Why is water said to be “incompressible” when sound can travel through it? Doesn’t sound imply compressions and rarefactions?
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Everything is compressible. An incompressible material would transmit a push at one end into movement at the other instantly, allowing faster-than-light information transfer and thus breaking the laws of physics.
just because something is “incompressible” doesnt mean sound cant travel through it. compressions and rarefactions still happen at a more microscopic or otherwise less visible way. solid material still transfers sound for example and incidentally does so faster than gasses or liquids (typically anyway)
It’s like throwing a tiny packet of salt into a lake and saying “the salt did nothing”. It did but it’s such a tiny difference you would need crazy high tech equipment to measure.
It including all liquids and solids are compressible but it’s by such a tiny amount that it’s not worth taking into consideration for most applications.
Water at 1000psi compresses about 0.5%.
Water at 15,000psi (bottom of the deepest ocean) compresses around 5%.
A medium doesn’t need to be compressible in order to propagate sound. When you jostle water molecules they push against other water molecules and move them around. That movement Carrie’s sound.
Liquid are usually said to be incompressible in comparison to gasses, it’s a valid if simplified comparison, ideal gas law does not apply to liquids though both are fluids.
It doesnt hold in absolute though, everything is compressible if you apply enough pressure.
Imagine water like a table we’re sitting at. I slam my fist on the table and you can feel the vibration. I kick the table at the edge so it hits you in the chest, it doesn’t compress.
To start with, it is compressible. You are quite right that the fact sound can travel through it is a clue that it is. But it isn’t very compressible.
For example, if you go down to 6,000m depth in the ocean, the water is under a lot of pressure (about 600 times atmospheric pressure), but is only about 5% denser. That means there’s only about 5% more water in a tank of water at that pressure than there would be at the surface, so even 600x pressure increase doesn’t do much actual compression.
So why do people say that it is incompressible?
As far as I know, two sorts of people say this, and they have different reasons for saying it.
One sort of person – and I think pretty much all the other answers are about this sort of person – is an engineer or physicist or something similar. “It makes the equations easier” is what one person said. Engineers etc will often ignore small things that don’t make much difference to their calculations. They might as well assume it is incompressible for most of their work.
The other sort of person – and I am guessing this is where you heard it from, though I could be wrong, it is where I heard it from – are science and physics teachers. Why do they say this?
Well, it’s a style of teaching. If you want children to memorise “facts” for tests it is much easier for them to memorise: gas – compressible; liquid – incompressible. You don’t teach them to think about the world because if they really did they’d wonder how sonar works; how they can hear their friends with their head under the water in the swimming pool; what “water pressure” in submarine action films is about etc etc.
When discussing compressibility, we are really wondering how much the material will push back if we forcefully shrink it by a certain amount. Think about a very loose spring. You can forcefully shrink it by a huge amount, before the force it exerts back becomes large. A very strong spring only needs to be shrunk by a tiny amount before the force it exerts becomes large.
Water is like a really strong spring. You can make it a tiny bit smaller with a huge press, but the force it exerts back (and hence the force your press needs to exert on the water) increases extremely quickly. Therefore, it cannot be shrunk by much by most presses (though for instance, if you threw it into a neutron star it’d definitely get strunk).
A strong spring can still transfer information in the form of vibrations, it’s parts just don’t need to be expanded/shrunk by very much to do so in the process. The same holds for water.
EDIT: If my math is correct, the a kilogramme of water at the bottom of the mariana trench will be a few % (4% or so) smaller than at sea level, due to the immense pressure there.
Water is said to be incompressible because it doesn’t compress very much. This means that when sound waves travel through water, the water molecules don’t move closer together (compress) or further apart (rarefy) very much.
Don’t think of it as compressing. Think of it as the molecules bumping against each other. Really fast in solids, ok in liquids, slow in air.
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