Why is water said to be “incompressible” when sound can travel through it? Doesn’t sound imply compressions and rarefactions?
In: 93
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.
Latest Answers