Why is water said to be “incompressible” when sound can travel through it? Doesn’t sound imply compressions and rarefactions?

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Why is water said to be “incompressible” when sound can travel through it? Doesn’t sound imply compressions and rarefactions?

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Yes, and water is compressible to a degree, as noted by it’s ability to conduct sound. So it is almost entirely, but not completely incompressible.

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Answer: it’s incompressible a bit like glass is incompressible. Still, glass propagates sound too. It’s because all materials really have the ability to propagate a change in the pressure on one side to the other side.

An experiment you can make: put a bar of steel on a table. Push it from one side. It moves from the other side too! The pressure of your hand propagated through the steel to the other side.

The water does this with sound too.

Pressure and compressibility are two separate concepts.

It is possible to vary the pressure without reducing the volume of a substance.

Imagine an anvil.

Put a 10lb barbell on it. This exerts a pressure, but did you change the volume of the anvil? Probably, but not by any measurable amount.

Put a second 90lb barbell on top of the 10lb barbell. You now have 10 times the pressure of just the 10lb barbell. Did the anvil change volume? Same answer.

Sound is not a wave of fluctuating densities, it is a wave of fluctuating pressures.

And as an aside, we say water is incompressible, not because you cannot compress it, but that it undergoes such minimal compression that we can almost always ignore any compression that occurs.

Saying that water in incompressible is an approximation. Everything can be compressed– some things (gases) more than others (liquids).