Why Does Sound Propagate?

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I was only defining a wave, that is all there was no mention of molecules or anything similar. The wave could have been sound or an EM wave. The point was simply to define (and give an example) of a wave. I also specifically made NO mention of "how." How is a significantly more complicated question. I just simply defined a wave.
 
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Standing Around


Fair enough. Does the wave have to move? If a wave and its reflection interact (or, I suppose, if there are two waves of identical frequencies, such as in a laser hologram), it's possible to have a standing wave.

Standing waves are sure to raise their ugly head at some point in all this.

You mention EM waves. I'm probably going to weant to make a little side trip (hopefully, it can be a little one) regarding the propagation of EM waves and how the process may or may not relate to the propagation of sound.
 
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Propagation of EM waves is well understood. The "mechanism" that causes propagation or radiation is currents. When you ask "how" thtas when it gets a little murky.

The wave doesn't have to move, I made no such claim I simply showed an example of movement. A standing wave occurs when a normal incident wave and a scattered wave are related by a reflection coefficient of 1, the resulting total electric field is independant of time and and the E-field is only a function of position, thus the term 'standing wave.'

I suspect that creating a standing wave with sound is significantly more difficult than it is in EM. Though I can't be sure because I know very little about sound propagation.
 
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Relating Molecular Speed to the Speed of Sound

Everything here will relate to this graphic:

**broken link removed**

The first thing to note is that the graphic is a highly simplified 2D depiction of a 3D scenario. Think of the arced line as a segment of a sphere (like a ping pong ball cut in half) and the red line as a disk that's glued across the open edge.

If a pin is pushed through the center of the disk until it contacts the inner surface of the ball, the length of the pin represents 1100 mph. The nominal, average 1100 mph molecular speed is derived from the web page that's been referred several times in this thread that shows the speed of air molecules. The center of the disk represents the point of disturbance (the green dot).

The blue line represents the direction we'll envision the sound to be traveling. It's important to note that there's no direct relationship between the molecules traveling directly along the blue line and the direction the sound is traveling.

Any molecules traveling along the blue line are traveling in that direction at 1100 mph. Any molecules traveling along any of the other lines (red, yellow and black lines) are also traveling at an average speed of 1100 mph but, in the direction the line is pointing.

The question is, what is the average speed, in the direction of the blue line, based on the vector summation of all the directions the sound can radiate from the point of distrubance. It's impractical to show them all so just a couple of representative lines are shown.

The red line is perpendicular to the blue line so it's vector adds zero speed to the average speed in the direction of the blue line. The thin black line adds only a small amount of speed in the direction of the blue line. The yellow line is halfway between the red and blue lines (45 degrees) and its vector adds more speed in the direction of the blue line.

In fact, since there's half the vectors that are greater than 45 degrees and half that are less, the average speed of the molecules, in the direction of the blue line (again stress, "in the direction of" not, "along" the blue line), appears to be about 0.7 times the 1100 mph. That's about 770 mph, which is the speed of sound.

The 1100 mph value is a just a close estimate of the speed of the molecules so the speed of sound in the example is also off by a couple of mph.
 
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Dinosaurs are easy for 3rd graders...not so easy for scientists.

Emphasis mine:

3iMaJ said:
Propagation of EM waves is well understood. The "mechanism" that causes propagation or radiation is currents. When you ask "how" thtas when it gets a little murky.
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Well, yeah, you say that. But, there's been no shortage of people who say that about sound propagation...and still I find out the subject is just rife with falacies and misconceptions and misinformation. Unfortunately, I'm finding that lot of figuring out how sound propagates is having to painfully and repeatedly sift through all the chaff that doesn't make sense to get to the grains of truth.

And, sound is pretty mechanical and can be thought of in Newton terms. Electromagnetics is more esoteric even in concept. So, I'd be willing to bet money that if we all were to try to break down and conceptualize EM waves as I'm trying to do in this thread with sound propagation, we'd discover that EM waves are anything but well understood...even on the most fundamental level.
 


Perhaps you'll find a number of people who don't understand EM here as well as they would like or you would like, but I can assure you there are people (and a number of books infact) that understand EM propagation quite well.
 
I would like to know why crashsite things the slinky spring toy is a bad example of how sound travels.

3v0
 
3v0 said:
I would like to know why crashsite things the slinky spring toy is a bad example of how sound travels.
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I'm glad you asked.

If a person drops a rock into a pool of water, several things happen. We can think of two of them. One is that the "sound" of the rock hitting the water speeds away from the point of where the rock hits at sonic speed of thousands of feet per second. The other is that some water is displaced creating a slow wave structure that also moves away from the same point but, those waves take seconds to move even only a few meters.

The slinky spring does a good job of dlemonstrating the slow wave propagation which can easily be defined and visualized as the interchange of potential and kinetic energy. That's true whether one end of the slinky is waggled side to side (and the energy moves along the spring in a traverse manner) or if the Slinky is pushed or pulled (and the energy moves along the spring in a longitudinal manner).

The executive ball and string gadget does a better job of demonstrating the concept of sound propagation. I'm sorry but, doggone-it, it just does. It shows how adding energy (with a specific direction) into a system propagates rapidly through a medium (at a speed that is independent of the speed the energy is added) and how that energy "reappears" on the other end.
 
The executive toy is more properly know as "Newton's Cradle". Here you can see one in action.

YouTube - Newton's Cradle

Take a long rod touching a ball at the far end. If you tap the rod the ball will move. How fast does the energy move in this case? The energy transfer is quite nearly instantaneous. Much faster then the speed of sound.

You have the same problem with Newton's Cradle if the moving ball(s) strike a row of touching balls.

The slinky is a good model because is not a solid. It shows the compression waves in the coils. The speed at which energy propagates through the slinky is actualy more realistic then the near infinite speed show by Newton's Cradle.

3v0
 
Rock-a-bye


I'm not sure what putting a long rod against the ball and tapping it will show except that a solid rod approximates the continuity of the balls when they are touching each other.

The Slinky is more graphic but, it just doesn't show sound propagation.

I was hoping to get some other people to weigh in on this but...
 
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I personally dislike analogies and prefer to attack problems directly. But if I had to choose an analogy from the choices discussed here, I would choose the slinky. The propagation of sound in a gas has its own unique features that need to be understood. Once those are understood you can go back and make the connection with the analogs whether Newton's Cradle, steel rod and ball, or slinky. They all transmit energy and so possess certain common features. But that's really turning the problem on its head. Its better to attack the problem of propagation of sound straight on.

In Friday's post where Crashsite talked about compression cycles, and rarifaction, and adding energy I thought that Crashsite was finally moving in the right direction. He was missing one major concept but it seemed possible that he might discover it soon. However, with yesterday's post about vector summation, it seems things are back at square zero. The post was more like numerology than physics and the so called vector summation wasn't.
 
Newton's Cradle is not useful in modeling sound

crashsite said:
I'm not sure what putting a long rod against the ball and tapping it will show except that a solid rod approximates the continuity of the balls when they are touching each other.
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It is a very very good approximation. It shows the Newton's Cradle model is useless. No modeling of sound. Just pushing a stick !

The slinky shows propagation via compression waves. The way sound travels. It does not duplicate the speed but neither does Newton's Cradle.

Was was the purpose of posting the child's cradle image?

3v0
 
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crashsite said:
In other words...why does the sound packet speed away from the emitter? What propels it?

More important, is there a simple, 8th grade science class explanation of that reason?
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how about taking an analogy into consideration...like ,, if u hit ball towards a wall ,first the ball goes away from you at a speed with which u hit .. then after colliding with a wall it returns to u..or there can be lot if balls in a straight and perfect line and (theoritically) when u hit one ball hits and pushes next ball, it is same with the sound.. great way understanding a thing is to find a analogy
 
This must all be analogous to somelthing...

skyhawk said:
I personally dislike analogies and prefer to attack problems directly. But if I had to choose an analogy from the choices discussed here, I would choose the slinky.
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I don't mind using analogies...so long as there are no misconceptions about what the analogy shows. To simply say that the Newton Cradle thing is a good analogy for sound propagation is bad practice. To note that it's a good model for showing how energy can be added to a system and then have that energy propagated through the system at a speed that is unrelated to that of the disturbing force is a pretty good use. It's also pretty good to use that analogy for expaining the fundamentals of how impedance mismatches relate to reflections and provide the mechanism by which the energy can be extracted from the system.

The Slinky model is a good analogy for something like the water wave and can be used to visually demonstrate both traverse and longitudinal waves and how they move through some medium. The trouble with the Slinky is that it demonstrates none of the facors related to sound propagation. When introduced into a discussion about sound propagation it mostly just muddies up the waters.


I can't disagrree with that.


Pray tell me the major missing concept!

There's a maxim, in the training world, that technical training is not an Agatha Christy mystery. You don't hold back information to heighten suspense. Of necessity, the instructor does hold back information to ensure that it's presented at the correct point in the presentation but, if it's needed to understand the subject, there's no point to tease it while keeping it a mystery.

skyhawk said:
However, with yesterday's post about vector summation, it seems things are back at square zero. The post was more like numerology than physics and the so called vector summation wasn't.
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At this point I feel fairly comfortable with the vector analysis. I've been touting the vector angle (no pun intended) for most of this thread.

There is one fact about sound propagation that definitively tells me that it must be directly and pretty much exclusively related to molecular speed.

When you realize that the speed of sound is essentially unaffected by pressure, that must mean that sound travels just as fast when air molecules are further apart or closer together. Failing some sort of "tunnelling mechanism", sorcery or other supernatural phenomena, the speed of sound must be as fast as a molecule can carry it before striking another molecule (also moving at a similar speed) and in that manner propagating the sound.

My graph analysis assumes the molecules to be moving randomly at a nominal speed of 1100 mph (due to heat). The molecules are moving in all different directions. The disturbing force (as it's being handed off from molecule to molecule) is moving radially outward (propagating) from the point of disturbance only. But, we can be selective as to which direction we sense that propagation (the direction to our detector).

But, just because we can be selective about what direction our detector is, we cannot control the amount of sound energy that's being propagated in the direction of our detector by molecular interactions along axes other than the one between the disturber and our detector. We are receiving the vector sum of the molecular speeds (with the energy from the disturber being carried along with them).

I know that's a pretty involved concept but, envision what the total speed of the molecules in any given direction is when the vector amplitudes and directions are averaged.
 
1 Corinthians 13:11

3v0 said:
It is a very very good approximation. It shows the Newton's Cradle model is useless. No modeling of sound. Just pushing a stick !
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I'm not quite sure what your logic is. Are you saying that if a sound is propagating through the air and suddenly encounters a huge molecule of some kind, the whole concept of sound propagation, as we know it, is invalidated?

Well, I can see some parallels between the Newton's Cradle toy and how sound acts but, I can also see the futinlity of nit-picking the differences between steel balls and air. So, I'll take the lessons that I feel the toy has to offer and let it go.

3v0 said:
The slinky shows propagation via compression waves. The way sound travels. It does not duplicate the speed but neither does Newton's Cradle.
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Same with the Slinky. Where you see parallels to sound propagation, I see only waves (and you know how I view wave analysis when it comes to the subject of sound propagation). Again, we could easily spend a lifetime nit-picking the differences between Slinky springs and air.

So, it's probably best to take the lesson from the two toys that we view their actions differently and let this one go, too.

But, I do worry that both you and Skyhawk (and others) are thinking too much about waves (at least when considering sound propagation).

3v0 said:
Was was the purpose of posting the child's cradle image?
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I just thought that Newton, as a baby, would probably have been rather uncomfortable in a, "Newton's Cradle".

Okay, I can see that fell flat...so, I've removed the picture.
 
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Refining the Analogy

Emphasis mine:


The fly in that ointment is that, regardless of the speed with which the ball is hit (in sound, perhaps it's the speed with which a speaker cone is moving some air), the effect of the ball (not the ball itself) always speeds away at the same speed (in sound, at Mach 1). Then the ball seems to 'magically" reappear at some distant point (like at your wall).

The questions are, how do all those things work?

Let me take the liberty of making a small editorial comment. I don't know what the future of the Eglish language usage may be but, at least at the present time, there is a perception of people by their use of language. Maybe it's unfair but, the truncation of "you" to "u" fairly screams, 12 yo girl. That's kind of too bad because, otherwise, your usage looks fine.
 
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But, I do worry that both you and Skyhawk (and others) are thinking too much about waves (at least when considering sound propagation).
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Not me. I'm thinking about conservation of mass, momentum, and energy.

A few days ago I did a simple analysis of the propagation of a rarefaction discontinuity. Consider a cylinder filled with air fitted with a pressure sensor at one end and a piston at the other. When you withdraw the piston at a constant speed a rarefaction discontinuity propagates toward the other end at the speed of sound. Easy to see why just using the conservation laws.
 
Crashsite:

Air is a compressible. The compressibility and rebound from compression are important in understanding sound in air.

I agree with skyhawk regarding the piston and air sensor.

We have to look at how the sound/compression (they are the same thing) moves as fast as it does.

Perhaps if you stopped telling us how wrong we are about waves and tried to understand the application of wave theory to sound you would be closer to understanding

3v0
 
Oh, balls!

skyhawk said:
Not me. I'm thinking about conservation of mass, momentum, and energy.
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It's true that at every step one must ensure that at least Newton (and sometimes Einstein, when relativistic effect are prominent enough) are not offended.


I'm not surprised to see this post after your, 'ping pong ball in a tube' video. But, I'm not sure that what you are measuring is what you are measuring.

In the video we see the ball drop and, as the air compresses below it, it acts like a spring. But, that spring action has no speed correlation to the sonic wavefront that propagates down the tube while the ball is dropping. Are you measuring the air pressure change as the ball is dropping? Are you measuring the instant-by-instant change as the sonic wave changes after reaching the bottom of the tube, where your sensor is, at Mach 1? Are you measuring the rebound caused by a gross pressure change of the air mass? Are you measuring the rebound as a function of the impedance discontinuity that causes the reflection of the pressure (ala the Newton's Cradle toy...that I proeviously promised to stop talking about)? Are you measuring the thermal interactions of the air molecules that make it all possible? And, on it goes.

Certainly, the video does not visually show anything related to the propagation of sound. That's happening way too fast for even a video camera to capture it and, besides, the air is transparent so the video doesn't even show what 's happening to the air mass, except by inference of what the ball is doing.

There are many thing that happen as stuff moves through the air. I'm intrigued with how sound does it. I'm also intrigued by how the ping pong ball does what it does in the tube. I'm also intrigued by the way a smoke ring can maintain its shap while wafting through an air mass. I'm also intrigued by how a supersonic event interacts with the air. But, intrigued though I may be, I can't let myself get confused about how each one works by trying to apply parallels to the principles of how the others work.

When I see comments about sound propagation being applied to the movement of ping pong balls in a tube, I do worry...
 
Use the piston and pressure sensor as described in skyhawks post. You can then find out how long it takes the sound/pressure to travel.

I predict that you will see a pressure change at a time set by the speed of sound.

3v0
 
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