Why Does Sound Propagate?

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Propagation vs. Pressrue (nearing a vacuum)

Hmmmm, this seems to have turned into pretty much a monolog. I guess I can assume that either this community has gotten bored with this topic or has gotten fed up with my ramblings or (most likely), my logic and reasoning are so complete, impeccably perfect and fastideously accurate that no comment or challenge is needed or warranted.

So, anyway...

I had started another, related thread awhile back that eventually got folded into this one. The question asked was how sound propagation dies off as air pressure is reduced (and, indeed there is no sound propagation at all in a vacuum).

I think the molecular displacement view of sound propagation gives a pretty good shot at advancing at least a fairly defensible explanation of what may happen.

If we start with the notion that not all the molecules in a medium are necessarily involved in the propagation of sound and simply extend that thinking, I believe that the mechanism for the way sound falls off with reduced pressure can be presented.

Under "normal" (nominal) conditions, there are enough molecules and they are dense enough that a lot of them are involved. I was hoping to get at least one taker to do the statistics on this and create a graph or chart or perhaps a Java or Javascript routine to show some real values...but, alas (so far anyway)...nada. Anyway, even in the absence of hard data, it's pretty easy to visualize that not all the molecules are going to be affected by the disturber directly or by interactions with molecules that have been influenced by it.

As the pressure is reduced, the molecules get further and further apart. The speed which sound propagates is not affected because the molecules are still traveling at the same speed for the same temperature. But, the likelihood of them colliding becomes less and less. Therefore the power, as integrated over time, becomes less and less.

Since the power in sound propagation is contained in the thermal energy of the molecules and their collisions, when there are very few molecules involved, there is very little power being propagated.

The power may fall off in a linear manner as the pressure is reduced. But, statistics can be funny things so it wouldn't surprise me if it's defined by some other slope or curve. Yes, that is a broad hint to some math genius.
 
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Quit hogging the spotlight! Okay, it's yours.

If you go anywhere in a sound field and listen, you can hear the sound. Likewise, if you place a transducer into the field, you can convert the sound to an electrical signal. For that to happen the eardrum or microphone diaphragm must vibrate in sympathy with the originating sound source. How can that happen if the medium itself is not oscillating?

I've been kind of hogging the "sound stage" as it were. I really would like to see someone else break with the classical notion of sound propagation and explain the "how" of how that works (or follow the classical notion of sound propagation to explain how it works). I'm going to refrain from posting for a couple of weeks and see what might shake out of this. I hope to see some cogent intervening posts but, if not, then I'll dive back into it again.
 
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The last 12 posts on this thread have been yours, I think you are talking to yourself, which is a bad sign.
 
Persistant cuss, eh?

ericgibbs said:
The last 12 posts on this thread have been yours, I think you are talking to yourself, which is a bad sign.
Click to expand...

You're probably right. But, only by actually sitting down and typing it out can I be sure that at least what I want to say is said. My main thrust is to try to figure it out. While it would be nice to get some expanded info/input, all I can do is run it up the flagpole and see what happens.

But, thanks for caring enough to share.
 
How does the information stay intact with respect to a delay in time and distance? ECHOE

Look no further than the working of a radio signal,
frequency modulated, or amplitude modulated and how the information is carried unhindered over great distances and time.
 
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The Carrier Concept

12345678901234567890 said:
How does the information stay intact with respect to a delay in time and distance? ECHOE
Click to expand...

Whatever answer is given has to address that question.

12345678901234567890 said:
Look no further than the working of a radio signal,
frequency modulated, or amplitude modulated and how the information is carried unhindered over great distances and time.
Click to expand...

I believe there are parallels that can be drawn between sound propagation and EM propagation but, you have to be careful not to try to oversimplify those parallels. One is pretty much "mechanical" and the other "electrical" so, they do have different characteristics.

Both involve putting some sort of intelligence onto (or into) a medium that will convey it over distance and time. In the case of sound propagation (the subject of this thread), the question is how that happens.

I believe that I have at least the seed of the answer but, I seem to be a minority of one on that opinion...possibly of two...as user J_friend did post a quote from another website awhile back that indicated that someone else may be thinking along the same lines as myself.
 
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I agree 100% what you said about the difference in the two and to not oversimplify, if it were that simple I think the answer would have been found by now,

but this is like the question I asked on another forum how does light travel in space when there is no atmosphere but a complete vacuum, but that's another dilemna question, that would need it's own thread about,

but on a serious note when you really think about it, it is quite amazing how that information (echoe) does stay intact with good clarity, after a delay in time and at long distances, almost as if it was in it's own envelope, being sent out,

I remember as kids when we would yell and get a echoe, while the echoe was returning we would shout again and get a echoe back in the elapse of time the first echoe was being heard so the echoe information coming in did not get hindered from the new information going out, they both were in there own little envelope of time and sound frequency...


here is something else to coinsider about sound, an orchestra, each instrument has it's own frequency of sound, yet the waves don't get all befuddled and sound like a conglomerate of noise, but you can pick out the individual instruments, as they are all playing at the smae time,

so is it possible that every frequency has its own linear path through the atmosphere, or that just like 2 objects cannot occupy the same space at the same time is it possible that sound acts like a substance like light,

and can travel intact and be bent and dispersed....Well this is way off now but just somemore things to look into concerning the ability for sound to behave with such predictable outcomes, over great distances and elapse of time...


A good experiment to do to see if sound is a substance or just a vibtation , would be to set up a vacume chamber and place a speaker hooked to an amplifier on to a oscilloscope, then use a audio frequency generator.

to transmit a soud to the speaker, in this vacum if sound is a solid substance the speaker should still pick up the vibration not as a air pressure but as a force against it and it should register on the oscilloscope.

I know this is way off base of what we understand about sound being a vibration in air molecules.

Just some thoughts...

Here is somemore thinking questions, you can hear yourself swallow, or munching on food, thats sound originating from within not passing through air, or putting your ear tight against the wall and still you hear pretty clear through the wall the tighter you put your ear the less air movement to occur yet the sound gets louder, tap on the wall and the vibrations are heard as distint sound
and so on and so on...

It seems like to be able to answer your original question, you have to take every available situation concerning how sound behaves, or how we percieve sound, and then draw conclusions.

these just a few more thoughts...
 
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12345678901234567890 said:
I agree 100% what you said about the difference in the two and to not oversimplify, if it were that simple I think the answer would have been found by now,
Click to expand...

As I think I've been pretty clear in this thread, I don't think the "experts" have sound propagation figured out. Anyone who believes that the medium oscillates (in the traditional manner of dflecting and resorting forces) must account for some tiny disturber (such as vocal cords) moving literally hundreds or even thousands of tons of air.

12345678901234567890 said:
but this is like the question I asked on another forum how does light travel in space when there is no atmosphere but a complete vacuum, but that's another dilemna question, that would need it's own thread about,
Click to expand...

You're not the first person to question this. In fact, the notion that there needs to be some sort of "material" for light or radio waves to travel through is so strong that a cocept was invented for it. It's called, "The Ether".


That is a key point and one that gets conveniently ignored in the classical way sound is taught (based on the way it was taught to me and the way it's described in the Wikipedia). But, the sound can't be throught of as an envelope of information. It must be thought of on an instant-by-instant basis (with an "instant" being defined in the picosecond time frame). If you go back and read about the last 15 or so posts (as was commented about as me talking to myself, you'll see a pretty good wrap up of what I think happens...but, it really does need to be considered in the context of the rest of this thread).


It's not that every sound has its own frequency or waveform or path but, that every sound has it's own characteristic and point of origin. The question then becomes that of how those individual sounds can co-exist without becoming, as you say, "befuddled". The molecular displacement explanation of sound propagation explains that very eloquently. I believe that anyone who is willing to clear their minds of the "conventional" descriptions and definitions associated with sound propagation can "see" it.

I've been hoping that someone will leap in and explain it for us. I've promised to step back for a bit but, if nothing is forthcoming, I'll dive back in and explain it.


For such an experim4ent to work, you'd need to define sound in a completely different manner. Therefore, it would no longer be sound.

For example, the electrical signal to the voice coil will radiate some energy and that could be detected...but, it's not "sound".


Sounds can pass through just about anything that has molecules so, all that is really just how sound travels through different materials and across interfaces between differing materials. That gets into things like impedance mismatches. If you have the grit to plow through the posts in this thread, pay special attention to the ones that refer to the "executive toy" or "Newton's Cradle" toy. There are some hints about those impedance mismatches and how they work in a practical sense.
 
There is so much to read, but it is a very intriguing topic for discussion, maybe sometime I'll look over the whole thread, to gain a better understanding of this subject.

Thanks for your explanations concerning some questions I put forth about this. That helps me to better understand those questions I asked.
 
Okee dokee. Good luck.


Like virtually all science, it's obviously not critical to either understand or not understand sound propagation. The fact that virtually no people on earth, past or present, have even a sense of scientific curiosity and still manage to live and breath and procreate just fine is a testament to the relative importance of it.

But, I had a problem that involved sound propagation I was trying to figure out and all of a sudden, 2+2 just wasn't equalling 4 based on what I had been taught. As I got further and further into this thread I began to see some things fall into place but, for them to make sense, they needed to be thought of in a different way.
 
Feynman on Sound Propagation (Direct Question)

Feynman seems to be the gold standard of college physics courses so, I'm going to ask a very direct question and see if I can get a very direct answer. In fact, there can only be one of three answers.

Does Feynman's presentation of sound and sound propagation (that is, his explanation before he launches into mathematical modeling), substantially follow the Wikipedia article on sound and sound propagation (ie: that a disturber creates compression waves causing the medium to oscillate and thus propagate the sound by the interchange of potential and kinetic energy) or is it more like my theory (ie: that the disturber passively allows the energy inherent in the medium itself to cause molecular displacements {in a picosecond time scale} to propagate the sound) or is Feynman's views on sound and sound propagation something completely different than either?

In any case, I would appreciate it if you could go back to one of your "Feynman" physics books and give a synopsis of what he says. That info is very hard to find on the internet because textbooks are a money-making enterprise for publishers so they allow very limited freebee access to their contents. However, for discussion, copyright laws do allow you to quote passages. This link gives a guideline for "fair use" of copyrighted materials:

**broken link removed**
 

Can a disturber be a moving boundary of air that has a different temperature, pressure, moisture, or molecular composition than the medium - even if the medium is another boundary of air?
 
The Feynman question has still been posed...

I can't say that I'm happy that the mod decided to move my "Feynman question" to this thread...or that it failed (so far) to result in a "direct" answer but rather picked up an irrelavent "Jasonbe" comment instead.

But, regardless, the question has been asked if Feynman's physics text forms the basis for the Wikipedia article on sound and sound propagation. I suppose all I can do at this point is hope against hope that it will still manage to elicit a cogent answer.

And, once again I fill this thread with yet another gripe that has nothing to do with the topic.
 
The "Monologue" Continues...

About 3 weeks ago I asked this question and invited others to join in and take a stab at answering it. I'm a little disappointed but I guess I can't say that I'm even a little bit surprised that there were no takers.

So anyway, to refresh memories:

"If you go anywhere in a sound field and listen, you can hear the sound. Likewise, if you place a transducer into the field, you can convert the sound to an electrical signal. For that to happen the eardrum or microphone diaphragm must vibrate in sympathy with the originating sound source. How can that happen if the medium itself is not oscillating?"

Actually, this has been pretty much answered a few times (only by me) but, here's the "direct" answer.

The medium is not oscillating. There are no compression waves. It's all based on statistics. The molecules are largely in random motion. Whether there's sound present or not, any instantaneous snapshot of the air molecules will reveal no pattern. But, when there is a sound present, there is a bias to the randomness of the positioning of the molecules.

Let me try this analogy. The signals downlinked from the GPS satellite constellation, up some 11,000 miles, are very weak. In fact, as received on earth, they are below the noise floor. But, by knowing what to look for and using iteration, the signal can be coaxed out of the noise. That takes time. That's why it takes awhile for a GPS receiver to "lock" onto enough satellites to get a position fix.

A similar thing happens with sound. Each positional variation of the molecules is mostly random but, integrated over time, the pattern emerges. I explained (complete with color-coded molecules so, I don't know how to make it any more basic) how the positional bias is impressed onto the molecules and propagated.

Now the question is, how are they "decoded" to make a receiver such as an eardrum or microphone diaphragm vibrate?

As is always the case, the receiver is somewhat more complex than the transmitter. The same is true with sound. The air molecules (sticking with air for simplicity), are always impinging on the receiver at an average, nominal speed of Mach 1. That's true whether there's any sound energy present or not. The molecules are moving due to their thermal energy and impinge on whatever they come into contact with.

Becasue the molecules have mass and motion, they have kinetic energy. They push against whatever they come into contact with...again, whether there's sound present or not.

When the original disturber was pushing outward (into the air mass), the bias on the molecules was to have them go where they were going a bit earlier (remembering that we're talking about a picosecond, molecule to molecule interaction time scale here). Therefore, the effect will arrive at the receiver also a bit earlier. This tends to have an additive effect with the randomness of the molecular motion and that will push the receiver in the same direction as the original disturber was traveling (the receiver is receiving more energy per unit time) but, delayed by the amount of time it takes for the effect to propagate from the disturber to the receiver.

When the original disturber was traveling inward (away from the air mass), it took a bit longer for the molecules to reach it and that effect was also propagated outward to the receiver. At the receiver, there's a subtractive effect. There are fewer molecules striking the receiver per unit time.

Let me pause here for a reminder. Even when dealing with moving an air mass, in the rarefaction (vacuum, intake, "creating a void" or whatever you want to call it) phase or cycle, the disturber just moves. It doesn't do anything except create a space that some pressure pushes the air into.

In the case of sound at the receiver, there is also no mechanism for "pushing" the air back toward the disturber. It's the air pressure on the other side of the diaphragm that does that task. We get a very graphic demonstration of this when we have a cold and our eustachian tubes are swollen and plugged up and everything sounds kind of flat and dead.

This brings us back to our old friend, the "impedance mismatch". Once the molecular displacement has been established by the disturber and is propagating through the medium (at a nominal Mach 1, of course), there is no compression. There is no pressure gradient. There is no logitudinal or traverse wave. There is only a displacement of the molecules from where they would have been if there were no sound or if the sound pattern were different or if an additional displacement from some echo or other sound source is present and vector sums onto the molecules.

It's only where an impedance mismatch is encountered that the positional pattern is converted back into a physical vibration. Where people get confused is when they can go anywhere in the sound field with an eardrum or a microphone and detect sound. Of course you'll detect sound. You've inserted an impedance mismatch at that spacial position and converted some of that molecular positioning to mechanical vibration. Ther very act of "hearing" changes the environment.
 
Feynman will have to reside here as well

I had requested to discuss Feynman on sound propagation in a separate thread but, the moderator nixed it. So, whatever discussion about that will have to be done here.

I will point out that I disagree with his decision and feel that trying to maintain two running threads under one topic is a bad way to go but....

Not my choice.
 
The Cricket's Leg (Sound Propagation Thread)

On a still evening, if you are on one end of a football field and a cricket chirps on the other end, it's pretty easy to hear. Likewise, if someone is standing the same distance away on the other side of the cricket, they will also easily hear the chirp. At a radius of 600 feet, a tiny little cricket leg is able to cause a lot of air to oscillate.

I looked up the number. Air weighs about 0.07 lbs per cubic foot. Even a math moron like myself can figure out how much air that little cricket leg can make oscillate. Let's figure a hemisphere of air with a radius of about 300 feet. When you work it out, the answer is about 1500 tons.

We are being asked to believe that a cricket leg can make 1500 tons of air oscillate by the shear power of the exchange of kinetic and potential energy pushing a wave through it. Now, really. Even if that's the Mr. Universe of crickets, does that make any sense at all?
 
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Waves (Feynman thread)

Gee...where to even start.

I guess the first thing to say is that I can only hope that some (even one) of you will try to at least come to the defense of Mr. Feynman when I say that he's wrong. And, I will be saying that he's wrong.

I'll start with the title of, Chapter 47, "Sound. The Wave Equation". That really does pretty much set the tone for the chapter. The first thing Feynman says is how important waves are...and, he's right. Waves are very important.

Waves are also very comforting; even soothing and contemplative. Waves provide order and structure and provide the basis for precise mathematical modeIing. I myself, love waves. But, like anything, you have to be careful not to get too emotionally or psychologically attached. Waves can be like an opiate; adictive. Once you get into the adiction, it can be hard to put down the pipe.

There's no polite way to say it. Feynman is a wave junkie. A user and an abuser. There are times that you need to consider other answers and Feynman is so hooked on waves that he couldn't do it. And, like Dr. Timothy O'Leary did with LSD, he has led a generation of physics students down the primrose path of waves. I see it here and I see it in the Wikipedia article on sound.

I really have to apologize for putting such unrelated stuff into this thread on sound propagation but, it needs to be said and (hopefully) argued and discussed and, like I said earlier...it's not my choice of venue.

Let me close this post with a question. How much energy does it take to make 1500 tons of air oscillate a nomial 0.001 mm at 1 kHz for 0.1 seconds? How many ergs or joules or watts or Newtons or whatever your favorite units of power or force are? How much energy or force does a cricket have to expend or apply to make 1500 tons of air oscillate?
 
Feynman: The Man - The Legend (Feynman thread)

I'm sure that the tone of my posts has many (perhaps all) of you convinced that I despise Mr. Feynman, his mother, his sister and his dog. Nothing could be further from the truth.

But, there are some things to consider and, since this thread on sound propagation seems to have become a "catch-all" (thanks to the moderator's diktats), let me tag up here and make some of the "social stuff" clear.

I'm not here to socialize or make friends. My goal is to learn how stuff works and anything that gets in the way of that (like this kind of crap, for instance), is of little interest to me. To the "socially sensitive" it may seem like I don't like, Feynman and that I'm out to smear his good name. That's not true. Actually, even though I've never had a class based on his teaching, I like the guy and I like his writing style and his approach to dealing with problems. It kind of mirrors my own. But, he's a lot more knowledgable and better at it than I am.

Still, his approach is to kind of figure stuff out as he goes along. You can see it in the text of his lectures. I think that mostly he gets it right; but, not always. That's the case for his lecture on, sound propagation. He's locked into the wrong venue for explaining how it works and, as a result, he's struggling pretty much all the way through that chapter (the one I referenced a few posts ago...Chapter 47, Sound. The Wave Equation).

While I'm in the "teen girl" twitter, tweet, be my BFF mode here, I'll append this adjunct: You're watching a football game (American football, of course) and it's the final 2 minutes of the 4th quarter with the score 52 to 0. Both teams will still be fighting it out as though it were a tied game. The most disrespectful thing the team that's on top can do is let up and let the other team make a score.

The most disrespectful thing I could do regarding Feynman is to let him slide by on his laurels when I think he's wrong on sound.

Now, you guys have a choice, too. You think I'm wrong and you think Feynman is right. You can close ranks and just clam up and sigh heavily while I drone on in what has obviously become a monologue or you can show your respect for Feynman by using his techniques and methods to show that he's right and that I'm wrong. You can take the tack that you don't give a diddly-damn about how many tons of air a cricket can make oscillate, by pushing on molecules, or you can work through Feynman's math and conclusively show me that it's a reasonable value. Or...you can take the tack that I'm just so wrong that it's a waste of time to even consider any of it.

I'll be zeroing in on other aspects of Feynman's analysis of sound and sound propagation that just don't square with the facts as I continue with this now, dual thread.

I'll be happy to field any comments about this post but, really, I hope this will be the end of the "personalizing" and that we can get back to topic.
 
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Active vs. Passive (Sound Propagation Thread)

Whether dealing with moving an air mass or propagating a sound, it's important to keep a perspective on just what the action taking place is. One of the things to consider is the role the disturber and the medium play. That is the subject of this post.

To keep things as basic as possible, I'll stick to air as the medium.

I'll begin with the air as a mass.

When compressing some quantity of air, the surface of the disturber (usually some sort of piston or reed) moves toward the air mass and physically pushes on the molecules. The disturber actively interacts with the air. The disturber is active and the air is reactive. Continuing to think only about the air as a mass, those molecules push on other molecules and that effect continues outward, at some speed that is determined by the speed of the disturber. Unless the disturber is moving supersonically, it's always a subsonic speed and usually a fairly low one. The pressure gradient created by the air mass that's pushed outward fairly quickly equalizes and difusses.

When rarefying some air (such as yiou might find in a vacuum pump, the intake of a car engine, sucking on a soda straw, etc.), where the surface of the disturber is moving away from the air mass, the disturber is just creating a void. If there is air, at some pressure, in the vicinity, the air pressure will push air molecules into the void created by the disturber in order to even out the pressure gradient created. The disturber itself is completely passive and the air is...I would say, reactive. In the rarefaction case, as the pressures equalize, the effect also difusses fairly quickly.

But, in the case of the rarefaction, there is something else that needs to be considered. It takes energy to move the disturber...sort of. Anyone who has ever operated a hand vacuum pump knows that it takes energy to pull that vacuum. But, that's a special case that involves either a closed or restricted-flow system and so, other than to note it, wont be discussed here.

Now, consider the disturber when it comes to sound propagation.

The molecules are always moving at an average, nominal speed of Mach 1, relative to the surface of the disturber. When the disturber is moving (subsonically) toward the air mass, the molecules impinging on it, due to their thermal activity, do so regardless of the speed of the disturber (or indeed, whether the disturber is even moving at all). The only thing that changes is the timing and thus the spacial positioning of the molecules as they interact with the disturber. The disturber itself is completely passive and the air is active. Because the effect is that the molecules are displaced rather than simply moved, as a group, that displacement effect continues to remain intact for much longer times.

The same thing happens when the surface of the disturber is moving away from the air mass (subsonically). The molecules are moving faster than the disturber so they are able to continue to impinge on it with the only effect being that the timing and thus the spacial positioning of the molecules varies. The disturber itself is completely passive and the air is active.

Regardless of the direction the disturber is moving, when the effect is the displacement of the molecules, the effect still continues for the extended time and, when it does diffuse, it does so in a different (and substantially more complex) manner than an air mass does.

In the case of a supersonic disturber (such as a bullet or the tip of a bull whip)...well, I can't comment on those because I haven't thought enough about them to have figured it out.
 
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