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-   -   Compression of water (split from Waveguides) (http://www.diyaudio.com/forums/everything-else/126943-compression-water-split-waveguides.html)

MJL21193 25th July 2008 01:56 AM

Quote:

Originally posted by thoriated


Sure there is "enough energy in a sound wave to compress water", at least a very tiny amount (relative to air). Say, one part in a million, billion, trillion? Good to go. Ok? But a primary reason sound travels so fast in water is that it is relatively so incompressible.

Consider a spherical sound source immersed in water. In the real world, there is no way for it to propagate *without* compressing the water that surrounds it as it expands. Also, if the water didn't compress at all, the speed of sound in water would be infinite.

Think about it.


Hi,
Again:
Sound travels by making things vibrate. Air vibrates, water vibrates, solids vibrate. What is vibration? Mechanical motion, in this case, the molecules in the medium are moving and bashing into each other. they are not getting smaller or changing shape, just moving. This is why water transmits sound better than air and why steel transmits sound better than water - the molecules are closer together.

A spherical sound source immersed in water will make the water molecules move (vibrate). They move in the direction they are pushed. Nothing is being compressed, not even a minute amount.

I have thought about it, like 30 years ago when I was in elementary school and I learned the basics of sound transmission and (unrelated at the time) how it takes incredible force to compress water.
Think about hydraulics and how it works. Think about how sound goes through steel. Think about how easy it is to move water - you can pour it into a glass; try to put more water in the glass by compressing it (good luck with that one).
Observe, using your bathtub as the lab, how easy it is to produce waves in water by dropping objects into it. These waves show on the surface, but run deep. They are identical to sound waves, except they are on a lower frequency.
After much deep thought and experimentation, you should come to the well know conclusion that these mediums are not being compressed at all.

Is there not one reader of this thread that understands this? I encourage him or her to jump in.


PS: Don't send out invitations if you don't want me to attend. :)

thoriated 25th July 2008 02:08 AM

Quote:

They move in the direction they are pushed. Nothing is being compressed, not even a minute amount.
I'm sure you agree that any movement here will require some force.

Water is slightly compressible, as I believe you acknowledge.

The movement you mention in the quote above requires some force to exist which will necessarily cause some compression of the water, because that is how the compression of water is defined, in part. Apply force and you will get some compression of any material including water.

And recall, that I posted that if water was incompressible that sound in water would have an infinite velocity, which we all agree is not the case. Right?

I can trot out the math up to the differential equation level to prove each and every thing I'm asserting here.

Quote:

After much deep thought and experimentation, you should come to the well know conclusion that these mediums are not being compressed at all.
A couple hundred years ago, some of the best minds in existence did exactly that, and not only did they realize that sound travels through water by compressing it, they verified this through experimentation and made it a part of scientific knowledge.

MJL21193 25th July 2008 02:22 AM

Quote:

Originally posted by thoriated

And recall, that I posted that if water was incompressible that sound in water would have an infinite velocity, which we all agree is not the case. Right?

I can trot out the math up to the differential equation level to prove each and every thing I'm asserting here.


Compressibility has absolutely nothing to do with sound. Zero.
Density has everything to do with sound transmission. More dense mediums will support better sound transfer, due to the reduced distance between molecules. Molecules in everything move (even the densest substances), and when they do they bump into each other, transferring energy.
Force, of course, is involved. Anytime you want to do some work force is needed. How much force is required to move an object as opposed to crushing an object?
When you compress something, you are forcing it into a smaller space. Consider the boundaries of the incident - how much force is needed to compress a millilitre of water in a bathtub full. What would contain the compression? In a compressor, the closed cylinder contains the compression of the pistons motion. This is not the case if the great wide open.

Once again, think about it.

FrankWW 25th July 2008 04:00 AM

John, it's time you went back to school on this.

If you want to start a thread on the nature of sound waves and about the different kinds of wave I might discuss it with you.

Please understand the basic non-mathematical description of a sound wave is

compression - rarefaction - compression - rarefaction
hotter - colder - hotter - colder

In any fluid. Including water.

Nobody here is going to let go of it because it's correct.

How the initial disturbance is created is irrelevant.

You might be, I don't know, but you might be, misled in your thinking because sound wave radiation is actually very efficient. (Tom Danley's message to you is spot on with it's little springs.) If there are no obstacles for the wave which cause it to lose energy through friction, then it seems to travel indefinitely but of course the sound becomes fainter and fainter as it expands through greater spherical volume and its energy is spread out thinner.

It takes very little energy to compress and rarefy water (and air) enough to generate sound we can hear. In the extreme cases of certain natural and mechanical sources we can hear sounds over remarkable distances. I heard a volcano explode 400 kms away, on a quiet night I could hear the very low frequency cavitation of ships' propellors one or two miles away as it bent my home's single glazing and rattled the sashes (very old house).

When there is no masking wind noise it's possible to hear bird and animal colonies miles away. Marine animals can communicate with sound through water over remarkable distances. In these latter cases the energies involved are remarkably small.

Nonetheless, the process in the air and water is compression - rarefaction - compression - rarefaction ....

No more from me here on this topic.

Please read:

http://en.wikipedia.org/wiki/Sound

http://www.gmi.edu/%7Edrussell/Demos...avemotion.html



http://www.kettering.edu/~drussell/D...aves/Lwave.gif

dlr 25th July 2008 04:11 AM

Quote:

Originally posted by MJL21193
Compressibility has absolutely nothing to do with sound. Zero.
Density has everything to do with sound transmission.

Once again, think about it.

Since this isn't going away...sigh.

You complained of a lack of response by another, yet you chose to totally ignore the link I provided that describes unequivocally what the velocity of a sound wave in a liquid depends upon. It shows your lack of understanding, but in the hopes that you will actually read the page and, if you insist, show how it is in error, the page is again:

Velocity of sound

Equation (2) shows that the speed of sound in a liquid (water does fit the bill here, the book topic is oceanography) is directly proportional to the square root of the specific heat ratio divided by both the density and the compressibility. Read that again. Density and compressibility. You claim emphatically that compressibility has nothing to do with it. The reference shows that claim to be without merit whatsoever.

If you choose to ignore discussing a relevant reference, it will be apparent that you have no desire but to ignore facts other than those you claim.

So which will it be, discuss it or ignore it? Your claim is counter to what everyone else recognizes and the linked text. Tell me, how is the referenced text wrong?

Dave

MJL21193 25th July 2008 04:33 AM

Quote:

Originally posted by FrankWW
John, it's time you went back to school on this.


Please read:

http://en.wikipedia.org/wiki/Sound

http://www.gmi.edu/%7Edrussell/Demos...avemotion.html

Nothing new in the first link.
The second link looks very nice. Read the qualifier for compression though: "(ie, it is a pressure wave)". Increase pressure, decrease pressure.
Water can be pressurized, I've already touched on that. Pressure is not the same as compression.

MJL21193 25th July 2008 04:41 AM

Quote:

Originally posted by dlr

Since this isn't going away...sigh.

You complained of a lack of response by another, yet you chose to totally ignore the link I provided that describes unequivocally what the velocity of a sound wave in a liquid depends upon. It shows your lack of understanding,


I did look at that link before, but chose to not comment because I didn't see the relevance. It shows a formula using a variable k, with k representing the compressibility of the medium. The medium (water) has very low compressibility, therefore it will have negligible impact on the outcome. For air, which has good compressibility, the impact would be significant.
BUT, this is for solving the velocity of sound through the medium, not how it travels through such.

dlr 25th July 2008 04:44 AM

Quote:

Originally posted by MJL21193



I did look at that link before, but chose to not comment because I didn't see the relevance. It shows a formula using a variable k, with k representing the compressibility of the medium. The medium (water) has very low compressibility, therefore it will have negligible impact on the outcome. For air, which has good compressibility, the impact would be significant.
BUT, this is for solving the velocity of sound through the medium, not how it travels through such.

The fact is that compressibility is a factor. You said it has none. You just won't acknowledge the obvious.

You're hopeless.

Dave

Sheldon 25th July 2008 04:53 AM

Quote:

Originally posted by MJL21193
Sound travels by making things vibrate. Air vibrates, water vibrates, solids vibrate. What is vibration? Mechanical motion, in this case, the molecules in the medium are moving and bashing into each other. they are not getting smaller or changing shape, just moving. This is why water transmits sound better than air and why steel transmits sound better than water - the molecules are closer together.

A spherical sound source immersed in water will make the water molecules move (vibrate). They move in the direction they are pushed. Nothing is being compressed, not even a minute amount.

I have thought about it, like 30 years ago when I was in elementary school and I learned the basics of sound transmission and (unrelated at the time) how it takes incredible force to compress water.
Think about hydraulics and how it works. Think about how sound goes through steel. Think about how easy it is to move water - you can pour it into a glass; try to put more water in the glass by compressing it (good luck with that one).
Observe, using your bathtub as the lab, how easy it is to produce waves in water by dropping objects into it. These waves show on the surface, but run deep. They are identical to sound waves, except they are on a lower frequency.
After much deep thought and experimentation, you should come to the well know conclusion that these mediums are not being compressed at all.

Is there not one reader of this thread that understands this? I encourage him or her to jump in.


John,

Elementary school physics doesn't address compressibility of water, because in everyday practical life it is considered incompressible. But do we stop there? Going on in school introduces many concepts that aren't needed in everyday live, but they are needed for a deeper understanding of how things work. Quantum mechanics isn't taught it elementary school. Does that mean it can't be true?

From the Wikipedia entry on compressibility of water: The compressibility of water is a function of pressure and temperature. At 0 °C in the limit of zero pressure the compressibility is 5.1×10-5 bar−1.[10] In the zero pressure limit the compressibility reaches a minimum of 4.4×10-5 bar−1 around 45 °C before increasing again with increasing temperature. As the pressure is increased the compressibility decreases, being 3.9×10-5 bar−1 at 0 °C and 1000 bar. The bulk modulus of water is 2.2×109 Pa.[11] The low compressibility of non-gases, and of water in particular, leads to them often being assumed as incompressible.

From sound transmission in the ocean: Because sound wave transmission speed is directly proportional to pressure, the speed of sound increases as the pressure increases with depth.

You said it yourself, that transmission speed is a function of density (true for a given substance). If you increase the density (i.e. compress the liquid), sound travels faster. How does your motion model explain this?

And here is the physical proof: http://www.grc.nasa.gov/WWW/RT/RT199...5juergens.html

For the schlieren system, a fiber light source, which is used as a point source, is collimated with a 6-in. concave spherical mirror focused offaxis. This mirror provides an economical, large field of view but produces astigmatism and field curvature. The collimated light is projected through the ultrasonic beam being studied. Since the ultrasonic beam produces pressure gradients in the water, the water’s index of refraction is changed slightly. The gradient in the water’s index of refraction causes the light to stray slightly from its path. The collimated beam is then focused to a point using another 6-in. concave spherical mirror. A knife-edge is placed at the focal point of the mirror to block the rays whose path has been altered by the varying index of refraction from the ultrasonic beam. A video camera is placed after the knife-edge, allowing the ultrasonic beam to be viewed on a monitor.

Sheldon

They are measuring directly changes in the density of water as ultrasonic (high frequency sound, but sound all the same) sound compresses the water. Simple motion does not change the refractive index of a liquid.

MJL21193 25th July 2008 04:54 AM

Quote:

Originally posted by dlr



You're hopeless.

Dave


Now, why do you have to be like that? I'm not voicing what I think of you here, am I?
You want to prove me wrong?
Here's how:

Change the laws of physics.


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