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Old 28th July 2008, 02:12 AM   #121
SY is offline SY  United States
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OK, now you're conflating two things you wanted to keep separate- water that's unconstrained and has a compressible gas above it and surface waves versus volume constrained liquid with compressional waves. Which do you want to talk about?

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Let's hear your version of how energy is transferred through a medium?
Um, how many more times do you want me to repeat it?
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Old 28th July 2008, 02:15 AM   #122
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Just curious, but does it matter which way the winds blows
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Old 28th July 2008, 02:31 AM   #123
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Sign convention.
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Old 28th July 2008, 02:33 AM   #124
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Quote:
Originally posted by SY
OK, now you're conflating two things you wanted to keep separate- water that's unconstrained and has a compressible gas above it and surface waves versus volume constrained liquid with compressional waves. Which do you want to talk about?



Um, how many more times do you want me to repeat it?

Once again, re-read what I have written. I said nothing about surface waves at all. The point of the analogy was show the difference in force required to compress VS displace. That's the MAIN POINT that I'm trying to make

The whole topic is about how sound travels through water, whether it's open with sunny sky above or trapped in a sealed tank. You all say it undergoes "compression", I say it does not. I say there is a pressure change. You say (believe) that compression and pressure are the same. I say they are not. It's all here for anyone to read, I'm not holding anything back.

I'm curious about your thoughts on how sound travels through a medium without moving the molecules. That is what you said.


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Old 28th July 2008, 02:43 AM   #125
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Old 28th July 2008, 03:02 AM   #126
SY is offline SY  United States
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Quote:
I'm curious about your thoughts on how sound travels through a medium without moving the molecules. That is what you said.
I call shenanigans. I not only said no such thing, I said precisely the opposite.
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Old 28th July 2008, 03:02 AM   #127
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Quote:
Originally posted by MJL21193
Imagine, if you will, that you have a material that is fairly dense. It isn't infinitely dense, but it does require a great deal of force to make it more dense, even in a "local" way.
Water meets that description. And you don't need to specify "local" as it behaves the same locally as it does globally with respect to compression.

Quote:
Originally posted by MJL21193
material is at rest, at a specific temperature under a specific pressure.
Now, examine how sound travels. It is a form of energy that will produce vibration, right down to the molecular level. The energy from our sound wave will make molecules vibrate in our material.
Increasing the vibrational state of water, simply means you've heated it up. Heat doesn't travel in waves near acoustic wavelengths through water. There is no alternating cool and warm areas. Sound moves by compression. A small amount of the energy is lost as heat during wave propagation over distances we are interested in, due to the friction of compression and release, and eventually it is all converted to heat.

Quote:
Originally posted by MJL21193
The molecules need room to move. The problem is that the energy (or force) from the sound wave is not enough to push the molecules closer together in this material. It tries to push molecules closer together, as in compression, but it can not as there isn't enough force.
Do you know how much force is required? Or the flip side; do you know how much compression is required? If you don't, how can you make that statement? Here, by the way, is how you calculate it for various media. You may not like it though, because it requires terms for density and bulk modulus (read; elasticity, or if you like; compressibility - ouch, there's that horrible term). http://hyperphysics.phy-astr.gsu.edu...ouspe2.html#c1

This was on a site already noted about a page back, BTW.

Quote:
Originally posted by MJL21193
What it does instead is displaces the molecules around it (like getting in a tub that is too full, water overflows - that is displacement). This takes a lot less force.
That means nothing in the context of acoustic waves transmitted through water, unless you can construct the equations which predict things like the speed of sound in various media, including liquids and solids. I see SY has already asked you for your calculations.

Quote:
Originally posted by MJL21193
Now we have the "compression" part of the sound wave exciting the molecules in our material, making them move, vibrate rapidly. The increased activity, under the relatively unchanged condition (temp, pressure) translate to a region of high pressure in our material. This is "seen" as a cluster of rapidly vibrating molecules.

Along comes the "rarefaction" part of the sound wave. This tries to reduce density within our material, but it also lacks the force necessary to accomplish that. Instead, it pulls back on our excited molecules. The result of this is a region on low pressure within our material. This would appear as an area of relatively still molecules.
"Rapidly vibrating molecules" as opposed to "not so rapidly vibrating molecules" are radiating energy in the form of infrared energy (as SY has noted). Yes, heat applied locally, as with a pulsed laser can cause a local rise in pressure that can be fast enough to create an acoustic wave, which can then travel through water as sound, i.e., a pressure wave. Thermal energy is not transmitted anywhere near as fast as acoustic waves, nor as alternating cool or warm regions (as noted earlier).

Quote:
Originally posted by MJL21193
As the sound wave travels through the material, it loses energy in the form of heat. Given the small amount of energy the sound wave has in the first place, this will not be very much heat. BUT heat is heat and it will effect a temperature change.
OK.

Quote:
Originally posted by MJL21193
What happens to any material when it gets warmer? I won't wait around for a sensible answer, but give it now myself: as the material gets warmer, it's molecular activity goes up (more vibration). This equates to a lower density per unit volume. Admittedly, this will be a very slight decrease in density, as the temperature increase would be slight as well.

I ask you - what is so ridiculous about what I am saying here? I dare anyone to read with understanding what I have written above and tell me what I'm saying isn't plausible.
I read it. I don't have any problem understanding what you are saying. Heat is infrared radiation. Infrared radiation is much much shorter in wavelength than sound. At about 35 degrees centigrade, the wavelength is about 10 microns. I'll leave it to you to calculate how many orders of magnitude that is from the wavelength of acoustic waves. Water is a very good absorber of IR, so heat travels through water almost entirely by conduction (and under the influence of gravity, by convection, but that's another matter), which, as noted earlier is very slow compared to sound waves.

John, you've got the math behind the physics, you've been given even experimental results from direct measurements on the density changes in water as sound waves pass through it (which you said, was due to motion - though you can cite no evidence that refractive index has anything to do with bulk movement. Don't you think the scientists who were capable of devising the experiment might have thought of that if it were indeed the mechanism).

I'm truly intrigued. Why are you clinging so hard to this misperception? And that's exactly what it is. You PERCEIVE that water is "too hard to compress" for compression to be the mechanism for sound waves traveling through it. But science and technology requires that we go beyond our gut perception of things and develop models that can be tested. We all have misperceptions, but darn, why so vested in this one? Everything you need to do the calculations is in this thread. Get out your pencil and prove it too yourself. If you are still not satisfied, make up you own world - hey, L Ron Hubbard did and it was quite lucrative for him. Just don't design anything based on it, where your life might depend on the outcome.

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Old 28th July 2008, 03:34 AM   #128
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First off, you have misunderstood a lot of what I am saying.

Quote:
Originally posted by Sheldon


Water meets that description. And you don't need to specify "local" as it behaves the same locally as it does globally with respect to compression.
The term "local" wasn't mine, it was Sy's. Local is supposed to refer to the compression at the leading part of the wave.


Quote:
Originally posted by Sheldon

Increasing the vibrational state of water, simply means you've heated it up. Heat doesn't travel in waves near acoustic wavelengths through water. There is no alternating cool and warm areas. Sound moves by compression. A small amount of the energy is lost as heat during wave propagation over distances we are interested in, due to the friction of compression and release, and eventually it is all converted to heat.
Sound travels through a medium by making the molecules move. I said "vibrate" where I should have said "move". Is that more understandable?
Replace "rapidly vibrating" with "rapidly moving".



Quote:
Originally posted by Sheldon
[B]
Do you know how much force is required? Or the flip side; do you know how much compression is required? If you don't, how can you make that statement? Here, by the way, is how you calculate it for various media. You may not like it though, because it requires terms for density and bulk modulus (read; elasticity, or if you like; compressibility - ouch, there's that horrible term). [B]
I know that there is quite a bit more force needed to compress something than to displace it. That's the main point that I'm trying to get you to understand - you guys are as obtuse as you make me out to be.
Put away the other terms you are using, they don't apply to what I'm saying here.
Instead of seeing the whole story, you fixate on tiny details and argue about those. You miss the forest for the trees.
Read my post #107 again and come back with some sensible comments please.
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Old 28th July 2008, 04:15 AM   #129
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Quote:
Originally posted by MJL21193

The molecules need room to move. The problem is that the energy (or force) from the sound wave is not enough to push the molecules closer together in this material. It tries to push molecules closer together, as in compression, but it can not as there isn't enough force. What it does instead is displaces the molecules around it (like getting in a tub that is too full, water overflows - that is displacement). This takes a lot less force.
Ok, this is me trying to keep an open mind and understand what you are saying...

In a sound wave molecules are not compressed but rather displaced? Where do the displaced molecules go? What happens to the water that has overflowed onto the floor?


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Originally posted by MJL21193
Hi Glen,
More of a leg man, myself, but thanks for the diversion.

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Old 28th July 2008, 05:51 AM   #130
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by MJL21193
The term "local" wasn't mine, it was Sy's. Local is supposed to refer to the compression at the leading part of the wave.
You used the term local in your post. I don't see where ownership enters into the discussion. And it's not relevant to my point, which is that the compression is governed by the same rules, whether local, as a pressure wave, or general, as in compressing bulk water.

Quote:
Originally posted by MJL21193
Sound travels through a medium by making the molecules move. I said "vibrate" where I should have said "move". Is that more understandable?
Replace "rapidly vibrating" with "rapidly moving".
No, it's not more understandable. "Moving" is so general in this context as to have no useful meaning. Moving relative to what exactly, how fast, how far, in what direction? On the other hand pressure waves are exactly described as to these definitions. Its speed relative to point of origin is described, it's radiation pattern is radial from a point source (small point, relative to the surrounding medium and distance measured), it's reflection and difraction patterns are described (which is where this whole thing started). And your "moving"? Surely if that's how it works, you must be able to find some source that gives the physical and mathematical description. No? After all the work done in physics, including the work that helps people design the parts we use here, you'd think that if your conjecture is correct that it would be easy to find validation for an authoritative source (no, not the grade school text). Found it yet?

Quote:
Originally posted by MJL21193
I know that there is quite a bit more force needed to compress something than to displace it.
Absolutely true. So how does that establish that sound waves traveling in water are not compression waves? These things can be quantified and measured - and have been. That's why I asked you if you knew how much force is required. And yes, that was somewhat of a rhetorical question, given your refusal to come to terms with long settled physics. But there's nothing obtuse about it. I gave you the force required to compress water, early in the discussion. I gave you the equation to plug that number into in the last post. Do it. You will find that you can calculate the speed of sound in water. It works, and doesn't violate any laws of physics, such as requiring forces to be transmitted at the speed of light. Surprised? Coincidence? Now try and find a competing theory that allows the same calculations without requiring compression. Good hunting.

Quote:
Originally posted by MJL21193
Put away the other terms you are using, they don't apply to what I'm saying here.
Instead of seeing the whole story, you fixate on tiny details and argue about those. You miss the forest for the trees.
Read my post #107 again and come back with some sensible comments please.
No, the terms we are using don't apply to what you are saying. Because what you are saying is physical nonsense.

I responded to every issue you raised in you post. My comments aren't sensible to you because they contradict your position.

You finished with: "I ask you - what is so ridiculous about what I am saying here? I dare anyone to read with understanding what I have written above and tell me what I'm saying isn't plausible."

Based on grade school understanding (your term), or to a layman with no training in physics, acoustic compression waves in water might not be obvious. But many people have tried to explain how it works, have pointed to detailed descriptions of the mechanism, given experimental evidence, relevant equations and constants to calculate the movement of acoustic pressure waves in water, against which nothing that can be mathematically derived or computed, in fact no citations of any kind which support the proposed alternative mechanism. What is ridiculous? I didn't not bring up that pejorative, but since you asked, it's your continued refusal to move from your position despite the weight of evidence against it. And no, in light of the evidence in this thread, what you are saying isn't plausible.

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