Yes, this is caused by the fact that each time any form of "signal" - that propagates as a waveform - is reflected, scattered or deviated as soon as the properties of the propagation media change. When listening underwater we also have such a discontinuity (or impedance mismatch) between the water and the eardrum. The eardrum and the bones of the inner ear have been "introduced" to match the characteristic impedance of the fluid in the chochlea to the characteristic impedance of air. If the outer transmission medium is water then there is again a mismatch.
If a change of the relevant properties of a medium is continuous and monotonic, like the air temperature normally is (and on which the velocity of air depends), then the waves are "bent" (upwards for the usual situations without inversions etc). Wind can further increase or decrease the bending of the waves with the well known effect that it "travels better" downwind and worse upwind.
So far for the off-topic.
Back to topic:
I don't know if this figure has been mentioned so far but the relative compressibility ( i.e. - dV/ (V * dp) of water is 0.5/GPa.
Regards
Charles
There is no compression without displacement. There is no displacement without compression.
I think John's point is well taken in respect of the fact that the behaviour of molecules in a gas is widely different from that in a liquid, but my feeling is that his attempt to characterise sound propagation as 'areas of high and low pressure as a result of excitation of the molecules (increased pressure) from the positive portion of the wave...' etcetera is arbitrary. Why should the pressure manifest itself exclusively as 'increased excitation' rather than a reduction in the mean free path?
w
I think John's point is well taken in respect of the fact that the behaviour of molecules in a gas is widely different from that in a liquid, but my feeling is that his attempt to characterise sound propagation as 'areas of high and low pressure as a result of excitation of the molecules (increased pressure) from the positive portion of the wave...' etcetera is arbitrary. Why should the pressure manifest itself exclusively as 'increased excitation' rather than a reduction in the mean free path?
w
despotic931 said:
The impedance mismatch between water and air proves just how small the forces are that we are talking about. The displacement in the medium is tiny.
As phase_accurate said, sound traveling through air will bounce off of the waters surface - the impedance mismatch. A portion of the sound wave through water will actually bounce off it's own surface, back downwards. Energy transfer from a more dense to a less dense medium (or vice-versa) is always less efficient.
The train tracks are steel. Steel has a very high density and is highly elastic, therefore sound travels through it faster and further. It has the same impedance mismatch problem with air as water, and this will not be improved no matter what the mechanism for sound travel is. Different issues these are, and have not much to do with how sound travels.
The forces are small, and the displacement will be measured in molecules, not litres.
phase_accurate said:
I posted above that at pressures less than 1GPa, water is treated as incompressible in fluid dynamics. When one considers the forces involved in sound transfer, and how they would be measured in microPascals, it's easy to see that the mechanism for sound transfer through water is not compression.
wakibaki said:
Relative. A infinitesimally small amount of compression as opposed to a notable and effective for energy transfer displacement.
Sound energy through a medium will make the molecules move, of this there should be NO doubt. This movement translates to high and low pressure. Every reference I have found says this - high and low pressure.
Think about it: Reducing the "mean free path" as in compression, will not efficiently transfer energy. Displacing the next molecule in line by pushing it is an efficient means of energy transfer. You need to keep in mind that the molecules are as close as together as they can be at the given pressure and temperature. You would treat them as if they are in physical contact with each other. Picture a pail full of steel ball bearings. Push one and energy will travel through the next, and so on. No compression has occurred (not that could be quantified anyway), but work was done all the same.
sreten said:
Oh, we are back to that again. Figures.
Like 7 year olds in the play ground:
"you're stupid!"
"NO, you're stupid and you're ugly too!"
"Am not!"
"Am to!"
Take the time to demonstrate your superior understanding by posting something here that either questions some of the points I have made (like those above) or give some definitive reference that contradicts what I am saying.
Prove to me how well you understand the process.
John's hypothesis make a prediction, thus it's falsifiable. His prediction is that if a volume of water is constrained, it cannot pass sound waves; the boundary conditions don't allow displacement, and if there can be no compression, there can't be sound transmission nor can the medium support standing waves.
Bold prediction, that. And trivially easy to find counter-examples.
Bold prediction, that. And trivially easy to find counter-examples.
SY said:
Ok, what are the counter examples?
Show me the proof and I'll easily concede - I'm not too stubborn or proud to admit when I'm wrong. I'm wrong about things everyday.
As I posted earlier, the water (or any liquid or solid for that matter) that is totally constrained (as in no room for volume expansion due to flexing container) should not support a sound wave.
Do you suggest that sound will easily travel through such a system?
MJL21193 said:
Yes.
This thread is amazing, the speed of sound in any medium is ~ the square root of the bulk modulus/density. The bulk modulus is a measure of compressibility. Water has a bulk modulus 4 orders of magnitude larger than air but water is 1000 times more dense, gee the speed of sound is ~ square root of 10 different. Rocket science in action.
Here's a link again, the physics and astronomy dept. of a major university not a Wiki page.
http://hyperphysics.phy-astr.gsu.edu/Hbase/sound/sprop.html
amazing for sure
It is fascinating that over 35 members of this esteemed forum (as well as many many online references) are posited as incorrect on this issue, as opposed to one individual with a purported transcendent perspective. Must be a severe case of testosterone poisoning....
Another reference.. see, for instance, section 2
"Sound waves in water
A sound wave propagating underwater consists of alternating compressions and rarefactions of the water. These compressions and rarefactions are detected by a receiver, such as the human ear or a hydrophone, as changes in pressure. These waves may be man-made or naturally generated."
http://en.wikipedia.org/wiki/Underwater_acoustics
John L.
It is fascinating that over 35 members of this esteemed forum (as well as many many online references) are posited as incorrect on this issue, as opposed to one individual with a purported transcendent perspective. Must be a severe case of testosterone poisoning....
Another reference.. see, for instance, section 2
"Sound waves in water
A sound wave propagating underwater consists of alternating compressions and rarefactions of the water. These compressions and rarefactions are detected by a receiver, such as the human ear or a hydrophone, as changes in pressure. These waves may be man-made or naturally generated."
http://en.wikipedia.org/wiki/Underwater_acoustics
John L.
MJL21193 said:
Hi,
There is only one 7 year old round here.
I am not interested is discussing your "points".
Or proving anything to someone who ignores the plainly obvious.
Like :
The simple fact the speed of sound in water with
its density indicates the compressibility of water.
And :
Sound travels through an elastic medium as a wave.
Anyone who wants to argue about the latter statement is plain daft.
There is nothing to discuss.
/sreten.
We've all seen the demo in school with the tank full of poly balls and a speaker at the bottom to provide 'thermal excitation'.
I think this is the problem. Steel balls are a good analogy in some circumstances, but water molecules are more like tacky, fuzzy, fat little boomerangs constantly slipping around one another like a bucketful of fish.
Otherwise what you suggest is patent nonsense John.
If it were possible to suspend a sonar transducer in a body of water whose volume was absolutely constrained, the transducer would still launch pulses into the surrounding water. This is because it is only in contact with water immediately surrounding and 'knows' nothing of the molecules at a greater distance. The transmission of sound in water is not infinite in speed, as has already been pointed out. Even in the case of a radio wave launched into co-ax, the wave propagates regardless of the termination, until it encounters the termination.
However small it is, and however you dislike to acknowledge it, it is this compression, which you implicitly acknowledge as equivalent to reducing the mean free path, which permits the passage of sound through water. A progressive crowding and uncrowding of molecules passes through the water. If waves of 'excitation' were passing through the water to create the regions of higher and lower pressure, this would be equivalent to regions of higher and lower temperature. What would constrain the thermal energy to flow in only one direction?
Undoubtedly an examination of the propagation of sound waves in air could describe the instantaneous temperature at a point where sound waves are passing as varying, but since understanding of the gas laws underpins much of physics the convention is that this is thought of in terms of compression, as the velocities of uncollided molecules remain unchanged and the temperature of the medium is unchanged after the passage of the wave other than to the extent that the medium is lossy, plus it makes things much more difficult to understand if you don't.
w
MJL21193 said:
I think this is the problem. Steel balls are a good analogy in some circumstances, but water molecules are more like tacky, fuzzy, fat little boomerangs constantly slipping around one another like a bucketful of fish.
Otherwise what you suggest is patent nonsense John.
If it were possible to suspend a sonar transducer in a body of water whose volume was absolutely constrained, the transducer would still launch pulses into the surrounding water. This is because it is only in contact with water immediately surrounding and 'knows' nothing of the molecules at a greater distance. The transmission of sound in water is not infinite in speed, as has already been pointed out. Even in the case of a radio wave launched into co-ax, the wave propagates regardless of the termination, until it encounters the termination.
However small it is, and however you dislike to acknowledge it, it is this compression, which you implicitly acknowledge as equivalent to reducing the mean free path, which permits the passage of sound through water. A progressive crowding and uncrowding of molecules passes through the water. If waves of 'excitation' were passing through the water to create the regions of higher and lower pressure, this would be equivalent to regions of higher and lower temperature. What would constrain the thermal energy to flow in only one direction?
Undoubtedly an examination of the propagation of sound waves in air could describe the instantaneous temperature at a point where sound waves are passing as varying, but since understanding of the gas laws underpins much of physics the convention is that this is thought of in terms of compression, as the velocities of uncollided molecules remain unchanged and the temperature of the medium is unchanged after the passage of the wave other than to the extent that the medium is lossy, plus it makes things much more difficult to understand if you don't.
w
John
can i suggest that instead of a box full of steel balls you sit in a box with a large newtons cradle and play with that when you think about sound transmission.
the frame/volume stays the same size, the bonds/wires are movable and the balls compress in a very definite way.
http://www.find-me-a-gift.co.uk/images/product_images/prt004.jpg
Nick.
can i suggest that instead of a box full of steel balls you sit in a box with a large newtons cradle and play with that when you think about sound transmission.
the frame/volume stays the same size, the bonds/wires are movable and the balls compress in a very definite way.
http://www.find-me-a-gift.co.uk/images/product_images/prt004.jpg
Nick.
scott wurcer said:
Hi Scott,
I have seen the link you pointed out several times. Thanks.
Many are hung up on the speed of sound. I'll elaborate on what I said earlier:
Bulk modulus is a measure of Incompressibility, not compressibility. It also refers (more aptly, in this instance) to a materials volumetric elasticity, or the tendency of an object's volume to deform when under pressure. We can all agree (I think) that water will deform quite readily to applied force. It's lack of elasticity slows sound travel through it as a medium.
Take lead. It has a very high density (more than 11 times greater than water) and a high bulk modulus of 46 GPa - 21 times greater than water. For solids, where speed of sound is concerned, Young's modulus is the relevant number, and that is only 16 GPa. Greater density and higher resistance to compression, yet sound travels slower through lead than water, due to it's low elasticity.
On the other hand, take diamond. It's density is only 3.5 times that of water, yet sound will travel through it at nearly 10 times faster than sound through water. Why is that? Much greater elasticity - 1220 GPa.
Now, is it reasonable to suggest that further compression is possible in either of these materials with what amounts to a pee-shooters worth of energy? The bulk modulus of diamond is somewhere around 450 GPa. This is a colossal amount of force.
How will a handful of microPascals of force (typical sound wave) sweep through this material and actually effect density changes? Magic?
Semantics, maybe just poor rhetoric
The reciprocal of the bulk modulus is called the compressibility of the substance.
A common statement is that water is an incompressible fluid. This is not strictly true, as indicated by its finite bulk modulus, but the amount of compression is very small. At the bottom of the Pacific Ocean at a depth of about 4000 meters, the pressure is about 4 x 107 N/m2. Even under this enormous pressure, the fractional volume compression is only about 1.8% and that for steel would be only about 0.025%. So it is fair to say that water is nearly incompressible. Reference: Halliday, Resnick, Walker, 5th Ed. Extended.
What do you really want, John?
The reciprocal of the bulk modulus is called the compressibility of the substance.
A common statement is that water is an incompressible fluid. This is not strictly true, as indicated by its finite bulk modulus, but the amount of compression is very small. At the bottom of the Pacific Ocean at a depth of about 4000 meters, the pressure is about 4 x 107 N/m2. Even under this enormous pressure, the fractional volume compression is only about 1.8% and that for steel would be only about 0.025%. So it is fair to say that water is nearly incompressible. Reference: Halliday, Resnick, Walker, 5th Ed. Extended.
What do you really want, John?
Re: Semantics, maybe just poor rhetoric
Another wiki nugget. Tell me something I don't know.
Here's a solid reference: Fluid Mechanics. Attached below is a page from the book.
It's important to understand the forces involved. At pressure greater than 1GPa water can be considered compressible.
1GigaPascal doesn't sound like much? That's equivalent to ~145,000 psi. Big enough number?
Ed LaFontaine said:
Another wiki nugget. Tell me something I don't know.
Here's a solid reference: Fluid Mechanics. Attached below is a page from the book.
It's important to understand the forces involved. At pressure greater than 1GPa water can be considered compressible.
1GigaPascal doesn't sound like much? That's equivalent to ~145,000 psi. Big enough number?
Attachments
sreten said:
Ed LaFontaine said:
Apparently it is permissible to call a member "daft" here.
Ed, are you daft?
Did you miss the part where I quoted a reliable source that says that at pressure less than 1GPa water is treated as incompressible? Gee, it's 2 posts up.
Nothing is absolute. Read it over and over and over how I have said that water is compressible.
For about the 100th time: it takes more force than a sound wave can supply.
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