruler moves at whatever rate the spring/inertia system dictates as a result of the first ruler's movement (may be slower, may start out slower but then actually get faster than the original movement as momentum gathers, etc). The third ruler is again once-removed from the nature of the original push. Conceptually, the energy of the initial push lasts forever, but as it travels down an infinite ruler chain, it gradually fuzzes out (the area of compression or rarefaction becomes wider and shallower as it travels) until it gets lost in the jiggling. There is still a real "push" happening through the system, but it's so small as to be unmeasurable. (Of course in the real world of energy bouncing around there's a lot to get lost in.) This maybe also explains why lower frequencies travel further, since the lower the frequency the less it depends on the next ruler to "catch up" to the movement in time to represent.

The crucial point is: the interplay of forces and masses involved (the nature of the attraction/repulsion springs, the mass of the rulers and their concomitant inertia, the average gap between them) results in the "speed" of the "wave" down the chain, results in what energies and frequencies we consider "jiggling" and what we consider "displacement", and results in the rate at which the character of the initial push is lost as it "fuzzes out", and so on. If you tweak these variables you can see that the system is going to act wildly different: imagine mile-long rulers, or millimeter-long rulers; imagine springs that keep the rulers a mile apart, or just a millimeter, imagine springs that don't have a linear relationship of force-to-compression, imagine rulers spinning around, and so on. This tweaking results in very different pictures of how different initial pushes are treated by the system, whether or not they propagate, and how.

I think this all extrapolates pretty clearly from one to three dimensions. Things certainly get more complicated, but it's the same ideas. An air molecule's graduated "sphere of influence" is the volumetric extent of its repellent field (which has a measurable size, just as the ruler and the ruler's magnetic repulsion has a measurable width). "Air pressure" is the degree to which all those molecules are pressed together, compressing the springs. Temperature is the average jiggling kinetic energy the system already has as they bounce off each other (or at least shake around a bit) in no particular coordinated direction (I'm sure there's something wrong with my definition of temperature, there, but suffice to say...) The idealized compression "wave" is a disturbance of a sufficient number of those jiggly molecules in aggregate that radiates out from its source as a growing sphere. It's all springs and masses bouncing around. (Why does sound travel faster or slower in X, how does temperature affect it, etc. Given all the above, these questions are clearly going to be non-trivial to answer, but perhaps educated guesses could be made just from the above descriptions.)

So the essential question, of "how and why does sound propagate through air" perhaps could be answered thus: The balance of the forces of repulsion of molecules from each other, the character of that repulsion, the inertia of the molecules, and the average spacing they have from each other, results in a dynamic system of particles amongst which disturbances of appropriate magnitude and speed (if we're speaking cyclically, frequency results in speed) will result in perceptible propagation of a "wave" through that system. Disturbances of too-little magnitude, or too-high frequency at a given magnitude, or disturbances that for other fluid-dynamic reasons result in non-directional turbulence instead of coordinated compression/rarefaction, won't."

As to "how is the perceived integrity of the sound maintained through space", that answer also seems to follow... if you grok how one "wave" through the rulers model could "pass through" a second wave coming the other direction, then it should be clear that whatever pattern of pushing/pulling happens on the first ruler is rolling down the line of rulers (again, crucially, tweak the springs and masses in your mind until this becomes plausible). The waves bounce off the theoretical wall at the end (the last ruler is repelled by the wall, of course), and the "reflected" "waves" "pass through" each other on the way back to the first ruler where, if they haven't been overly damped or otherwise degraded, they reasonably replicate the motion of the original ruler (actually, the opposite of the motion, but it's the same difference with sound).

I think the crux of your confusion is your internal tweaking of the springs, inertia, etc, of air molecules. Your internal system's variables are not dialed-in in a way that lends itself to seeing this phenomena happening in air. Try being playful with the variables and see if it doesn't make more sense.

Nervously awaiting your response,
-C

P.S. It's funny that I found this thread, because I was recently running through some calculations about speaker cones: I was wondering how close the front of a speaker comes to the speed of sound when moving. Turns out, not very close (at sane sound pressure levels, anyway). Phew. Sound is definitely a trip, for all kinds of reasons. The aspects that confuse me are the frequency-domain type analyses: talk about people using equations as bogus explanations, geese.

 

[3v0]

First there is no pleaseing crashsite.

Second will wait for the Readers Digest Condensed version.

 

[chconnor]

...

Second will wait for the Readers Digest Condensed version.

Yeah, it's crazy long. It took so long to get down I didn't have the energy to edit it...

-c

 

[3v0]

Yeah, it's crazy long. It took so long to get down I didn't have the energy to edit it...

-c

LOL

I skimmed it. Sounds like you and I are on the same page.

 

[crashsite]

The Curmudgeon Speaks (again)

First there is no pleaseing crashsite.

That's not true. A more accurate statement would be that "there's no pleaseing (sic) Crashsite with 'canned' explanations that don't answer the question or that attempt to 'answer' the question with formulae that merely quantify the phenomena".

I do feel that some progress was being made with elements of, notauser's SHM approach but, I feel (perhaps incorrectly) that it while it was doing a good job of explaining the "wave aspects" of sound, it wasn't getting to the heart of the issue of the "propagation" of the sound. I'm still hungry for info on how the SHM explains the propagation aspects.

While I'm not sure that I agree with all that, chconnor says in his epic, I like and appreciate his approach. That is, to apply commonly known laws of physics to the question and then to try to come up with an explanation of how they might work to satisfy the question.

I've been continuing to mull this question and came to some of the same conclusions as, chconnor but, I haven't distilled it down to a "postable" dissertation (ie: one that wouldn't be so 'rambling' and 'disjointed' as to do nothing more that royally piss off the regulars here).

While it may be the blind and ignorant leading the blind and ignorant, I have renewed hope that posting my analysis may at least have a critical audience and hopefully can get some renewed general interest in getting to the original core question..."Why does sound propagate".

But, I still do need to structure my follow-on efforts to try to keep a good focus! That translates to some degree of delay. Patience all.

 

[crashsite]

d=rt (oh no...math!)

With air, I imagine a space filled with particles that are repelled from each other (and by virtue of those repelling forces, effectively "attracted" to voids).

These molecules are "jiggling" very rapidly: they are moving, and the kinetic energy they possess (temperature, I'm told) has them traveling in one direction a very short time only to bounce off the field of a neighbor. This motion, which I'll call "random" at the risk of falling off an existential cliff, doesn't result in displacements or shifts on any kind of large aggregate scale.

I believe that this is the crux of the whole sound propagation thing. But, a piece of information I am missing and can't seem to find (at least, I haven't been able to pose a question either here or in any of the search engines I've tried, that leads to anything even approaching the answer to the missing piece). The question is: Are the air molecules (let's stick with, air just to avoid having to be constantly referring to, "the medium", "the substance", etc.)...anyway, are the air molecules always moving at Mach 1?

We know they are moving at some rate so, why not Mach 1? Yes, I know I've already posed this question but, it lies so centrally at the baseline of my analysis that if the answer is that they do NOT move at Mach 1, it will cause me to have to completely revise my thinking. And, that's okay...so long as it's based on sound (no pun intended) physics.

User, notauser says that the molecues are moving at gigahertz rates and that sounds pretty fast until you think of them also moving only micro-inches. If you work out the rate of vibration and distance of travel, for a given temperature, are each of the individual molecules intrinsically moving at Mach 1?

If they are, then the sound propagation thing becomes a question of vector direction. In undisturbed air, all the vectors sum to zero (within the miniscule granularity posed by chconnor). Any disturbance then gives a vector bias to the molecules. Since the molecules can only travel at Mach 1, as they collide (within the atomic level definition of atoms being able to collide), they impart this vector bias to the next adjacent molecules.

If the air molecules are not traveling at Mach 1 (especially if they are allowed to travel at random rates for a given temperature), the whole theory falls apart.

 

[3v0]

I do not see the mystery.

When no sound or noticable displacement of the air mass is present, the speed of air molecules is dependant on the air temperature (by definition). Further we can only talk about the average speed. The speed of this movement only relates to the speed of sound in that it alters the whay in which the molecules interact. The average of the all displacements is effectivly zero.

The crux of sound propagation is the ability of each molecule to influence the position of nearby molecules from a distance. This allows a slow moving speaker cone to indirectly displace molecules at the speed of sound. Again the average of the all displacements is effectivly zero, but over a larger period of time. Chconnor's row of rullers helps to illustrate this if you think of the molecule as a point at the center of the ruler. The body of the ruller provide the interaction between the points.

The only qustion that remains is in regards to the forces that cause molecules to displace each other at a distance.

 

[crashsite]

"Steady as she goes, Mr. Hardy." (Admiral Nelson)

I do not see the mystery.

The crux of sound propagation is the ability of each molecule to influence the position of nearby molecules from a distance. This allows a slow moving speaker cone to indirectly displace molecules at the speed of sound. Again the average of the all displacements is effectivly zero, but over a larger period of time. Chconnor's row of rullers helps to illustrate this if you think of the molecule as a point at the center of the ruler. The body of the ruller provide the interaction between the points.

The only qustion that remains is in regards to the forces that cause molecules to displace each other at a distance.

I can see serious problems with this approach and it's also the parts of chconnor's dissertation that I tend to...I wont say, "disagree with" but, rather question the completeness of. It's obvious, both from a thought excersise view and from known physics, that the molecules are both moving and interacting. We agree there, right?

The problem is having molecules that are vibrating at random speeds somehow imparting a very specific sound propagation rate and, worse, a rate that is much faster than the proverbial speaker cone is traveling. In your analysis, why doesn't the sound propagate at different speeds depending on the rate the speaker cone is moving? Why does it, in fact, always move away from the cone at Mach 1, regardless of the rate of the air disturber?

You answer almost matches mine in that you conclude that the speaker is moving the molecules at some specific rate but, in your answer, at the rate of the speaker cone. But, then the effect sort of magically (through some undefined process) zips away at Mach 1. I'm saying that the mechanism for the sound wavefront to move away at Mach 1 is the crux of the issue of sound propagation, not the local movement of the molecules by the speaker cone...or the displacement of the rulers.

(add on) To put it perhaps more succinctly, it's not how the speaker cone moves the air molecules at that interface but, rather how the air molecules then move each other to propagate the effect.

I think that's where the usual science teaching falls down and why everyone here seems to be so hung up on the wave nature of sound rather than those instantaneous effects that cause it to propagate.

 

[Sceadwian]

Because it's not the velocity of the cone that produces the speed of sound per say it's the pressure that builds up from the speakers displacement of the medium, the pressure wave itself is what travels. The speaker compresses the air in front of it and then releases that pressure very fast, because it does this relatively quickly high pressure pulses are created, it's those pressure waves that are actually propagating and that's only limited by the speed of sound in that medium.