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

648 views

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

In: 93

21 Answers

Anonymous 0 Comments

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.

Anonymous 0 Comments

[removed]

Anonymous 0 Comments

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.

Anonymous 0 Comments

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

Anonymous 0 Comments

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.

Anonymous 0 Comments

Imagine an “incompressible” steel rod. Hold one end of it, and consider someone pushing it at the other end, the pressure of them pushing it is transferred to your hand. Now imagine someone pushing and pulling that steel rod … you are still feeling the push and pull of the rod, right? In fact denser mediums transmit sound faster because there is less energy wasted compressing empty space. In a vacuum, there is nothing to compress at all so no sound can travel.

The thing is that whatever is observing the sound is NOT incompressible. So say you are under water and someone drops a rock onto another rock, the compression from the shock of the impact travels through the water and compresses your eardrum so you hear it.

Anonymous 0 Comments

water is compressible but, for the sake of equation simplicity, it is considered to be incompressible.

Anonymous 0 Comments

Most of the time when water is referred to as “incompressible”, it’s in the context of fluid dynamics. In that context, we’re talking about objects traveling through a medium (like air or water) at some speed. The typical medium in aerodynamics is the atmosphere, which is also considered “incompressible” at low speeds but must be treated as compressible at speeds that approach or exceed the speed of sound. Most engineering problems involving water vehicles are solved just fine by assuming water is incompressible, because the speed of the vehicle is very low compared to the speed of sound in water. It’s a useful approximation that makes the mathematics simpler to deal with, nothing more.

Like all approximations, the assumption will break down at some point, but last time I checked, we’re not designing many things that travel through the ocean at supersonic speeds…

Anonymous 0 Comments

It is in comparison to gasses which can have, molecularly speaking, many times their number squeezed into a tighter space versus having barely any more at all.

As anyone on board a submarine can attest, water very much DOES compress…just not a lot…and it gets really angry and moves really quickly when it decompresses.

Anonymous 0 Comments

Everything is compressible. Water is just much harder to compress than air. Think about it like a spring, air is a very loose spring, so it’s easy to compressible, and water is like a very tough spring, so it’s very hard to compress. Even rock can be compressed and if you remove weight from on top of it (say a glacier melts) the rock expands and pops back up and can cause minor earthquakes without a fault line nearby.