Why Does Sound Propagate?

Back to "one view"

I've given this matter some thought and have come to a conclusion that will undoubtedly make some of you happy. I've decided not to pursue the matter of Feynman's lecture on sound and sound propagation.

I really would have liked to have had a discussion about it in its own thread but, I can see that the moderator was right in nixing that idea. It would have almost certainly just turned into another monologue, pitting me against a mute adversary.

I know what Feynman said and I've cited the link for anybody that wants to go view or, if they've taken a course based on his teachings, to review the chapter. I'm just offhand guessing that none of you did either.

So anyway, there is still more ends to tie up in this "Why Does Sound Propagate?" thread and I'll just concentrate on making sure that the "makes sense" version gets aired.

Don't mean to offend but I wrote this thread off like 8 months ago =P

Not a Problem

Sceadwian said:

Don't mean to offend but I wrote this thread off like 8 months ago =P

Click to expand...

Okay. Noted. And, I'm not a bit offended.

But, thank you for caring enough to share.

The Cricket Leg Resolved

The question was posed as to how a little cricket leg could make 1500 tons of air oscillate. Of course, the whole notion of a cricket leg making that much air oscillate is absurd. Therefore there must be a different mechanism at work than having a moleule nudge the next and the next...(see attachment):

Of course, there's a different mechanism at work than is described. To propagate the chirp, the cricket leg is only obligated to move within the air mass. Remember that the disturber is always passive when it comes to its role in sound propagation. The energy in the air does all the work of propagating the sound.

That doesn't mean that the cricket leg doesn't expend any energy at all. It still has to move some air adjacent to the leg but, that's "moving air" not propagating sound. The cricket leg must also overcome the resistance of opposing muscles and the joint covering as well as the mass of the little leg itself.

Even though the energy in the air is providing the power to propagate the sound, it's still not producing any pressure or waves. What's being propagated is a statistical displacement of the molecules within their random motion due to their thermal energy.

Therefore, regardless of the amount of air, it takes no more energy to propagate a sound than it takes to move the molecules around when there is no sound. Even with that relative ease, the sound doesn't continue propagating forever. There are always losses in a system and the air is no exception. I'll talk about some of those losses later.

By direct observation we know that, by using the energy of the air itself, the sound created by a cricket can propagate through 1500 tons of air with a fair amount of ease.

Audio as an Artifact

Way back, earlier in this thread, I stated that any waves observed in the air were merely artifacts of the process of sound propagation. Of course, that view was summarily dismissed by the "experts" here. But, facts are facts and I want to revisit that aspect in more detail.

The notion of artifacts (things that only exist as a by-product of other processes) is not particularly unfamiliar to people who are involved in electronics. Heterodyning in radio receivers; mixing a local oscillator with an incoming radio signal to create an IF (intermediate frequency), is a good example.

Perhaps an even simpler example is when you tune a guitar string by plucking two strings and listening to and nulling out the beat note created by any frequency difference between them. In that case, the non-linear nature of the ear itself is used as part of the tuning mechanism (a study of interest in its own right).

A somewhat more involved example but, one that might be more germain to the modern, computer crowd, is related to the artifacts that can be produced when digitizing audio using PCM (pulse code modulation). Nyquist found that you can encode an audio signal with full fidelity if you use a sampling clock of at least twice the highest frequency you want to digitize.

So, to digitize a 20 kHz signal you can use a sampling clock of 40 kHz. But, that comes with a problem. Any frequencies above 20 kHz create "alias" signals (artifacts of the original signal) that come back down into the audible spectrum. To avoid those artifacts, the sample clock is raised to 44.1 kHz with the understanding that agressive filtering, beginning at 20 kHz, will reduce the power of any audio signal above 22.05 kHz to a low enough level that any artifacts generated will be negligible. The 44.1 kHz clock is the standard used to encode most WAV files and for audio CDs.

The upshot of all that is the notion of signals not really being there but, under the right (or wrong) conditions, spontaneously appearing. In the case of sound and sound propagation, the audio (or other sonic) signal is present only as a statistical positioning of molecules until some situation or condition causes them to seemingly appear as mechanical vibrations. Of course, in the case of sound, the desired situation is to create those artifacts because they are what allow us to hear those sounds (as well as do other tasks).

But, to try to force the notion that sound is waves or pressures or oscillations or vibrations at inapproprate times is just plain wrong.

Just Checking

Sceadwian said:

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

Click to expand...

I was searching back through some of the old posts in this thread for something else and I ran across this one. In light of intervening conversations, I was wondering if this is still your position on this topic?

I'd say I still believe that basic quote as being very close to matching the real world. However I have not kept up on all the posts in this thread, nor do I ever intend to. While I respect pure curiosity and research into the basic fundamental reasons for various real world phenomenon I don't think you've used any solid scientific methods to draw a clear and concise theory to have any true basis for a continuing discussion, and this thread has become completely unreadable.

If you have any desire for me to continue the conversation I would request that you draw up a basic theoretical summary state it in a single paragraph (a very short and concise one without large numbers of references) come up with some serious basic goal (again very short) maybe a testable hypothesis or two, and post it in a new thread as this one should be put to rest.

Given the length of of this thread and the length of the posts in it I would like to know what so far has been acomplished scientifically because flights of fancy or linguistic machinations don't interest me =) Then again I've missed many posts. However structure is the foundation for the advancement of knowledge, and this thread has none.

Okay, Thanks...

Sceadwian said:

I'd say I still believe that basic quote as being very close to matching the real world.

Click to expand...

Okay, thanks. Just curious. I like to keep my ear to the ground so to speak as to what people are thinking.

Sceadwian said:

If you have any desire for me to continue the conversation I would request that you draw up a basic theoretical summary state it in a single paragraph (a very short and concise one without large numbers of references) come up with some serious basic goal (again very short) maybe a testable hypothesis or two, and post it in a new thread as this one should be put to rest.

Click to expand...

I would like anyone who has anything of value to add to comment.

I guess the proof for me (at least that the prevailing theory on the subject is wrong) is that I find the notion of a cricket leg making 1500 tons of air oscillate is absurd. Therefore there must be a better explanation since it's not particularly unusual to hear a cricket chirp at a range of 300 feet. There are posts that make that statement make sense.

Anyway, thanks for the response but, sorry...there is no one short paragraph answer. Wish there was.

Perhaps THIS Explanation Will Help...

In trying to think about ways to make this topic make more sense, I was thinking about the "moving air" aspect of it. More specifically, about when moving air is the appropriate consideration and when to consider alternative explanations.

Close to a speaker cone, the cone is both moving air and propagating sound. Air is actively interacting with the cone to move the air mass and passively interacting with the cone to propagate sound.

If the sensation of sound comes from having the eardrum vibrate, then having a direct connection of the eardrum to the speaker would certainly do it. If you are close enough to the speaker that you directly experience the air being pushed by the speaker (and the ambient pressure pushing the air back on the rarefaction cycle), you also have a direct coupling of the speaker cone to the eardrum, via the intervening air molecules. In both those instances you can hear the sound from the speaker because either the speaker cone or the air near it is vibrating and that vibration is coupled to your eardrum.

This, I believe is where people get the notion of the air mass oscillating or vibrating as being the mechanism for propagating sound. And, it is an element of it.

But, is that "sound propagation"? Let me be more specific. Is that "sound propagation" as it's usually defined? That is, as a sonic "wave", traveling at Mach 1 through the medium. Or is it some volume of air that's being moved back and forth rapidly enough by the speaker cone to vibrate the eardrum fast enough that the vibration is sensed as "sound"? There is a difference.

Let's say that teen daughter is up in her room playing her Jonas Brothers CD as loud as her little 3 Watt radio and 6 inch speaker will go and her dad, downstairs, is yelling, "Turn that down!". Is that little speaker causing the whole mass of air in the house to oscillate? Even if it's a 2000 square foot home with average 8 foot ceilings (at 0.07 pounds per cubic foot of air)....and, the daughter has her bedroom door closed? Is the air that's vibrating dad's eardrums being directly pushed on by air from the speaker on a molecule-pushing-molecule basis?

Or is it that by the time the sound gets to dad downstairs, it's no longer air pressure that's directly pushing on dad's eardrums but, rather sound that's been propagated by a different method? Perhaps, by statistical displacement of the molecules in the air with dad's eardrums representing an impedance mismatch that acts to convert the molecular displacements back to mechanical vibrations?

Both the moving of the air mass and the displacement of the air molecules are initiated when the speaker cone vibrates. Very close to the speaker (and, probably when wearing earphones or ear buds), the vibrating air mass will dominate. But, that effect diffuses and dissipates quickly with distance and is replaced by the longer range molecular displacement.

Because the molecular displacement is propelled by the energy in the air itself, it has no problem propagating throughout the entire house.

crashsite said:

Because the molecular displacement is propelled by the energy in the air itself, it has no problem propagating throughout the entire house.

Click to expand...

Have you even shown or proven the idea that preexisting energy moves sound ?

Just read this last page of posts and don't understand where the confusion is, if I throw a small stone in a lake I can move the whole lake and see it happening. So, yes, the teen daughters radio is moving all the air in the house. Same with the cricket.

Mike.

This threat has fallen completely off the track. I started by giving the concept (or concepts) that Crashsite has been advocating serious consideration, but after the periodic slams to various establishments (mine included) and the nonsensical and complete non believe without rational or mathematical foundation I've completely lost my taste for this thread. Delete the thread and start over with a concise problem statement instead of aimless ramblings of things you do or do not believe. Frankly I don't care what you believe or don't believe experimentation contradicts you in many regards, not to mention the ensuing mathematics.

I don't see what the problem here is either...

Like Pommie braught up, if I throw a rock in a lake the energy of the rock gets transfered to the water and radiates away from the source in the form of waves. Waves after all do not represent physically moving the medium from one place to another, but rather transfering energy from one place to another. As the energy radiates from the source, the area over which the energy is spread increases and therefore the energy is more spread out which causes smaller waves the further we go from the source.

Bam same idea for sound.

crashsite, it doesn't make 1500 tonnes of air move, it makes 1500 tonnes of air oscillate... Remember, the movement is actually a traveling pressure difference, it's not like the entire mass of air moves in one direction that would require an insane amount of energy, pressure waves simply nudge a little bit at a time out of the way and put it back, though that's a bit imprecise. If you think of sound under water the numbers are even more boggling, it still happens. It can be directly observed using Schlieren photography.

Water

Pommie said:

Just read this last page of posts and don't understand where the confusion is, if I throw a small stone in a lake I can move the whole lake and see it happening. So, yes, the teen daughters radio is moving all the air in the house. Same with the cricket.

Click to expand...

I'll respond to just this one post directly since the last three said about the same things and, I'll weave in comments to the other posts within this response.

I'm glad you guys brought water waves back into it.

Water waves are nice. You can see them, observe them and, once you get it that they don't really go anywhere except up and down with one wave providing the energy for the next, pretty easy to understand. But, impressive as they look, they don't contain much energy. In fact, the sum total off all the waves created by dropping in a stone never can exceed the energy that the stone put in initially. In fact, it's always a little less. Because the water waves are a surface pehnomenon, not a lot of mass is involved.

With water waves being so visible and understanable, it's pretty easy to get fixated on them and come to the conclusion that they (and their counterparts in air) are the answer to all things related to propagation. So much so, in fact, that even the famous physics professor, Mr. Feynman succumbed to the temptation.

In further fact, it's easy to forget that, in addition to those pretty ripples, when a rock is dropped into the water, a sonic shock wave is also generated that travels a few thousand feet per second and that sonic event is not a surface phenomenon! Using the same initial energy supplied by the rock to create the ripples, that sonic wave has to somehow move through...well, let's just say that water is a lot heavier than air and now it's a cubed function for the amount of water involved. Add to that the fact that the sonic wave travels not just faster but a lot further than the water waves do.

Where do you suppose the energy to propel that sonic event comes from? Virtually all the energy of dropping in the rock can be accounted for in the surface wave ripples.

Like coming to grips with the fact that the surface ripples are due to the exchange of potential and kinetic energy of the water as it rises and falls, at some point you also need to come to grips with the fact that the energy that drives the sonic event is contained within the medium itself.

But, coming to grips with the notion that the energy is in the medium is only part of it. You also have to figure out how that energy is tapped. I've explained how it works in this thread (in a conceptual, non-mathematical way).

I do agree that it might be good to delete this entire thread and restate only the conclusions but, I guess if the choice were mine, I'd leave it as is because I think there's things to be learned by seeing the false starts and corrections and arguments and ye verily, even the gripes. Do I have it all figured out? Not by a long shot. But, I do believe I'm on the right track.

The numbers still don't come out right

Sceadwian said:

crashsite, it doesn't make 1500 tonnes of air move, it makes 1500 tonnes of air oscillate... Remember, the movement is actually a traveling pressure difference, it's not like the entire mass of air moves in one direction that would require an insane amount of energy, pressure waves simply nudge a little bit at a time out of the way and put it back, though that's a bit imprecise. If you think of sound under water the numbers are even more boggling, it still happens. It can be directly observed using Schlieren photography.

Click to expand...

You're right. Even if the cricket were brute-force moving the air, it wouldn't make the entire 1500 tons of air oscillate all at once but, it does make...well...a rough approximation might be gained by considering how long the chirp lasts and how fast the sound propagates and considering that it propagates throughout a hemisphere of air (plus whatever is absorbed by the ground). At 1100 feet per second for sound propagation and a chirp lasting a tenth of a second, that's still way too much air for the cricket to move without getting help from the energy in the air.

I believe the cricket only moves a tiny bit of air, near its leg, and the air does the rest of the work.

You're kind of making the same argument that Feynman did. In order to simplify his math he simplified the path to a simple and direct one from the source to the receiver. But, then he neglected to say that that simple path needed to be replicated a zillion times to account for all the directions the sound really goes.

No crashsite you are incorrect, all the energy comes from the crickets leg, 100% of it, the air actually attenuates the sound slightly as it travels it provides no energy, it actually absorbs some. You can believe anything you want, you are however mistaken.

It's easiest to visualize by the pressure difference that's created, because that's ALL that's occuring. the low pressure created pulls in air from nearby air spaces to equalize the pressure difference as soon as it occurs, this causes the space the air that was sucked into that space to now have a lower pressure, so the air next to it goes towards it to equalize and so on and so forth radiating outwards from the originating low pressure zone in every direction (that it's capable of moving) The pressure wave exponentially dissipates over time because it's radiating from a single point in ever direction.

You seem to think that that huge mass of air is actually moving all in one direction which would of course take an incredible amount of energy, which is not the case. It's easier to think about the the pressure wave itself. Different types of medium propogate and attenuate these pressure differences very differently. The speed of sound in water is about 4 times faster than that of air because it's denser, it also transmits lower frequency sound over significantly greater distances because of it's properties.

Tryng yet another direction

Sceadwian said:

No crashsite you are incorrect, all the energy comes from the crickets leg, 100% of it, the air actually attenuates the sound slightly as it travels it provides no energy...

It's easiest to visualize by the pressure difference that's created, because that's ALL that's occuring. the low pressure created pulls in air from nearby air spaces to equalize the pressure difference as soon as it occurs...

Click to expand...

A problem with this subject is all the phenomena that are simultaneously taking place and the different factors that contribute to them. For example, solids, liquids and gasses (and even plasmas) propagate sound but, each has it's own characteristics. Even among the differents states of matter there are variations that affect the propagation of sound. For example, the speed of sound in helium is faster than in air, even though both are gasses.

A big part of the problems of explanations is that so much time and effort is needed to just specify what's being discussed...even before discussing the topic itself. That's why most people tend to try to stick to air and to even simplify what's going on there. Unfortunately, I'm not immune to the cumbersome nature of this in making my case.

If we have a steel rod of some length we can pretty easily envision a condition as described by the wiki author in his aritcle on sound propagation. That is, the molecules being tiny steel balls separated by tiny springs. Push on one end of the rod and you start compressing the springs. That "propagates" that push down the rod and it takes some time for the effect of the push to reach the other end. Stiffer springs transfer that energy faster and the effect of the push travels faster down the rod.

There's a pretty good analogy in electricity. Electrons enter one end of a wire and push on electrons in the wire and they in turn push on others and the current flows. It flows very slowly.

If you consider the steel rod to have inert steel balls that are only moved by the force being applied by a push or a tap, how fast does the effect propagate? When thinking simplistically, it travels at the speed of sound through the steel. I'm not convinced of that. For one thing, I know that the steel balls are not motionless until moved by an external force or, within the rod, by the pressure from an adjacent spring. The balls (molecules) are absorbing heat (thermal energy) and are at least vibating in place and continually flexing the springs (molecular bonds). I also know that the speed of sound is directly related to the molecular speed of the molecules due to their thermal energy.

In the wire, the electrical effect, traveling at some significant percentage of the speed of light, doesn't have to wait for the electrons to catch up. There is a separate phenomenon taking place, related to but independent of the movement of the electrons. In the rod, there's a similar condition. The slower process of molecules moving molecules and the faster process of sound propagation.

I'm going to stop here for this post. There is obviously a lot more to consider. And, it certainly wont be contained in one or two short paragraphs with mimimal references.

-

Getting back on track

Hmmmmm...we seem to be back into the "mute/monologue" mode again. That's okay. There's still more that needs to be said so.......

Continuing with the theme of the previous post:

The speed of sound is directly related to the velocity of the molecules in the material. Just how is the question.

In Feynman's chapter on sound (and sound propagation), he makes a very interesting comment at the very end of the chapter. It's interesting enough that I'll not just try to quote it here, I'll actually show you what it looks like in print:

**broken link removed**

Bless him. In his heart of hearts he knows that the speed of sound is directly related to the velocity of the molecules. But, he's so addicted to waves and the notion of sound as pressure waves that even though he knows that the answer is molecular velocity, he spends the entire lecture (chapter) trying to work his way through an explanation of pressure waves. The issue needs to be explained in terms of molecular velocity.

That's essentially what I'm trying to do in this thread. Put sound propagation at least into the correct venue. Once I realized that the speed of sound is directly related to the speed of the molecules, I also realized that it's not waves and it's not pressure.

I see the challenge as four-fold:

1. Explain how the sound information gets impressed onto the molecules.
   
   
2. Explain how the sound information propagates through the medium.
   
   
3. Explain how the sound information is extracted from the medium by the receiver.
   
   
4. Finally, verify that it happens essentially the same way in all states of matter.

The first three have already been explained in this thread but, will be reviewed and a future post will cover the fouth.

I suppose, realistically, there's also a fifth challenge and, one that's probably greater than all the others put together. To convince wave addicts that there might be another way.

crashsite said:

Once I realized that the speed of sound is directly related to the speed of the molecules, I also realized that it's not waves and it's not pressure.

Click to expand...

Why ....... ?