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Old 25th July 2008, 03:55 PM   #31
Daveze is offline Daveze  Australia
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Quote:
Originally posted by MJL21193
Second, we are not talking about air, we are talking about water. The cool thing about water is that anything that you add to it, that will stay in suspension, will actually decrease density.
Actually, water does not possess this property you describe. The density of a quantity of water can be increased dissolving something in it, eg comparing the buoyancy of fresh water to that of salty water.

Quote:
Originally posted by MJL21193
The springs analogy is fine, if you understand that the springs go both ways, as one shortens the opposite side lengthens. Molecules are moving and they move faster when excited by the force from a sound wave. Molecular movement denotes a density increase, not the other way around. As molecule move more rapidly, they become further apart, temperature goes up.
Actually, it works both ways. An increase in density can cause molecular movement, which in turn causes an increase in density...its how a wave propagates through a medium.

Quote:
Originally posted by MJL21193
Energy into a system will produce heat, not refrigeration.
So I'm in an ordinary mood but you're wrong again. Its about where you apply energy, what the system is and how you define refrigeration. If you apply energy to the right location of a heat pump system you will get a refrigeration effect at one location (relative to its surrounds) and a heating at another (relative to its surrounds).

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Originally posted by MJL21193
Ok, please explain the other factors that influence sound transmission through a medium, that are not directly related to density.
I'll start holding my breath...now.
The primary other factor, which others have alluded to, is the compressibility or stiffness properties of a substance. Fundamentally speaking: its the nature by which a substance's structure deforms and reforms.
- For a gas, the chaotic nature of the particles motion means that it easily deforms but does not reform so readily (high compressibility). This means that the pressure wave moves slowly because the substance is not resisting the changing condition (the local high/low densities).
- For a liquid, the particles are still chaotic but not to the same extent. They can be compressed but do not like to be and return to their original state more quickly than a gas. This translates to a pressure wave moving more quickly through the material.
- For a solid, the particles are fixed (crystalline) and consequently highly resistive to changing their location. It is then the ability of the crystal structure (rather than particles) to deform and then reform, which is why there is a difference in the speed of sounds that Bratislav mentioned. Simply speaking, steel is generally made up of many sub-crystals that deform relative to each other, whereas diamond is a single crystal with bonds such that it is incredibly stiff.

I've noticed that because I took so long to form a response, several other people have replied with similar concepts. Hopefully I, at least, provide some assistance.
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Old 25th July 2008, 03:56 PM   #32
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Quote:
Originally posted by SY


Now consider a single frequency sound wave in the fluid. There will be a change of average intermolecular distance along the direction of the wave, with the a.i.d. being greater than unity in places where the sine wave has its peaks, and less than unity where the sine wave has its troughs. The average over the fluid is still unity- this is very important to note, the domino thing is inapt, the First Law of Thermodynamics still applies here.

OK, now take a tiny little cube of material from the peaks. Count the molecules. The particle density will be greater than unity. Take an identically sized cube from the troughs. The density will be less than unity. And if for the same volume the particle density differs from unity, the mass differs from unity.

Now most people would say that if two identical volumes from the same fluid show different densities, there's been a compression in one. But if you don't like that definition, here's an alternative one: for a given number of molecules, the resulting volume at the peaks will be smaller than the volume of the same number of molecules at the troughs.

The water suffers no overall compression or expansion, but locally, it indeed expands and contracts by either definition. Don't confuse AC with DC.


Ok, compression is making something smaller. A volume of water, a car, a office building. In order to compress something, you need to do some serious work. You need to bring substantial force to bear.
It doesn't matter how small the sample of water is, compression is still not taking place. Molecules are being pushed in response to the pressure exerted from the sound wave. When you push a single molecule closer to the next one, compression is not happening - they still occupy the same amount of space. Density is not increasing, a void is left behind by the molecules forward movement.

Do this:
Construct a very strong, sealed tank filled with water (no air inside). Put a transducer in the side of the tank, with it's diaphragm in direct contact with the water and it's back open to the room. See how much the water inside the tank will be compressed by the forward motion of the transducers diaphragm. The diaphragm will not move. It cannot possibly compress the water trapped inside.

Now, take that same transducer and put it inside the sealed tank. It will produce sound that will go through the water and reach the outside air. No problem. Why? Because the water is not being compressed by the diaphragm, it is merely moving the molecules in the water. In fact, efficiency will go down as the water heats up due to increased molecular movement. Instead of compression, we will see the opposite, the water will expand.
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Old 25th July 2008, 04:03 PM   #33
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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.
They are pushed by applying force, you agree. Where you are wrong is that you believe there exist materials that will not compress at all when a force or pressure is applied to them.

There is also no threshold in a non quantum-dynamic system where a material will 'not compress' below a certain pressure.

So, you're just out of luck here. If you admit that sound consists of pressure waves in fluids and gases, and materials compress due to pressure, you must also admit that sound compresses materials.

No rational way around it.
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Old 25th July 2008, 04:07 PM   #34
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In order to compress something, you need to do some serious work. You need to bring substantial force to bear.
You're just making that up out of whole cloth. Any force, even an extremely 'tiny' one will cause some compression, as I just stated.
Also, force is not 'work'.

In fact, I challenge you to find any reference on the internet or elsewhere that backs up in any way your excerpt above. You can't do it, because it's untrue. Every valid reference I've ever seen to 'incompressible materials' makes sure to mention that it is really just a catch phrase for a definitely quantifiable but reduced level of compressibility for that material that otherwise obeys the same relationships to force as 'more compressible' materials.
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Old 25th July 2008, 04:13 PM   #35
Eva is offline Eva  Spain
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MJL21193 I have a question for you...

Given a constant mass of water (a constant amount of molecules), does it's volume change with temperature?

If your reply is "no", then, why water expands as it gets closer to freezing even to the point of breaking too rigid containers?

If your reply is "yes", then try to visualize sound propagation, in water or any other medium, as travelling temperature gradients (temperature, density, pressure, in the end it's all related).
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Old 25th July 2008, 04:14 PM   #36
Daveze is offline Daveze  Australia
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Quote:
Originally posted by MJL21193
Ok, compression is making something smaller. A volume of water, a car, a office building. In order to compress something, you need to do some serious work. You need to bring substantial force to bear.
It doesn't matter how small the sample of water is, compression is still not taking place. Molecules are being pushed in response to the pressure exerted from the sound wave. When you push a single molecule closer to the next one, compression is not happening - they still occupy the same amount of space. Density is not increasing, a void is left behind by the molecules forward movement.
But you are increasing the density, just not where you're proposing to measure density. The bulk density remains the same, true. The local density effects, ie molecules moving nearer or further, are what are concerned for the transmission of sound.

Quote:
Originally posted by MJL21193

Do this:
Construct a very strong, sealed tank filled with water (no air inside). Put a transducer in the side of the tank, with it's diaphragm in direct contact with the water and it's back open to the room. See how much the water inside the tank will be compressed by the forward motion of the transducers diaphragm. The diaphragm will not move. It cannot possibly compress the water trapped inside.

Now, take that same transducer and put it inside the sealed tank. It will produce sound that will go through the water and reach the outside air. No problem. Why? Because the water is not being compressed by the diaphragm, it is merely moving the molecules in the water. In fact, efficiency will go down as the water heats up due to increased molecular movement. Instead of compression, we will see the opposite, the water will expand.
Poor, poor transducer. You're asking an ant to do the job of an elephant. Experiment: Take a beer bottle. Consume beer. Refill with water to the brim. Hold filled bottle by neck. Take palm. Slap top of bottle in forceful manner. Watch your feet. Seek medical attention if required. Review your super high speed camera and observe the time that it takes between the slapping and the shattering. That time is the speed that the pressure wave propagates through the fluid. That speed is the speed of sound in water.
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Old 25th July 2008, 04:15 PM   #37
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Quote:
Originally posted by Daveze


Actually, water does not possess this property you describe. The density of a quantity of water can be increased dissolving something in it, eg comparing the buoyancy of fresh water to that of salty water.
I stand corrected. Yes, salt water will be more dense.


Quote:
Originally posted by Daveze


Actually, it works both ways. An increase in density can cause molecular movement, which in turn causes an increase in density...its how a wave propagates through a medium..
I think not. Molecular movement will decrease density.


Quote:
Originally posted by Daveze


So I'm in an ordinary mood but you're wrong again. Its about where you apply energy, what the system is and how you define refrigeration. If you apply energy to the right location of a heat pump system you will get a refrigeration effect at one location (relative to its surrounds) and a heating at another (relative to its surrounds)..
Feel the heat from your refrigerators compressor and condenser lately?


Quote:
Originally posted by Daveze


- For a liquid, the particles are still chaotic but not to the same extent. They can be compressed but do not like to be and return to their original state more quickly than a gas. This translates to a pressure wave moving more quickly through the material.
Depending on temperature, molecules are always in motion. As temperature goes up, density goes down, as does the speed which sound will move through it.
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Old 25th July 2008, 04:25 PM   #38
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I've cited reputable scientific sources that incontrovertibly define sound in fluids as a compression wave, yet 'mjl' cavalierly rejects them without explanation.

I thought grade schools were better thirty years ago than they are today myself, but not to the extent of believing that their oversimplifications for the benefit of young minds trumped the physics that they were (less than accurately) drawn from.
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Old 25th July 2008, 04:25 PM   #39
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Quote:
Originally posted by Eva
MJL21193 I have a question for you...

Given a constant mass of water (a constant amount of molecules), does it's volume change with temperature?

If your reply is "no", then, why water expands as it gets closer to freezing even to the point of breaking too rigid containers?

If your reply is "yes", then try to visualize sound propagation, in water or any other medium, as travelling temperature gradients (temperature, density, pressure, in the end it's all related).

Volume changes with temperature. As a liquid gets hotter, it will occupy more space. When water reaches it's boiling point, it will expand to 1600 times it's original volume. I mentioned this earlier.

Water expands below freezing, not above it. I talked about that too earlier.

As sound goes through a medium, it expands and dissipates, it losses energy due to friction. Friction, as everyone knows, produces heat. Heat results in expanding of the medium, not compression.
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Old 25th July 2008, 04:29 PM   #40
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