I thought that sound waves effectively were a series of compressions within the medium they were travelling through. This could well be wrong.
In: Physics
The phrase “liquids can’t be compressed” is only approximately true. They absolutely can, just by a much, much smaller magnitude than gases can
When people say that water can’t be compressed what that really means is that it can’t be compressed *much*.
With air if you raise the pressure from 1 bar to 2 bar then the volume will decrease by half. That’s pretty compressible. If you take a liter of water at 1 bar and subject it to 2 bar then you’ll have a hard time measuring that it has compressed to less than a liter–it’ll be by microscopic amounts.
For most purposes that means it’s good enough to just say water–and liquids in general–can’t compress. You can build a hydraulic system with hydraulic oil and reason about the volumes of hydraulic oil without ever caring about how that volume changes with pressure. But when it comes to something like sound that non-zero compressibility is what makes things work.
As a practical example, a friend of mine is a petroleum engineer who works with pipelines under the Gulf of Mexico. At one point one of the pipelines had some sort of a blockage (I forget the specifics) that required a bit of room. My friend suggested some maneuver that would compress the miles-long column of water enough to make the room, using thousands of psi, to which a college educated but junior petroleum engineer smugly countered “water is incompressible.” That cued whipping out the math on how incompressible water actually is and when you’re talking about thousands of psi and miles of pipeline the compressibility becomes enough that you can practically measure it on a human scale.
Water *can* be compressed, just not by very much. In fact, the compressibility of water is so close to 0 that we might as well call it 0, hence the falsehood of saying water is uncompressible. For the record, at the bottom of the Mariana Trench, 11 km below the surface of the sea, there is about 1,100 times the pressure as at sea level, and the water there only has about 94% of its volume at the surface.
For comparison, air would have about 0.09% of its volume at the same depth.
Everything is compressible, possibly excluding the abnormality of whatever exists at the singularly of a black hole, for which we have no physics. There is also some strange behavior with the degenerate matter in neutron stars, though there is some degree of compressibility there. Neither of those exceptions involve water or conditions anywhere close to anything you’ll find on Earth.
Maybe I’m wrong, but I’m surprised by the answers that (correctly) indicate that water is slightly compressible. While that’s true, I think that sound would propagate instantly in a fully incompressible medium (which I guess means there is no such thing as fully incompressible medium).
Take a piece of hypothetically incompressible matter (I don’t know, think diamond, but harder I guess), put it against the drum of a microphone and exert variable pressure on it. What would happen? The drum would move and the microphone would record sound.
You don’t need a medium to be compressible for pressure on one end to propagate to the other.
Liquids *can* be compressed. They just compress so little, they’re *almost* incompressible by comparison to gas.
Also, I might be wrong, but sound propagation through a medium happens despite its compressibility, not because of it.
Imagine a column of water and a column of air. For sound to travel through each column, vibrations at one end need to somehow be transmitted to the other end. If the medium is very compressible, vibrations don’t have much of a chance to travel. If it is incompressible, like a length of steel or a column of water, then what happens at this end is almost perfectly transmitted to the other end.
That’s why, for example, sound travels so much faster through water.
The Newtown Laplace equation puts the speed of sound at sqrt(Ks / rho), where Ks is the bulk modulus, or how stiff the medium is, and rho is its density.
To add to what others are saying, the density of the water, which part of what makes compression harder, is also what makes it easier for sound to travel compared to air. In water the sound vibrations can move from molecule to molecule very rapidly because the are close together. In the air the molecules are much further apart so it takes longer for the waves to propagate.
Imagine if you are trying to transfer a bucket of water over 1 mile. If you have a bunch of people spaced 6 inches apart you can quickly pass the bucket from one person to the next. But if you have fewer people spaced much further apart, say 6 yards, they have to run the bucket to the next person. The further the people are, the more work to transfer the bucket.
materials that are close to in-compressible still transmit sound, think of glass – it’s a pretty good sound transmitter. Water resists compression even more than glass (by 20 times), but water is also a fluid, so it can definitely transmit waves even if it wasn’t compressible at all.
Sound waves are longitudinal waves, meaning the particles in the medium vibrate back and forth in the same direction as the wave travels.
You actually can compress water, it just takes a lot more power to get anywhere close to a noticeable shrinkage, like so much power.
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