Compression of water (split from Waveguides)

[despotic931]

Take a bowl of water. Add sound energy. As explained above, energy dissipation = heat; heat equals expansion. Density decreases.
Take a steel ruler. Add energy by bending it. Energy in = heat = expansion. Density decreases.
In both cases, displacement is the answer. Molecules are displaced and the volume increases.

Ok John, in your example the displacement is caused by heat, cause and effect. It's not the mode of transfer but rather a byproduct of the events that are taking place.

Somehow you commented on the beginning and end of my post without reading the middle

Seems that John does this quite often...

This is trivial, patronising and a distortion of the truth. As ever, you focus on displacement to the exclusion of all else.

The primary mechanism in this case (thermal expansion) is the increased inter-molecular distance resulting from increased vibration. Yes, this results in displacement, but what you have written suggests displacement is the prime cause. Displacement is a result.

It is commonplace to ascribe cause to those events preceeding results.

You need to revisit thermodynamics.

w

I'm quoting this ^ because it is everything I thought to say when I read John's post that I quoted earlier in this reply. It seems we are in agreement again!

-Justin

 

[MJL21193]

nor do you own them or this forum. I'm not personally insulted, I just think you're playing games with the people of this forum and I'd like to know what the real goal of that game is.

No, I don't own the forum but I'm not being mean to you or anyone else here, am I? Say your peace without sarcasm and snideness and I'll respect you more for it and give more credence to your replies.
As for the games we play...well....

Somehow you commented on the beginning and end of my post without reading the middle where i actually agreed with you, but so what? Does saying the *average* density of a large volume of water changes one way or another prove that there are no local variations in density? No. It's a totally different issue AND it says nothing about your claim that sound doesn't travel by compression wave.

I did read your entire post and see where you agreed. I was more intent on answering your question.
My point was that there is displacement - I have been saying it since the first page. The proof of this displacement is the first law of thermodynamics:
"The increase in the internal energy of a system is equal to the amount of energy added by heating the system, minus the amount lost as a result of the work done by the system on its surroundings."
You cannot add energy without a temperature change. The temperature change brings about a volume expansion. Displacement.

What happens when you heat water on a stove? Increased energy = temperature increase = expansion.

What happens when you hit a piece of iron with a hammer repeatedly? Increased energy = temperature increase = expansion.

What happens when you bend a strip of metal (a ruler perhaps) back and forth several times? increased energy = temperature change = expansion.

The above are all readily demonstrable and really, they should be fairly common knowledge.

Let's talk about stress again. Say there is a plank of wood that spans a ditch. When you walk on this plank, it is dealing with stresses from the load bearing down on it. On the top of the plank there is compressive stress, tending to crush the wood into a smaller space. On the bottom there is tension, where the material will try to resist being pulled apart. This is the first area of failure as the plank breaks, because this is where it is weakest.
Although the top of the plank is under compressive stress, it doesn't actually compress very much at all. Rather, the equilibrium point between these two stresses shifts toward the top. This results in the bottom getting stretched to the point where it will pull apart.
What does this have to do with sound in water? It demonstrates yet again the path of least resistance.

 

[MJL21193]

It seems we are in agreement again!

How is that old saying: "great minds think alike"

 

[auplater]

not direct insults, but..

..you make statements with an authoritarian tone contradicting what almost all other posters have stated ad nauseum, as if somehow you're the professor and the rest are the unwashed masses.

Nothing could be further from the truth. repeat this phrase as many times as necessary

cause>>> effect... cause>>>effect. You have these completely reversed, be it stress>strain relationships, thermal loading, transmission of sound... etc.

 

[MJL21193]

The primary mechanism in this case (thermal expansion) is the increased inter-molecular distance resulting from increased vibration. Yes, this results in displacement, but what you have written suggests displacement is the prime cause. Displacement is a result.

See, you are coming to your senses!

First comes energy (sound wave). This creates motion, molecules will move. In order for the molecules to move, they need space to do this. Johns First Law of Least Resistance states: "when there is no constraints on a medium to expand in volume to accommodate molecular movement, it will do so, as this is the path of least resistance".

Homework assignment: Visit the grocery store and buy a tray of Jiffy Pop popcorn.
Have an adult present to supervise the next bit.
Carefully heat the Jiffy Pop on the stove, following the instructions. Take note of what happens when the popcorn starts to pop. Incredibly, instead of compacting itself within the foil pan, it actually pushes the foil top outwards! This is astounding! Once again, Johns First Law of Least Resistance in action!
Now, reward yourself with this tasty snack.

 

[Sheldon]

Well, it isn't obvious to the majority of contributors to this thread, judging by what I have read. The given explanation for sound to travel through a medium is by compression and rarefaction, with no displacement or volume increase.

If what you mean by majority is all but one poster, then yes.

Let's be clear about displacement in this context. No net displacement is of the medium is required. Of course, in order to compress water or create rarefaction, molecules must be displaced. But once the wave passes by, the molecules return to their initial position.

The standard explanation: As the sound wave moves through the medium, it pushes adjacent molecules closer together (compression), leaving a space behind (rarefaction).

It's far more than the "standard explanation". It's a product of rigorous physical and mathematical confirmation over many years and all that is known about the properties of materials. We couldn't make progress in sonar or predict behavior in any other area that requires knowledge of the transmission of physical waves if we didn't have a solid theoretical and mathematical foundation to go on. It's not "standard", it's the only explanation that fits all that is known about the physical properties of matter.

Even though one member here suggested that at the end (after the sound has passed through the medium) the overall density of the medium would increase (occupy less volume), most here seem to believe that the medium will conduct the sound energy without a change to the original state (volume) of the medium.
They say that no displacement takes place.

Displacement and volume change are two completely different things. Displacement refers to movement of something from one position to another and doesn't require a volume change. During sound wave propagation, water molecules are displaced, as explained above. A wave passing through a medium does not require an overall volume change. Compression and rarefaction occur simultaneously, with no net change in volume. Yes, the sound wave will not be transmitted entirely without friction, which means a gain in temperature and gain in volume - in an adiabatic system. Has nothing to do with the mechanism of transmission, other than than the wave will eventually die out due to frictional losses, and that the acoustic energy pumped into the system will primarily end up as infrared energy (heat).

I have been saying that this requires less energy to achieve than to push molecules closer together. I have been saying that this is the primary mechanism for sound energy transfer through the medium.

If you mean it takes very little pressure (force/area) to move water in an unconstrained system, as compared to the local pressure required to generate a sound wave, then yes, that's true. But then, you don't generate a sound wave. That's the whole point. And that's the point of impedance matching in this context. The energy applied is a function of force over distance. Force is a function of pressure times area. An underwater transducer that supplies the same energy as one in air requires a much smaller area and much less excursion, but much higher pressure (think of the pressure over the area of your fingertip compared to the same fore applied at the point of a sharp tack) - precisely because water is much less compressible than air. Mount a water proof speaker cone against water and you will get some sound transmission. But it won't be through pistonic action of the cone, which is much too weak for the pressures involved. The sound would be transmitted by the edge of the voice coil, which is a very small area compared to the cone, and so can generate much higher pressure with the same force.

I reply for those readers who actually might be interested in the physics and mechanics involved.

Sheldon

 

[despotic931]

How is that old saying: "great minds think alike"

Then why does no one think like you John?

Have an adult present to supervise the next bit.

Dang, well I guess I can't do my homework!

(P.S. I have a jiffy pop tin sitting right next to my computer, but can't remember why. Hmmm, it must have been fate. Sadly imo, this experiment proves nothing for your case. However, if you wanted to state that a sealed container of water would explode when put on top of my oven I would agree with you...)

Displacement and volume change are two completely different things. Displacement refers to movement of something from one position to another and doesn't require a volume change. During sound wave propagation, water molecules are displaced, as explained above. A wave passing through a medium does not require an overall volume change. Compression and rarefaction occur simultaneously, with no net change in volume. Yes, the sound wave will not be transmitted entirely without friction, which means a gain in temperature and gain in volume - in an adiabatic system. Has nothing to do with the mechanism of transmission, other than than the wave will eventually die out due to frictional losses, and that the acoustic energy pumped into the system will primarily end up as infrared energy (heat).

Basically the small change in volume that does take place is not a direct result of the sound wave but rather a result of the heat caused by the friction of the molecules compressing and expanding and bumping into each other. I think that displacement is being confused with tension (best explained here, wiki on tension, and here, wiki on reaction forces, and if you look here you can read what wiki has to say about physical compression). Tension is a direct result of the compression that takes place as the sound wave travels through water. That is unless you are talking about particle displacement, which unfortunately does not result in any volume change in the medium either, so the point is still mute. Cause and effect, compression and tension/rarefaction, and heat and expansion. BTW sheldon, I agree with everything in your post, I am just quoting it to back up my own argument.

-J

 

[sreten]

Hi,

How do you extract such a malaise of tedium from a simple facts ?

Waves propagate in elastic mediums - end of story .....
There is no such thing as an inelastic medium .....

It means waves will propagate at the speed of sound in the material.

Waves are like osciilating systems, like a mass on the end of a spring.

Said sytem works by interchanging kinetic and pontential energy,
the total always being the same and the two functions being orthogonal.
That whilst the mass/ spring is typically solved for position, i.e. displacement,
and velocity give the kinetic energy, a description in terms of spring compression
and rarefication and potential energy is equally valid.

Otherwise it would not oscillate - and basic physics would be garbage.

Waves are exactly the same, an interchange of kinetic and potential energy.

The two are again orthogonal, the only thing required is an elastic medium.

The only thin of interest is terms of understanding is the total energy,
and this is represented by the combined kinetic and potential energy.

The interchange between the two can be represented by the function for either.

Saying waves are one or the other is totally inane ....
Waves are both, by definition of wave propogation.

You can choose to see it as either ....

No net displacement or net density change occurs as the wave
passes, this should be mindnumbingly tediously obvious.
Local displacement and density change is the wavefront.


/sreten.

 

[scott wurcer]

If what you mean by majority is all but one poster, then yes.

Let's be clear about displacement in this context. No net displacement is of the medium is required. Of course, in order to compress water or create rarefaction, molecules must be displaced. But once the wave passes by, the molecules return to their initial position.

Thanks, clearly you're closer to this stuff than I. I don't think I made any wrong statements. The point being, even with a general feeling for how things "work" the standard theory feels right intuitively.

 

[wakibaki]

Have an adult present to supervise the next bit.

Does blood in the water affect it's compressibility?

w

 

[MJL21193]

Displacement and volume change are two completely different things. Displacement refers to movement of something from one position to another and doesn't require a volume change.

I defined my version of displacement above. I'm very specific about what I mean.
Here it is again: "here's what I mean by "displacement": In order for the medium to accommodate the increased motion of the molecules as they are excited by the energy input, the volume that the medium occupies will increase."

During sound wave propagation, water molecules are displaced, as explained above. A wave passing through a medium does not require an overall volume change. Compression and rarefaction occur simultaneously, with no net change in volume.

Once again, how do you do work, transfer energy and dissipate energy without creating heat? Energy in = increased motion = friction = A change of temperature = volume change (displacement).
This is key - put on your thinking cap.

Yes, the sound wave will not be transmitted entirely without friction, which means a gain in temperature and gain in volume - in an adiabatic system. Has nothing to do with the mechanism of transmission, other than than the wave will eventually die out due to frictional losses, and that the acoustic energy pumped into the system will primarily end up as infrared energy (heat).

Now you are contradicting yourself. Either heat is produced or not.
How can you say that the end result of the sound energy passing through the medium has nothing to do with sound transmission? You guys were chirping "cause and effect, cause and effect", right? What happened to your better understanding?

Think about it. How much compression are you talking about? Very little?
Well, how much displacement do you think I'm talking about? Enough to double the volume the water occupies? Not likely. A tiny amount. And, as mentioned above, this mechanism is the path of least resistance. It will take less energy to move molecules by displacement than it will to push them closer together.
Simple, logical conclusion.

 

[poptart]

I think the record is skipping. You need to come up with some new crazy thing to say to keep us interested, the temperature change thing is unimportant.

 

[MJL21193]

Said sytem works by interchanging kinetic and pontential energy,
the total always being the same and the two functions being orthogonal.
That whilst the mass/ spring is typically solved for position, i.e. displacement,
and velocity give the kinetic energy, a description in terms of spring compression
and rarefication and potential energy is equally valid.

Otherwise it would not oscillate - and basic physics would be garbage.

Consider this with your most open minded attitude: As I mentioned above, of the two stresses that a molecule will see when a force come to bear upon it, tension is the easier to overcome. Could this stretching of the bond length be the determining factor in how elastic a medium is?
Is it possible that this is number two (behind displacement) in the chain of events that transpire to move sound through a medium?

So here's what I have, from major to minor: Energy in > increased movement > displacement (volume expansion) > bond tension > bond compression.

No net displacement or net density change occurs as the wave
passes, this should be mindnumbingly tediously obvious.
Local displacement and density change is the wavefront.

I have mind-numbingly proved otherwise. Have you read a single word I wrote???

 

[scott wurcer]

I have mind-numbingly proved otherwise. Have you read a single word I wrote???

You have a creative definition of proof.

 

[MJL21193]

the temperature change thing is unimportant.

I think the compression is unimportant. How's that for crazy?

Sincerely, think about it. You are discounting the only provable mechanism involved: displacement, proven by volume increase. This is not science fiction.
Kinetic energy cannot pass through a medium with no net effect. Heat MUST be produced.

Heat is the end result, not the cause. Motion is the cause, heat is the effect. Everything in between is part and parcel of the process.
Understand?

 

[Sheldon]

We tried our best. It's hopeless.
John, feel free to make up new definitions to common words, make up brand new physics out of whole cloth, and ignore anything that you don't like, regardless of well understood and well characterized physical realities. Enjoy your universe of one. By.

Sheldon

 

[MJL21193]

We tried our best. It's hopeless.
John, feel free to make up new definitions to common words, make up brand new physics out of whole cloth, and ignore anything that you don't like, regardless of well understood and well characterized physical realities. Enjoy your universe of one. By.

Sheldon

I didn't make up any definition. Common words can have different meanings. The displacement I've been talking about (ever since this began) has a real life definition.
I haven't ignored anything relevant, that you can verify by going back through these pages.

Anyway, have fun. Glad to have had this discussion with you.

 

[MJL21193]

You have a creative definition of proof.

Proof:
Sound is a form of kinetic energy. It will pass through a medium by exciting the molecules to move faster and further. This results in a volume expansion because:
1/ It is fairly well know that when you use energy to create motion, friction due to this increased motion will produce heat. Heat means a temperature rise, a temperature rise equals a volume expansion.
2/ Energy cannot be created or destroyed. When the sound energy enters the medium, it moves through the medium and encounters the boundary. Some sound energy is passed to the surrounding medium, but due to the impedance mismatch, much bounces back. In fact, a good potion of the sound energy will not leave the medium via kinetic transfer and will be dissipated in the form of heat within the medium. This will result in a volume expansion and a temperature rise.

 

[Daveze]

Proof:
Sound is a form of kinetic energy. It will pass through a medium by exciting the molecules to move faster and further. This results in a volume expansion because:
1/ It is fairly well know that when you use energy to create motion, friction due to this increased motion will produce heat. Heat means a temperature rise, a temperature rise equals a volume expansion.
2/ Energy cannot be created or destroyed. When the sound energy enters the medium, it moves through the medium and encounters the boundary. Some sound energy is passed to the surrounding medium, but due to the impedance mismatch, much bounces back. In fact, a good potion of the sound energy will not leave the medium via kinetic transfer and will be dissipated in the form of heat within the medium. This will result in a volume expansion and a temperature rise.

Proof? No.
Hypothesis? Yes.

A proof would require either a swag of equations or a set of results from an experiment that really strongly supported your hypothesis.

 

[poptart]

Nobody is denying that friction causes heat or that heat cause water to expand, but it doesn't mean anything. The tires on my car get hot from friction against the road and they probably expand but this doesn't say anything about whether gas, electricity, or "tire expansion" is used to make it go (and that analogy it about as far from relevant or "proof" as the things you've been posting).

Sound waves are perfectly well understood, you haven't "proven" anything. Not one person here would say you were anywhere even close. All you've been doing is playing little semantic games focussing on different meanings of the same word to try and make it seem like you have a point but there is none.