Why Does Sound Propagate?

[Sceadwian]

Dk you may want to put as mall caveat in your explanation. As air molecules do in fact have to move for sound waves to propogate, sure they move forward and then back, aside from friction losses the net energy lost is zero, but air still has to move, it's still the medium

 

[dknguyen]

The foam is moving is a vibratory sense. Because you are smacking it, causing starting off an oscillating motion oscillating motion of the block as well as a compression. The slight forward movement from hitting it causes a rebound making it sort of oscillatory. Both this movement and the resulting decompression (from compression due to hitting) both serve to comrpess the next foam block.

But I do agree, there is something strange in accounting for the actual oscillatory motion. It requires the foam blocks to decompress away from where they were hit in order to pass on the compression, but also requires that the foam blocks decompress towards the direction in which they were hit in order to provide the oscillatory movement. Or it requires them to slide a bit in the direction they were hit (in addition to the compression) and then rebound back with some overshoot to their original positions either due to the next block doing the same rebound, or the next block decompressing backwards a bit or both. Perhaps, jello would be a better material to use in the analogy- something that is compressible but also has easily observable vibratory motion when hit.

Also, remember that the foam or jello blocks are supposed don't represent the molecule per se. It represents the molecule and the surrounding empty space it effectively occupies from ultra-fast kinetic movements. So a general movement of the block translates to an average net movement of the molecule, not the actual movement since it's buzzing around every which way inside the empty space. And a compression of the block represents an increase in pressure as the molecule is limited from it's normal range of motion by entering and/or impacting the "free space" that "belongs" to other molecules.

But my main point is to try and say the decompression is what propogates the pressure wave after it has been formed (another way to think of it is the fluid is trying to equalize pressure distribution by sending high pressure into areas of low pressure), and that the rate of decompression which determines the speed of the propogation of the wave is independent of how the actuator formed the pressure wave in the first place. I can very slowly compress a block of foam/jello, or I can very quickly compress it the block of foam/jello. BUt upon release, I have no control over how quickly it decompresses- that's soley up to the material properties itself.

 

[dknguyen]

I had thought of this but, then hit a snag when colder, denser air gives a slower speed of sound. That's when I started thinking about the relationship between temperature and the vibrational speed of the molecules and how that might explain the temperature vs. Mach speed.

Is colder air more or less compressible than warmer air? I think compressibility is at the root of the speed of sound, but compressibility is hard to visualize and denser gasses might have a tendency towards compressability that increases the speed of sound. So people generalize to density rather than compressibility which might hold true for many cases, but not all cases.

I am thinking mediums with lower compressibility will have a faster speed of propogation.

I am reading this:
http://www.vias.org/genchem/kinetic_12450_03.html

And in the absence of interactions between particles, a warm gas might be more compressible than cold air (more change in volume for applied pressure), but cold air *might* be some attractive interactions between the molecules which might outweigh the higher density causing more compressability than warmer air. Of course, this is all conjecture as I have no found an article yet that actually relates the two directly.

EDIT:
Here is your answer for that anomaly with cold/warm air densities and the speed of sound. It seems to imply that compressability and density are NOT implicitly related (though they may have that tendency) and the speed of sound depends on a balance between the two:
http://hypertextbook.com/facts/2000/NickyDu.shtml

"The speed of sound is determined by the density (ρ) and compressibility (K) of the medium. Density is the amount of material in a given volume, and compressibility is a measure of how much a substance could be compacted for a given pressure. The denser and the more compressible, the slower the sound waves would travel. Water is much more dense than air, but since it is nearly incompressible the speed of sound is about four times faster in water than in air."

 

[crashsite]

Compressibility

Is colder air more or less compressible than warmer air? I think compressibility is at the root of the speed of sound, but compressibility is hard to visualize

Again, my understanding of this is slight but, I think there's some general confusion regarding the compressibility thing. In aerodynamics we are told that air is an incompressible fluid but, anyone who has ever bought an air compressor knows that you can compress it just fine. If (big "if") I understand it, the difference seems to be that in an unconstrained environment, because the sonic wavefront can move away from the compressing force faster than the air can be compressed, the air cannot be compressed. This has beaucoup implications, especially as regards this thread and I don't know what they are. But, I think it may be a pretty central element to understanding the sound propagation thing.

The other side of the compressiblity issue is the rarefactions and you do question that in your, "foam" theory. As regards sound propagation, both compression and rarefaction act exactly the same...they zip away from the disturbing force at Mach 1 (and do so with the characteristics of that disturbing force remaining intact).

I'm no wheres near ready to repostulate my thoughts on all this but, I will say that there are more and more facts and factors that are leading me to lean more and more towards the notion of my, "vector bias" theory.

 

[crashsite]

The Pendulum is Like an Albatross Around My Neck

A pendulum swinging.... it takes the same amount of time for the pendulum to swing back to center whether you give it a strong initial push or a weak one because of the "self-balancing" effect:,,,

I don't like the pendulum. For the same reason that I was resisting, notauser's SHM assertions. It's naturally oscillatory. The very words, "simple harmonic motion" fairly scream, "OSCILLATION"!!! and that implies a lingering effect.

But, sound propagation is so free of such effects that two sound wavefronts can pass through each other without so much as one affecting the other. Here's a quote from a NASA site (emphasis mine):

"Disturbances are transmitted through a gas as a result of collisions between the randomly moving molecules in the gas. The transmission of a small disturbance through a gas is an isentropic process. The conditions in the gas are the same before and after the disturbance passes through. Because the speed of transmission depends on molecular collisions, the speed of sound depends on the state of the gas. The speed of sound is a constant within a given gas and the value of the constant depends on the type of gas (air, pure oxygen, carbon dioxide, etc.) and the temperature of the gas."

Any thoughts about restoring forces implies interaction and oscillation and that gets back to that "wave analysis" bias that is so strong...I believe, due to the way that science is taught in school.

dknguyen seems to attempt to dampen out the oscillatory nature of things by using foam but, is that just a damping mechanism rather than an acknowledgement that the oscillations don't occur in the first place?

 

[dknguyen]

I changed it to jello where you can see both the compressability and the oscillation. I wasn't using foam because of the dampening effect, I was trying to use it to make an analogy for compression and decompression.

 

[chconnor]

I don't like the pendulum. For the same reason that I was resisting, notauser's SHM assertions. It's naturally oscillatory. The very words, "simple harmonic motion" fairly scream, "OSCILLATION"!!! and that implies a lingering effect.

Although it is definitely a rough analogy, there are similarities: the "pendulum effect" also applies to oscillating masses on springs. These particles are indeed masses on springs, where the "springs" are the forces between them. So whatever a pendulum may scream, the basic idea is good; the main difference is that there are multiple masses on springs all linked together. The main simplification (among many) is in reducing the series of masses on a series of springs down to one mass/spring system at a time to try to see why the disturbing/restoring forces balance. If you had one particle sitting between two immovable particles and disturbed it, it would bounce back and forth (oscillate) (talking here in an idealized universe).

But, sound propagation is so free of such effects that two sound wavefronts can pass through each other without so much as one affecting the other.

You said "oscillation implies a lingering effect", by which I guess you mean that a given particle vibrating would keep vibrating, and since it doesn't, it must not be oscillatory, and thus the pendulum analogy must be bogus. Again: the point is not that the particle is oscillating like a pendulum, it's that there are similar balancing effects as in a pendulum system, specifically that restoring force is proportional to displacement. With sound, the initial displacement pushes against the next particle (or ruler, or chunk of foam). Since the system follows a similar pattern as with a pendulum, the initial push meets a restoring force proportional to its displacement. As a result, fast/large displacement takes the same amount of time to balance as a slow/small displacement. With the pendulum, "rebalance" means oscillate back to the center, maintaining the kinetic energy. With sound, "rebalance" means convince the second particle to move, adopting the kinetic energy itself. So, we're selectively taking only the "restoring force proportional to displacement" part of the pendulum analogy; otherwise, in sound, the force is transferring from one particle to the next, so you don't see the continual oscillation as with a pendulum.

Any thoughts about restoring forces implies interaction and oscillation and that gets back to that "wave analysis" bias that is so strong...I believe, due to the way that science is taught in school.

Agreed, it's a slippery slope. That's why I've been harping on a single pressure front instead of "5k tones" or whatever, because talking about oscillating sources biases us strongly to pendulums, sine waves, etc, and what we're really trying to get at is why, on a physical level, the pressure propagates at Mach 1. But restoring forces are nonetheless crucial to consider, I think. And even if I am right, the actual interplay of forces is clearly a lot more sophisticated than the model I've described, and I'll say again that its behavior may not be representable by a simple metaphor.

-c

 

[crashsite]

Got it!

Okay, I think I got it figured out. I was pretty confident of the “vector bias” version and it turns out to be right (can that be a raised eyebrow I sense?). The following analogy will show fairly simply and mechanically how it works. There are a few loose ends but, they are minor and in the details.

Imagine a large pool table covered with a random distribution of balls. The table is large enough that the balls never reach the cushions within the lifetime of the example.

You come in with a cue stick and randomly strike a ball at a random angle but, with a known amount of energy, representing a heat value. The ball will travel away from the cue stick and may hit other balls which will then also move, possibly striking other balls. The balls will remain in motion until friction eats up the energy and the process will stop. There is no restoring force; nothing to make the balls want to go back. They simply stop until the cue is again used to add some energy to the system.

But, if you continually add energy by walking around the table poking at the balls and you do it with sufficient frequency, you can keep the balls in motion. Of course, each time you use the cue stick, it’s always with the same force (temperature) but at a random direction and in a random place on the table. That means you don’t know which ball will be hit or at what angle. Sometimes the cue may merely strike thin air. But, on average, what you’ll end up with is a table full of balls in motion, moving in random directions and at random speeds…but, within a fairly narrowly defined speed range. The speed range being determined by the cue force, mass of the balls and frequency and the friction losses. Done right you can develop an equilibrium.

There’s still no restoring force. There’s nothing to make any of the balls tend to return to anywhere. There’s merely a continual addition of energy keeping the system active. To paraphrase, Martin Luther King: ‘Free at last. Free at last. Thank God Almighty! Free of SHM and restoring forces at last!’.

While the instantaneous speed of any given ball is random, there is an average speed for the system. This is important to keep in mind because you need to consider both the micro and macro implications of it.

If you have an additional force that you can add to the system from some known direction, you can influence the movement of the balls. That force may be your pesky brother-in-law messing with your experiment by poking at a ball with his own cue stick. The overall movement of the balls will still be random but, now with a directional bias. At least some of the balls will move away from the pest’s cue as they are struck by his ball and other balls that have also been struck or otherwise influenced by it. In fact, that bias will continue to move through the system of balls until the frictions of the table dilute and finally eat up the effect.

The balls ahead of the effect are still random. In order to act like sound propagation, the balls behind the effect also need to return to true random motion. That’s one of the loose ends and I don’t know enough about the process or the math that would be required to calculate it to answer how it would work…or indeed…if it does work that way.

So, if the propagation of the effect of the pest’s cue represents the travel of a sonic wave front through the system, on the micro level (across just a few molecules), the speed of sound could be any speed within the range allowed by the individual molecules in the system. But, on the macro level (across a lot of molecules), it would average out to the Mach speed predicted for the medium and temperature.

All that sound about right???

 

[crashsite]

Maybeline for Gnats

Speed of sound is ~340 m/s... i wouldn't say that's close?

Yeah, a gnat's eyelash. I was envisioning differences of thousands or possibly millions of times difference or more.

There's also likely to be some difference between the rate that the molecules are bibrating and the rate at which they impart energy to adjacent molecules. Don't know enough about the physics to say...yet.

 

[chconnor]

Thank God Almighty! Free of SHM and restoring forces at last!’.

Ok, let's talk pure particles. I like where you're going here, but of course I'm required to give it my spin. It now makes sense why you kept talking about temperature in this context.

First, we know that at any realistic levels, the speed of the average disturber (say, a speaker cone) is well under the speed of sound. Even at crazy volumes a speaker cone might be moving at Mach 0.5 for the highest-frequency / most-deafening sounds (my own very rough estimations, here), and usually far less (Mach ~0.005 for a hard pluck on the low string of a standard acoustic bass guitar, and a lot less for the resulting sound board vibrations). So the sound wave propagates faster than the initial disturbance, and not only faster, but at a constant rate for all initial disturbances, fast or slow (we know this, i know).

I like to think of the Pest sweeping a board on the table and pushing a lot of balls at once. The preserved momentum is indeed transferred through the balls following the disturbance; following the initial push, a "crowded zone" is created as the balls are pushed into an area that has balls in it already. The momentum of this aggregate wave, which determines how crowded this area gets as it is starting to propagate (aka how much "pressure" it has), is transferred to the next layer of frenetic balls when those balls impact the crowded area (impacts are the mechanism of momentum transfer). The frequency and rate of those impacts, and thus the rate of propagation of the momentum, is determined by the temperature. [This is basically a restatement of what you suggested, I believe.]

Faster or slower initial disturbances do have more momentum, and in the short-term view that may even result in balls moving faster or slower (i.e. if the Pest strikes hard with a cue, the momentum transfers faster than if he strikes slow). But soon the faster push of the Pest's board results in a more crowded region of balls: the momentum is distributed over more balls, so that it is still preserved, but each individual ball has a correlating slower speed. Then, the propagating factor is the much-faster effect of an ocean of active balls bumping against that pressure wave and transferring the momentum in the process. So I would say that the "balancing effect" is the way the faster initial push is distributed among more particles (which is really just a restatement of what i said earlier: that faster initial pushes result in higher pressure, and with more mass to move, the "restoring force", which in particle-land is just the tendency to equalize average density, balances).

So, you wondered at the end of your post how the behind-the-wave particles get back to rest. The balls behind the wave are back to normal because their momentum bias was passed on when struck. Since they were biased they were more likely to get hit from the opposite direction of that bias, and since the ocean of balls is moving rapidly (compared to their disturbance speed) this transfer happens efficiently. Higher temp = faster propagation because the momenta are transferred more often through more frequent impacts.

So the animation on page 2, and all the similar animations we've ever seen, are somewhat deceptive because they show the particles as essentially still except for the wave passing through. But it's understandable that teachers would accept this basic level of abstraction: treat air as a collection mass/spring systems, since in aggregate it behaves that way, and it's easier to visualize the passing of energy in such a system (for some).

As an aside, on the subject of the range of molecule speeds, see this - scroll most of the way down, find the graph and check out the blue line - the distribution of speeds.

I don't know about anyone else, but I'm learning a lot in this process (especially on the relationship of temperature to the propagation). I've certainly revised the way i think about sound a few times as a result. Kudos for pressing the issue, crashsite.

-c

 

[crashsite]

I'm beginning to see a light at the end of the tunnel...

It now makes sense why you kept talking about temperature in this context.

Yeah, chconnor. I think you're "getting it". But, rather than trying to do a bunch of quotes and responses, I'm going to just bull ahead here and generally follow your "spin" while continuing to add.

Of course, the speaker cone acts more like a board that sweeps gobs of molecules (already moving...or, more accurately, vibrating and knocking each other around) into the body of the molecules (which are also already moving) so, you have to think of the process on a much more macro scale than my simplified version.

But, whether you think about a single ball (particle) as the disturber or a whole bunch of them, the action is the same. But, you can't fall into the trap of allowing yourself to fixate on the molecules that are swept into the melee by...say, the proverbial speaker cone. You have to think about what happens once they get there and, here's why.

So far we've concentrated on the compression excursion of the speaker. It's easier to think about the actions and reactions acting on something rather than on nothing as we need to do on the rarefaction excursion of the speaker. But, from the sonic wave front's POV, it's the same. That's because the propagation of the wave front is not accomplished by the molecules entering the melee...it's accomplished by the molecules that are already there. It's sort of easier to visualize on the rarefaction side:

On the rarefaction excursion, molecules at the speaker cone interface are sucked out of the melee and it's the thermal activity of the system driving the molecules into the void that not only occurs but creates an effect that continues to influence the system and that influence is what propagates away as the sonic wave front. I know it's a little harder to get your head around having the void propagating away from the disturber at Mach 1 just as efficiently and essentially by the same mechanism as it does for particles but, it's essential that you "get it" if you expect to understand sound propagation. It might ease the mental transition to think of hole flow in the semiconductor world. Same concept.

But, whether it's the influence of particles or voids that's propagating, we have to think about it on a near instantaneous basis. At least down to a few picoseconds. Trying to think about it as waves in any way, shape or form inevitably leads to the kind of "wave analysis bias" that's taught in schools and that I've griped so mightily about being so prevalent here in these forums. It's like apples and oranges. If you throw an apple into the pond, by the time the wave develops, the sonic wave front is already an orange that's 1000 feet away from the splash...and ne'er the twain shall meet.

There's more but, that's enough for this one post.

 

[3v0]

I love this thread. You put a nickel in and away it goes...

 

[crashsite]

Alms for the Poor

I love this thread. You put a nickel in and away it goes...

Well, ya know...what can I say. I refuse to discuss "wave analysis" when the subject is, "sound propagation" so it must all be a lot of bunk. Nigel got it right when he branded the thread, "pointless".

Perhaps your best tack is to sadly, knowingly shake your head in pity for us poor deluded fools. Or, better yet...lay out, in diamond sharp clarity, what's really happening (in a way us unedumacated idjits can understand it) and get this thread resolved and closed out. I had invited Nigel to do that way back on about page 5 or so but, he has apparently declined.

 

[Nigel Goodwin]

I had invited Nigel to do that way back on about page 5 or so but, he has apparently declined.

Sorry, this thread is such a load of complete and absolute crap that I don't read it it's just coincidence I happened to notice this post.

 

[crashsite]

A Voice From Steerage

Sorry, this thread is such a load of complete and absolute crap that I don't read it it's just coincidence I happened to notice this post.

Gee, just when I thought we were finally making some progress...whoosh! A dose of reality. So, even though your post does nothing to shed light on the topic of the thread it does help keep us grounded and less likely to ge all upity. Thanks.

Good to see the Electro-Tech grapevine is functioning well...

 

[Nigel Goodwin]

Gee, just when I thought we were finally making some progress...whoosh! A dose of reality. So, even though your post does nothing to shed light on the topic of the thread it does help keep us grounded and less likely to ge all upity. Thanks.

Sound is a vibration, what to discuss? - apart from that the speed changes depending on the medium it travels through, and obviously it can't travel through a vacuum.

 

[dknguyen]

Personally, I don't see what crashsite is still confused about, but maybe he knows something about sound that I don't. After all, the different conceptualizations for flight I ran across only fell apart after I became aware of certain things like vortices or induced drag, or alternate explanations that were also correct but didn't seem to mesh with other correct explanations.

But as it stands, I'm quite satisfied with thinking that the diaphram produces the pressure wave but doesn't propogate it. THe propogation is done by the fluid itself as it tries to equalize the pressure difference making the propogation dependent on the compressibility, density/inertia of the fluid rather than the movement of the diaphram itself. So the speed is completely dependent on the fluid and not the diaphram.

As for how the wave (energy, collision front, pressure front, whatever you want to call it) propogates in one direction even though the molecules themselves only oscillating, never moving very far from their original positions...that seems really obvious to me. If that animation I posted ALLLLL the way back at the beginning doesn't help you see how the oscillating molecules are transferring collision in only one direction, then I don't know what will.

A large part of the problem (at least from my perspective) is that I understand what everyone else is trying to say when they respond to crashsite, but then crashite starts talking and I don't understand what he's trying to saying at all. Like take his last post:

But, whether you think about a single ball (particle) as the disturber or a whole bunch of them, the action is the same. But, you can't fall into the trap of allowing yourself to fixate on the molecules that are swept into the melee by...say, the proverbial speaker cone. You have to think about what happens once they get there and, here's why.

So far we've concentrated on the compression excursion of the speaker. It's easier to think about the actions and reactions acting on something rather than on nothing as we need to do on the rarefaction excursion of the speaker. But, from the sonic wave front's POV, it's the same. That's because the propagation of the wave front is not accomplished by the molecules entering the melee...it's accomplished by the molecules that are already there. It's sort of easier to visualize on the rarefaction side:

On the rarefaction excursion, molecules at the speaker cone interface are sucked out of the melee and it's the thermal activity of the system driving the molecules into the void that not only occurs but creates an effect that continues to influence the system and that influence is what propagates away as the sonic wave front. I know it's a little harder to get your head around having the void propagating away from the disturber at Mach 1 just as efficiently and essentially by the same mechanism as it does for particles but, it's essential that you "get it" if you expect to understand sound propagation. It might ease the mental transition to think of hole flow in the semiconductor world. Same concept.

But, whether it's the influence of particles or voids that's propagating, we have to think about it on a near instantaneous basis. At least down to a few picoseconds. Trying to think about it as waves in any way, shape or form inevitably leads to the kind of "wave analysis bias" that's taught in schools and that I've griped so mightily about being so prevalent here in these forums. It's like apples and oranges. If you throw an apple into the pond, by the time the wave develops, the sonic wave front is already an orange that's 1000 feet away from the splash...and ne'er the twain shall meet.

There's more but, that's enough for this one post.

What are you trying to say? What's all this about melee of particules, and particles/voids propogating? What does that have to do with any of this?

But from your mentioning about the speaker cone sucking, I will say this:
Science doesn't suck, it blows. THe speaker cone directly affects the motion of the air when it pushes out because it applies a force on the air. BUt that is not the case when it pulls in. When it pulls in, it is not "sucking" the air in by applying a force on the air. The diaphram can't suck any harder or weaker. It is just making empty space, and the air molecules apply forces on each other via pressure to fill this lower pressure area. This means the cone does not really propogate any sound outwards when it pulls in to "reset" itself for another push. Rather, I think it would propogate inward towards the cone.

Think about it...if you had a speaker cone that just pushed outward once...does it make a sound? Definately! A big loud thud. But what happens when the speaker cone just pulls in? You don't hear very much. Why? Because the high pressure front is from the air being pushed towards the lower pressure region (the empty space) formed by the cone, and the sound only travels through the empty space from the air to the cone, where it stops. It propogates towards the cone, not away from it.

What does all that mean? It means although the diaphram sets up the high pressure front, it is the the fluid, not the diaphram, that propogates the front (by trying to equalize pressure).

But I might not be answering what you are now asking because I don't know what it is that you are saying. I'm still running off what you said in the first few pages about why does the sound go outward not move back and forth with the cone like the air molecules do, and why the speed of sound is dependent on the medium and not dependent on the diaphram. I have not been able to entirely follow what difficulties you are having since. Is it how the random movement due to temperature plays into all the oscillatory motion that the molecules use to transmit the collision front between each other?

 

[Sceadwian]

A transducer displaces a medium, medium compresses, pressure waves move out at the speed of pressure waves in that medium. That's it. For most practical purposes the actual 'flow' of the molecules of air can be disregarded as inconsequential. Much like AC current flow through a conductor. The posts in this thread is a bunch of rehashing of words nothing useful is being conveyed.

 

[chconnor]

As for how the wave (energy, collision front, pressure front, whatever you want to call it) propogates in one direction even though the molecules themselves only oscillating, never moving very far from their original positions...that seems really obvious to me. If that animation I posted ALLLLL the way back at the beginning doesn't help you see how the oscillating molecules are transferring collision in only one direction, then I don't know what will.

Well, right, if you model the particles as idealized particles that have attractive and repulsive forces (due to pressure), it's pretty clear to see. Crashsite's point is that s/he didn't want to visualize things in terms of SHM, restoring forces, etc. They wanted to get down and dirty with the particles, and since even something like "pressure equalization" is a simplifying model, and non-trivial to imagine on the level of particles, it's understandable that momentum propagation would be as well. That animation you posted is great, if you accept that the idealized balls it depicts can just pass their momentum on to the next ball through the repulsive force between them. But when you want to understand that mechanism on a particle level, it's not so obvious how/why it happens, and why the intensity of the momentum is irrelevant to the speed of that propagation. You, me, and probably others kept throwing inertia and springs at crashsite, and s/he wanted billiard balls.

A large part of the problem (at least from my perspective) is that I understand what everyone else is trying to say when they respond to crashsite, but then crashite starts talking and I don't understand what he's trying to saying at all.

I think it's safe to say that all our communication skills could use some help.

What are you trying to say? What's all this about melee of particules, and particles/voids propogating? What does that have to do with any of this?

I believe he's talking about my post (melee = crowd), and I think I get what he's saying (more below) and that it is important.

Think about it...if you had a speaker cone that just pushed outward once...does it make a sound? Definately! A big loud thud. But what happens when the speaker cone just pulls in? You don't hear very much.

Wellllll, hold on another sec. You'd hear the same same big loud thud, actually. Try it by drawing a postive-only waveform in a thud shape in a digital audio program. Play it through, hear the thud. Reverse the phase (reverse the polarity so now it's negative-only, e.g. a rarefaction) and the same thud happens. Or grab the trusty slinky and make some rarefaction-only waves.

Or, hey, check it out, I just made two files: positiveexcursion.wav and negativeexcursion.wav (see links). In the first file, about 2 secs of silence, then an excursion to a positive value (speaker cone moves out), which holds for another few secs. (One can ignore the pop at the end of the file as the cone returns to zero.) The second file is the same but with a negative polarity (speaker cone moves in.) Note also that such a movements don't make thuds, but more of a pop (let's not even get started on frequency analysis...) Disclaimer: files like this may harm speakers at high volumes, use at your own risk.

**broken link removed**
**broken link removed**

Rarefactions propagate just like compressions. Although apparently there are, intriguingly, some differences: From Wikipedia: "Rarefaction waves expand with time; for most gases the rarefaction wave keeps the same overall profile at all times (it is a 'self-similar expansion'). Each part of the wave travels at the local speed of sound, in the local medium. This expansion behaviour is in contrast to the behaviour of pressure increases, which get narrower with time, until they steepen into shock waves."

Isn't that interesting? I mean, the fact that waves eventually distort over time is no surprise, but that rarefactions do it differently than compressions... not a shocker, but interesting to note. Assuming it's true. Anyway, it's beside the point.

On the rarefaction excursion, molecules at the speaker cone interface are sucked out of the melee and it's the thermal activity of the system driving the molecules into the void that not only occurs but creates an effect that continues to influence the system and that influence is what propagates away as the sonic wave front. I know it's a little harder to get your head around having the void propagating away from the disturber at Mach 1 just as efficiently and essentially by the same mechanism as it does for particles but, it's essential that you "get it" if you expect to understand sound propagation. It might ease the mental transition to think of hole flow in the semiconductor world. Same concept.

I do agree that the rarefaction is important to understand as well, but I'm not sure your explanation convinces me. To me, the propagation of the rarefaction is pretty easy to see (works on the same principle as the compression), but the way that the initial rarefaction imparts momentum is not as easy to see. I think of it this way: those bouncing balls are also trading momenta back and forth with the wall, or barrier (whatever is going to make the rarefaction) as they bounce off of it. When the disturbance happens, that movement imparts a negative momentum vector: the momentum of the balls bouncing off is now increased towards the disturbing barrier: with compression, the movement of the barrier is imparting positive momentum with every ricochet, with rarefaction, it is imparting negative momentum. The frenzy of balls in the next region out thus adopt that momentum towards the disturber and leave a similar void after them, and so on. Again, the intensity of the initial disturbance only determines how voided that area gets, and the overall temperature determines how quickly the next balls fill the void (and thus the speed of sound).

-c

 

[crashsite]

Thanks for the questions.

Sound is a vibration, what to discuss? - apart from that the speed changes depending on the medium it travels through, and obviously it can't travel through a vacuum.

We used to have kind of a running gag answer for people who'd ask a question but, we knew didn't really want a detailed answer (usually reserved for supervisors). We'd say, "Works fine, lasts a long time". Your thirst for detail is about half a step above that.

Personally, I don't see what crashsite is still confused about, but maybe he knows something about sound that I don't.

The post you quoted was a follow-on post to the one I had made the previous day and which user, chconnor responded to I thought very intelligently...even adding his own interpretations (which I largely agree with). I'm surely not going to requote the whole thing here since it's so readily available either on this or, at worst case, the previous page of posts in the thread. The post is titled, a bit tongue in cheekly, "Got It".

In that post I layed out the complete scenario and, I believe it would answer the questions you asked about what I was trying to say. But, if it doesn't, I'll be more than happy to give a play-by-play recap.

I would recommend also reading, chconnor's response as an extension of my post.

(almost immediate add-on) I see that, while I was generating this post, chconnor was busy with his own and snuck in just ahead of this one. So, there might be a little disconnect going on here depending on what he said...that I need to go see...