Why Does Sound Propagate?

[dknguyen]

BTW, I think the word "wave" is the word that should be used instead of packet...sounds wave...not EM wave since somehow those are packetized hehe.

It's not obvious at all. The tuning fork moves back and forth but somehow the waves keep moving outward. Shouldn't the "logic" of it be that the waves will also move back and forth in sympathy with the fork?

No, because nothing dictates that the wave or the particle is rigidly attached to the fork. The only reason the particle actually moves back towards the fork is because of the rebound. THe particles end up looking like they are moving back and forth with the fork because they rebound off of other particles to send them moving back toward the fork, but not the wave. THe wave itself is propogated as the particles hit neigbouring particles on the right which causes that particle to move toward the right hitting more particles, etc. After hitting a neigbouring particle the original particle rebounds so it moves to the left. Remember, the fork is only able to push the particles- it can't pull. THe only way a particle can move back towards the fork is from another particle on the opposite side (ie. a collision or rebound).

The propogation of the wave is NEITHER the motion of the particle nor the fork. It is the "infectious effect" of one particle causing another to move (the so-called transfer of energy) that is the wave. So while a particle may be oscillating in one spot, if this causes subsequent particles to oscillate down a line at a rate of X additional particles begins to oscillate per second- that is the wave.

Here is a way awesome animation that demonstrates it. Does this help? Because if it doesn't...there's nothing more I can do!

So immediately after the first two particles collide, the 1st particle rebounds to the left. But the 2nd particle proceeds to hit the third. Subsequently, particles further down the line are still motionless because the wave has not reached them yet, even though particles at the beginning of the line are still oscillating left and right, the wave- a disease known as "the shakes" is moving down the line.

The motion of the particle is *NOT* the wave itself. The energy of the wave is not transferred when the particle moves- it's transferred when the particles collide. So the particles oscillating left and right makes the frequency of the sound, while the inducing of neighbouring particles to move IS the wave. If a wave is the transfer of energy, and the collisions are when energy transfer occurs, then in the image below follow the collisions. THe collisions move in one direction, even though the particles just oscillate back and forth. So like a transverse wave, just because the particle is moving back and forth, doesn't mean that the wave itself is moving back and forth (or even in the same axis).

It looks like there is a higher density of particles propogating to the right which is the wave. BUt at the same time it looks like there is a less dense population propogating to the left. Almost like the relationship hole and electron flow or between the positive and negative crests of waves moving in opposite directions. But no particles are actually moving along with the wave. It's just the density (or higher pressure if you will) is moving. THe particles themselves don't stray very far from where they started before moving in the opposite direction again.

Speaking of more awesome wave animations:
**broken link removed**

 

[crashsite]

Okay...I think I've got it figured out...

I still need to collect some thoughts but, I think I have the core of it and will post in a few days for comments/feedback. One of the things that needs doing is to "unlearn" some of the stuff that is normally taught since it just tends to muddle things.

 

[Papabravo]

...This is not the first time I've seen this Mr. Feynman touted as being the trusted "final word" on a physics subject in these forums. If Mr. Feynman has answered the question, can you copy and paste that simple and understandable text into one of your posts? I fear that Googling for "Feynman" and "sound propagation" will not very likely take us to anything useful.

You're putting words in my mouth. His explanation is a useful one, and you may find it illuminating. You need to trot around to a library or a bookstore to find a copy. I cannot copy and paste several paragraphs from a copyrighted work, as you very well should know.

The other thing you might want to try is to postulate an alternate version of reality. Then try to confirm or deny that postulate by whatever means at your disposal. Whatever remains no matter how improbable must be the truth.

 

[3v0]

The speaker cone moves forward crashing into the air molecules which crash into more air molecules. This sets up the forward moving compression wave.

When the speaker retracts it is moving away from the air molecules. There is no magic string attached to pull them back so the continue moving forward. The molecules near the speaker may move into the now vacant space but that has little effect on the energy that is already moving away from the speaker.

3v0

 

[antonio1]

The speed of sound is due to the space between molecules. Explains why sound is faster in desnser air. Denser air allows the molecules to collide faster thus increasing the speed of sound.

Very strange, some people are saying in one way, other are saying in another way. Hayato was saying

"For example, n1 = 1 (air), n2 = 590 (water).
If sound travels at 340 m/s at air, then, a soundwave that is generate outside the water that enters the waters suffers refraction, its speeds goes down to 0.58 m/s.
Then you are able to see it propagating very slowly. You sacrifice speed, but gain amplitude, and the frequency stays the same."

which means the speed in water (with denser molecules) is lower than the speed in air (with not so dense molecules). But you are saying different denser faster, not so dense lower, speed.

What's the truth?

Something else maybe a stupid question but real from practice. If frequency of source does not change at passing from one medium to another, why when we stay with our ears in the water and somebody is talking outside, we can't recognize his/her voice? Don't you hear different sound which means different frequency?

Let be as clear as possible in this subject. If we have a lot of balls (with more or less distance between them, these are molecules) and we have sound source (maybe boom vibration, or voice or anything), the first molecules from first line at generating have 2 properties vibration (oscillation as frequency) and moving in one direction (with a speed). The question is, when first ball with vibration and speed strokes the closest ball how it transmits the vibration to the second ball (to be the same vibration with no atenuation) if the ball just strike the other ball? Also at this point, if the first ball has a vibration what's the direction of that vibration and how that direction affect striking the other balls? Let;s go deep inside to understand how, frequency, speed and amplitude are transmitted for the sound.

 

[dknguyen]

If frequency of source does not change at passing from one medium to another, why when we stay with our ears in the water and somebody is talking outside, we can't recognize his/her voice? Don't you hear different sound which means different frequency?

I don't ever recall hearing a different sound- just muffled and a lot of the higher frequencies in the sound are filtered out (because of mismatched impedance matching between water and air).

Very strange, some people are saying in one way, other are saying in another way. Hayato was saying

"For example, n1 = 1 (air), n2 = 590 (water).
If sound travels at 340 m/s at air, then, a soundwave that is generate outside the water that enters the waters suffers refraction, its speeds goes down to 0.58 m/s.
Then you are able to see it propagating very slowly. You sacrifice speed, but gain amplitude, and the frequency stays the same."

Huh? You don't gain amplitude when sounds passes between air and water. The speed of the wave changes, but not the frequency at which the particles oscillate. Remember, the time it takes for a molecule to oscillate between two end points and the speed at which they are travelling can change independently of each other by changing the distance between the two end points. So the velocity of the molecule can slow down or speed up while the oscillation period remains the same by increasing or decreasing the distance that the molecule must travel between end points.

I'm pretty sure sound travels faster through water than air. I don't know where the figure of 0.58m/s is coming from. Google is giving figures ~1500m/s for water.

The question is, when first ball with vibration and speed strokes the closest ball how it transmits the vibration to the second ball (to be the same vibration with no atenuation)

THere is attenuation...that's why sound gets quieter as you move farther away. It transmits the energy from the first molecule into the second molecule the same way as pool balls.

 

[3v0]

Antonio,
Speed of sound - Wikipedia, the free encyclopedia

"Although "the speed of sound" is commonly used to refer specifically to the speed of sound waves in air, the speed of sound can be measured in virtually any substance. Sound travels faster in liquids and non-porous solids than it does in air, and travels about 4.4 times faster in water than air"

This is in agreement with what I said.

We preceive sound because the waves of high and low air pressure moves the eardrum. Not because the air molecules are vibrating.

Air molecules vibrate more as they get warmer. When they convey sound energy the take and give momentum. Each travels in a single direction.

HTH

 

[crashsite]

The Actual Propagation of Sound

The SOUND of the dominoes has nothing to do with anything.

The speed of sound is due to the space between molecules. Explains why sound is faster in desnser air. Denser air allows the molecules to collide faster thus increasing the speed of sound.

This makes so much sense that I tried (unsuccessfully) to apply it to a well known phenomenum (sp?). Colder air is denser than warmer air. Ergo, sound should travel faster in colder air. Fact is, Mach number is lower in colder air. Not sure why and can't seem to find out.

A Problem in Method:

One thing that seems to almost be a constant here is the attempt to explain by quantizing. That is, to reduce the explanation to equations and formulas. I'll use Ohm's Law as my example of the problem with this approach.

Ohm's Law is great and absolutely essential for practical circuit design. But, Ohm's Law, while allowing one to quantize electrical parameters, does absolutely nothing to explain the nature or physics of voltage, current, resistance and reactance. Likewise, the equations used to put values on sound propagation do nothing to explain the mechanism of it. Like Ohm's Law, it's essential for practical design. It just doesn't explain how it works.

A Theory:

I reason (perhaps incorrectly) that sound propagation is a reaction process. For a sound (wave, impulse, auditory disturbance) to exist, there must have been a physical entity that moved in the medium to create it. Something that moved the molecules adjacent to it and those molecules move away from that source disturbance at the speed of sound (for the material), where the collide with the next molecules further out. But, it's a reaction process. The molecules don't simply have some energy they somehow impart to the next, adjacent molecules. Instead, they push against the molecules closer to the sound source. If a molecule pushes back on another molecule that's pushing forward, the net speed is zero so, the only direction the sound can go is outward away from the source.

Now, I know that the above description may not be readily apparent and, frankly, really needs a lot more fleshing out (read that as, 'contains a lot more confusing verbage') but, I think that if you think about it in that way, it kind of does make sense. Of course, my notion that sound should travel faster in colder air also makes sense...even though it's dead wrong.

Thoughts?

 

[antonio1]

Huh? You don't gain amplitude when sounds passes between air and water.

I'm pretty sure sound travels faster through water than air. I don't know where the figure of 0.58m/s is coming from. Google is giving figures ~1500m/s for water.

If I don't gain amplitude when the sound passes between air and water, why when you dive in the water you hear better the sounds outside the water than you hear the same sounds when you are outside the water??? Does this could happen because the speed is increased and the strike of molecules to the eardrum is harder because of higher speed?

That's clear that in denser material the sound increase the speed, and we elucidated that someone here said a wrong conception. Ok let's go forth.

Antonio,
Speed of sound - Wikipedia, the free encyclopedia
This is in agreement with what I said.

We preceive sound because the waves of high and low air pressure moves the eardrum. Not because the air molecules are vibrating.
HTH

If it is as you say here, not because of air molecules are vibrating, but because of striking (then moving) the eardrum, WHY we hear different frequencies? Just a strike of molecules to the eardrum must result the same frequency, the same sound, indifferent of source, but higher or lower sound depending of speed. How the different frequencies are transmitted as they are to our ears?

 

[antonio1]

This makes so much sense that I tried (unsuccessfully) to apply it to a well known phenomenum (sp?). Colder air is denser than warmer air. Ergo, sound should travel faster in colder air. Fact is, Mach number is lower in colder air. Not sure why and can't seem to find out.
Thoughts?

Why cold air is denser than warm air, how it's happen? Normally seems to be truth as much as the mountain air you feel stronger than the sea air. But what of theory makes this to happen when you warming the air? Normally we can say that warmer air has much more molecules movements and much more molecules dimensions but lower density how you explain?

Huh? You don't gain amplitude when sounds passes between air and water.

I'm pretty sure sound travels faster through water than air. I don't know where the figure of 0.58m/s is coming from. Google is giving figures ~1500m/s for water.

If I don't gain amplitude when the sound passes between air and water, why when you dive in the water you hear better the sounds outside the water than you hear the same sounds when you are outside the water??? Does this could happen because the speed is increased and the strike of molecules to the eardrum is harder because of higher speed?

That's clear that in denser material the sound increase the speed, and we elucidated that someone here said a wrong conception. Ok let's go forth.

Antonio,
Speed of sound - Wikipedia, the free encyclopedia
This is in agreement with what I said.

We preceive sound because the waves of high and low air pressure moves the eardrum. Not because the air molecules are vibrating.
HTH

If it is as you say here, not because of air molecules are vibrating, but because of striking (then moving) the eardrum, WHY we hear different frequencies? Just a strike of molecules to the eardrum must result the same frequency, the same sound, indifferent of source, but higher or lower sound depending of speed. How the different frequencies are transmitted as they are to our ears?

 

[3v0]

I will read both posts in more detail in a few hours. At this time I want to touch on two points.

crashsite:
I was wrong and you are correct regarding density. Denser material indeed slows the speed of sound. Further reading shows there is another property at work, stiffness. This is new to me as my textbooks were along this line.

Speed of sound - Wikipedia, the free encyclopedia

Some textbooks mistakenly state that the speed of sound increases with increasing density. This is usually illustrated by presenting data for three materials, such as air, water and steel. With only these three examples it indeed appears that speed is correlated to density, yet including only a few more examples would show this assumption to be incorrect.

It is interesting how easy it is to be wrong in spite of education. And also how picking quotes out of context can support a wrong position.

Antonio:
Again I will read you post in more detail in a few hours.

As the pressure waves pass the eardrum vibrates at the frequency of the waves. The pressure waves happen because the same bit of air next to the eardrum is compressed and decompressed at the frequency you hear. Obviousy the air molecules need to move some distance to do this. I suppose it is fair to call that vibration since it moves back and forth. However wrong or right my mind associates moleculur vibration with heat, temperature and at times color. I know it holds for metals but does it hold for varioous gasses as well ?

I am thinking that the reason metals glow at a given temperature at a given heat is due to the speed of their vibration. If this is the case it is clear that we do not hear in the red frequency. Brain food

3v0

 

[crashsite]

I think you answered your own question.

Why cold air is denser than warm air, how it's happen? Normally seems to be truth as much as the mountain air you feel stronger than the sea air. But what of theory makes this to happen when you warming the air? Normally we can say that warmer air has much more molecules movements and much more molecules dimensions but lower density how you explain?

I think you sort of explained it yourself. Basically, it's the Boyle's Law thing that more movement of the air molecules makes the air pressure increase if the air (or any gas) is confined or have the same amount of air fill a larger volume if it's not confined. But, if the warmer air molecules are further apart, I don't know why the sound travels faster.

 

[crashsite]

Thanks 3v0, that was helpful

Denser material indeed slows the speed of sound. Further reading shows there is another property at work, stiffness. This is new to me as my textbooks were along this line.

Speed of sound - Wikipedia, the free encyclopedia


It is interesting how easy it is to be wrong in spite of education. And also how picking quotes out of context can support a wrong position.

That's a very good Wikipedia reference. I fully agree that a lot of this is glossed over in basic science texts (perhaps even more advanced ones) which leads to confusion when someone (like myself) starts wondering about some of the details and suddenly things just don't add up right.

I wont pretend to understand all the author of the article wrote but, I at least feel like someone is addressing the real issues of sound propagation rather merely assuming that the descriptions in high school science books are, 'good enough'.

 

[antonio1]

OK, so talking about this interesting subject let's develop our discuss and let's go forth. In conclusion from our discuss finally for a denser material the speed of sound is lower. So, in trying to explain and to understand the crashsite wrotes ("But, if the warmer air molecules are further apart, I don't know why the sound travels faster. "), I can say that with warming material, the volum of molecules increase, also the speed of molecules increase and indeed the density increase but the speed of sound also increase even if the speed must decrease for a denser material. But in this case I think it is because of speed of molecules and if the air is denser this case is not comparable with the iron with denser molecule structure but no speed of molecules.

Another question rise now, why if I hit two rocks in the air we hear worst than in the water or worst than in the iron material?

 

[dknguyen]

Why cold air is denser than warm air, how it's happen? Normally seems to be truth as much as the mountain air you feel stronger than the sea air. But what of theory makes this to happen when you warming the air? Normally we can say that warmer air has much more molecules movements and much more molecules dimensions but lower density how you explain?

It's just like you said...warm air has more molecule movements so the air molecules bounce off of each other harder, and produce more space between them than colder air. Think of popcorn- popcorn that is just sitting there has less space between it than popcorn that is popping.

But that's a good point. The density of warmer air is less...but the in an enclosed space the pressure increases which might have something to do with it. Kind of counterintuive when you think about it. It could simply be that pressure has more to do with it than mass density and density of number of molecules per volume (though this is related to pressure). And in solids there is some equivelant measure of pressure...like how rigid the molecular bonds are.

If I don't gain amplitude when the sound passes between air and water, why when you dive in the water you hear better the sounds outside the water than you hear the same sounds when you are outside the water??? Does this could happen because the speed is increased and the strike of molecules to the eardrum is harder because of higher speed?

WHat exactly do you mean by "better"?

If it is as you say here, not because of air molecules are vibrating, but because of striking (then moving) the eardrum, WHY we hear different frequencies? Just a strike of molecules to the eardrum must result the same frequency, the same sound, indifferent of source, but higher or lower sound depending of speed. How the different frequencies are transmitted as they are to our ears?

Don't get confused between the vibration of the molecules due to temperature and the oscillating movement of the particles due to to sound. AIr pressure is caused by the absence and presence of air molecules. So as the air molecules move, this presence/absence change causing the air pressure to change of the same frequency as the movement. This is what our ears detect.

 

[dknguyen]

OK, so talking about this interesting subject let's develop our discuss and let's go forth. In conclusion from our discuss finally for a denser material the speed of sound is lower. So, in trying to explain and to understand the crashsite wrotes ("But, if the warmer air molecules are further apart, I don't know why the sound travels faster. "), I can say that with warming material, the volum of molecules increase, also the speed of molecules increase and indeed the density increase but the speed of sound also increase even if the speed must decrease for a denser material. But in this case I think it is because of speed of molecules and if the air is denser this case is not comparable with the iron with denser molecule structure but no speed of molecules.

Not sure why sound travels faster in warmer air than colder air. It could be that the kinetic energy in the warmer more than makes up for the lower density. It's probably a balance thing between two or more variables as is so often the case in physics.

The oscillating motion of particles in solids is different from that of liquids and gasses. DId you take a look at this link I posted earlier?
**broken link removed**

Another question rise now, why if I hit two rocks in the air we hear worst than in the water or worst than in the iron material?

THe impedance of air is mismatched with that of iron and water...is that what you are talking about? As a result, a lot of the energy gets reflected off of the boundary between materials. It's very similar to impedance matching in electrical circuits.

Or are you talking about how come if two rocks hit in different materials, we here it better or worse in that material? I'm fairly sure that there isn't a clearcut relationship between speed of sound and the efficiency at which it propogates. So sound could travel very quickly through one material but dissipate after a short distance, while sound might travel very slowly through one material but do so more efficiently and take a longer distance to dissipate. How well you hear the sound depends on how much of the sound energy has dissipated by the time it reaches your ear, not how quickly it travelled to your ear.

 

[Hero999]

Generally the faster the speed of sound, the faster it will dissipate because the medium has less resistance. Strike a tuning fork underwater and it won't resonate for very long, the chances are it will be over-damped so there'll be no oscillation.

 

[crashsite]

Sound Under Water

Another question rise now, why if I hit two rocks in the air we hear worst than in the water or worst than in the iron material?

I can't prove it but, I suspect that we hear things louder under water than in air because the liquid couples the vibrations to our ears more efficiently than air does. Also, our bodies are mostly made of water anyway, so the sound under water may also get into our auditory canal through direct conduction rather than merely by vibrating our ear drums, as in the case with sound in air.

 

[crashsite]

Resistance

Generally the faster the speed of sound, the faster it will dissipate because the medium has less resistance.

Is that true? In electronics, high resistance has less effect since less current can flow (thus less power dissipated) but, in mechanical systems, higher resistance means that it has more effect.

 

[killivolt]

Thru space.

If I take a wave produced in space.

If I take a wave produced in the atmosphere.

If I take a wave produced in the Ocean.

Differing density and variety of conditions. All be it different waves.


Q:What is the result and why will it behave the same or different.

I understand the absence of disturbance.

Still. There is propagation as explained.

For every action a reaction. Laws of attraction and repulsion.

Each body having an electrical quality and quantity.


IE. To exact the amount of energy directly into another body of the same size and shape equally within a dimension of space.

Will they or can they transfer energy of unknown quanta not measurable.




kv