Compression of water (split from Waveguides)

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#301
gtforme00 said:


To summarize:

A displacement (oscillating in the case of acoustic sources) causes a compression or rarefaction of adjacent fluid molecules. This compression or rarefaction (local change in fluid density) results in a local increase or decrease in pressure, which expands to or contracts inward from the adjacent fluid (which is at a lower or higher equilibrium pressure). The elastic nature of the compressible fluid provides the restoring force of the fluid back towards equilibrium.

And yes it is possible to have longitudinal vibrations (compression waves) in a solid metal bar, that's chapter 3 in the book.
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Nice work David.
This is a little too gray in areas, not black and white enough. Too much reference to "particles" and the use of inviscid (which is a fluid without viscosity, not necessarily water) is a little out of place as this is a term used in flow mechanics.
Pressure is used appropriately as is displacement, but applied to the typical "compression and rarefaction" particle model.
I feel they are talking more about gases, such as air, than liquids (their constant reference to "compressible fluid").

Yes, sound travels through metals in a longitudinal wave. No compression though - displacement.
 
#302
You are correct of course, water is not the sole fluid to which the text refers. The principles of acoustic propagation are detailed in the beginning chapters of the book, and these are general to all fluids which can be approximated as inviscid. Water can be considered inviscid for linear acoustic (and for that matter nearly all undergraduate level engineering) calculations. Like most other areas of engineering, it is an approximation to reality (that water has a very low viscosity), but empirical testing can prove out that assumption if the inviscidity comes into question. [Is that even a word? :scratch: ] Whether the medium is gas or liquid doesn't matter if the inviscid approximation is used. The book makes no distinction between air and water in the acoustic pressure equations, ostensibly because there need not be a distinction.

I'm not sure that you can avoid a discussion of particles in this topic, because it is at the particle level from which the variables must be derived. What are pressure and temperature if not a measure of particle kinetic energy?

-David

If you would humor me please, could you explain intuitively how an axial displacement can be exerted on a metal rod without both ends moving instantaneously? This example is analogous to the water tube example I proposed previously.
 
#303
gtforme00 said:

I'm not sure that you can avoid a discussion of particles in this topic, because it is at the particle level from which the variables must be derived. What are pressure and temperature if not a measure of particle kinetic energy?
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First, I have to say that it is a pleasure to talk to someone in a sensible and productive manner. Someone who is not acting like it's his theory that I'm questioning and feel the need (when intelligent recourse fails) to ridicule or belittle. Thanks David.

Particle theory is a handy way of explaining things that are, for the most part, not fully understood. It has convenient models to explain phenomenon, without being too specific.
I don't have a problem with this, but the explanation of how sound travels through a medium almost always uses the the term "particle" and that's where I have a problem. An air "particle" is very different from a water "particle". A metal "particle" is very different from a water "particle", yet these are all treated alike. They are not the same and the environment that they are in is not the same.

gtforme00 said:

If you would humor me please, could you explain intuitively how an axial displacement can be exerted on a metal rod without both ends moving instantaneously? This example is analogous to the water tube example I proposed previously.
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Ever tried to push something heavy with socks on your feet on a slippery floor? You end up pushing yourself backwards.
Imagine the sound energy comes to bear on the end of the metal rod. These molecules (particles 🙂 ) are excited to a higher energy state and try to push in opposition to the energy source. No room to move but backwards. The displacement happens here.
The backwards displacement clears the way for the energy to move through the metal, at it's normal speed of sound through this metal. Elasticity of the material will determine what that is.
 
#304
Not only did I read what you said the first time, I quoted it and refuted it and then you bizarrely quoted me quoting it right before you claimed I hadn't read it 🙄

You still haven't answered the question, explain how to calculate the speed of sound in your model. You can't so you've simply repeated the same non-explanation I had just quoted in the post before:

These conditions reduce the transfer efficiency through the medium, thereby reducing the speed that sound can travel through the medium.
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"Efficiency" is generally used to describe how much power gets passed along and this would affect whether the sound travelled a long distance or was dissipated after a shorter one but it does not in any way explain SPEED. Would you like to give it a another try? How do you calculate the speed of sound in your model? (and don't even start again with the sand because it's totally irrelevant)
 
#305
poptart said:

You still haven't answered the question, explain how to calculate the speed of sound in your model.
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Been there before. Take the measured properties of water and realistic acoustic pressure levels and YES the implied extremely small compression waves and all the numbers are self consistent with the speed of sound in water by the standard model. Then how does a completely different model get the same answer?
 
#306
poptart said:
Not only did I read what you said the first time, I quoted it and refuted it and then you bizarrely quoted me quoting it right before you claimed I hadn't read it 🙄

You still haven't answered the question, explain how to calculate the speed of sound in your model. You can't so you've simply repeated the same non-explanation I had just quoted in the post before:

"Efficiency" is generally used to describe how much power gets passed along and this would affect whether the sound travelled a long distance or was dissipated after a shorter one but it does not in any way explain SPEED. Would you like to give it a another try? How do you calculate the speed of sound in your model?
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Calm down friend. It's right here (though I shouldn't need to show you this, you should know it already). The square root (Elasticity/density)=speed of sound.
Our medium has density and elasticity, correct?
You asked about the "mechanism", remember?


scott wurcer said:


Been there before. Take the measured properties of water and realistic acoustic pressure levels and YES the implied extremely small compression waves and all the numbers are self consistent with the speed of sound in water by the standard model. Then how does a completely different model get the same answer?
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I thought you bowed out of this waste of time?
Now that you are back, perhaps you can explain to us how a small source of energy (sound) can compress a (relatively) incompressible fluid when this fluid is free to move and is not constrained in any way.
You have the floor.
 
#308
MJL21193 said:
The square root (Elasticity/density)=speed of sound.
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Incorrect. Proportional is not the same as equal. You are missing a few terms, which are conveniently provided in the prior citation.

MJL21193 said:
explain to us how a small source of energy (sound) can compress a (relatively) incompressible fluid when this fluid is free to move and is not constrained in any way.
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Again, a relative term (small) without any context. Gotta have numbers here, which are again conveniently provided, most recently (as well as pages ago), in Daveze citation above. As to being unconstrained, again incorrect. The medium, any physical medium, has inertia, otherwise it wouldn't take any force at all to move it. Wouldn't that be nice. Wouldn't take any gas at all to get the car moving. Unfortunately things would just fly all over the place in no predicable fashion. That wouldn't be so good.

Sheldon
 
#309
MJL21193 said:

I thought you bowed out of this waste of time?
Now that you are back, perhaps you can explain to us how a small source of energy (sound) can compress a (relatively) incompressible fluid when this fluid is free to move and is not constrained in any way.
You have the floor.
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Sorry to disappoint, I had to meet a 12:15 AM flight from Narita and had some time on my hands.

You continue to miss the concept of impedance match. The air that "slops" around when a speaker moves is mostly wasted energy, the actual acoustic waves launched by a speaker is a small fraction of the total energy. The same is true when you speak of sloshing around water because it's "not constrained" this is not the propagation of sound waves. There is an unfortunate need for semantic clarity on the word wave in this context.

Again no effort to put actual numbers on concepts. Water is 1/10000 th less compressible than air. I have no problem with a 1 Pa pressure wave compressing water to 1 - .00048ppm of its volume (I think I got the number right) why do you?
 
#312
I might add the several hydrophone configurations are designed around the same principles as microphone arrays and would not function if the sound wave propagated by displacement rather than pressure/compression.
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Ding! Ding! Ding! That's what I was trying to get at before. I'll be interested to see if John reads about hydrophone theory and testing rather than waiting to be spoon fed.
 
#313
I did not have this book in my home office yesterday, but have retrieved it at my work office today. I quote from Principles of Underwater Sound Third Edition by Urick. Chapter 1.5 on page 11 states:
Sound consists of a regular motion of the molecules of an elastic substance. Because the material is elastic, a motion of the particles of the material, such as the motion initiated by a sound projector, communicates to adjacent particles. A sound wave is thereby propagated outward from the source at a velocity equal to the velocity of sound. In a fluid, the particle motion is to and fro, parallel to the direction of propagation; because the fluid is compressible, this to-and-fro motion causes changes in pressure which can be detected by a pressure-sensitive hydrophone. In a plane wave of sound of pressure p, the instantaneous intensity, is related to the velocity of the fluid particles u by

p = ρcu

where ρ = fluid density
         c = propagation velocity of wave

The proportionality factor ρc is called the specific acoustic resistance of the fluid. For seawater,

ρc = 1.5 X 105 g/(cm2)(s)

For air,

ρc = 42 g/(cm2)(s)
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Furthermore, I quote from Underwater Acoustic System Analysis Second Edition by Burdic. On page 16 of Chapter 2 he states:
     The propagation of sound in water is a mechanical phenomenon and depends on the mechanical properties of an elemental volume area of interest. A net force across a volume element results in an acceleration opposed by inertial properties, and mechanical strain is created in the element related to applied force and the elastic properties of the medium. The total energy involved in these mechanical effects includes the kinetic energy of motion and the stored potential energy represented by internal strain.
     A localized variable source of mechanical force imposes unbalanced forces on neighboring volume elements. The propagation of the resulting motion-strain effects away from the source results in a longitudinal compression wave that transmits mechanical, or acoustic, energy away from the source.
     For an oscillating source, the wave consists of regions of compression, where the pressure exceeds the original equilibrium value, and regions of rarefaction with pressure less than the original value. These regions move, or propagate, away from the source at a constant rate determined by the properties of the medium.
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I hope these snippets help, if there are any areas you would like clarified (or further expounded on) please let me know and I will transcribe more material from these texts. I also have other texts available which are more field specific.

Regards,
David
 
#315
scott wurcer said:
You continue to miss the concept of impedance match.
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Interesting point. We hear sounds in air primarily via pressure on the tympanic membrane in the ear, which is a poor impedance match for the much less compressible water. Consequently, our hearing underwater is conducted to the auditory nerves via the bones in our skull and jaw.

Sheldon
 
#317
scott wurcer said:
I think the ultimate irony is that if you rolled the numbers propagation by displacement would probably involve a LOT more energy.
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I would think that frictional losses are typically much greater than those generated in elastic media, via compression/rebound.

The more interesting issue here is a sociological one. That is; denial of increasing volumes of clear physical evidence that challenges a prior belief - even absent anything comparable supporting that belief. I can understand why people resisted the hypothesis of Copernicus and Galileo , and later Darwin. Fundamental belief systems that explained our relationship with the entire universe were put into question. And now, I can understand why people would want to deny that human beings can or have changed the earth's climate system. The implications are profound and challenge our long held belief in our economic systems and the imperative of growth.

But I'm mystified and fascinated at the perceived stakes that seem to be driving the continuation of the non-acoustic compression position. It seems much larger than the issue of whether or not water is compressible enough to transmit sound waves. After all, John has even admitted that water is compressible - just not enough to account for acoustic transmission. But once water is compressible, there is no longer a fundamental physical principle at stake, much less an apparent matter of central faith. At this point, it's only a matter of the numbers working out - and that's easy to demonstrate.

I got into this discussion assuming it was a simple matter of misunderstanding, or mistaken impressions of the physical mechanism involved. No big deal, happens all the time. We've all been there. Someone who knows better than we, points out where the mistake was made, we ponder it, challenge it maybe, but eventually incorporate the new knowledge and move on having learned something new. We don't do that if there is something disturbing about accepting the new information. But I've never seen something so innocuous seem so painful. What could that issue possibly be here? I'm stumped.

John, if you are just yanking our collective chains, you are good man.

Sheldon
 
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