# I Sound waves in a 'compressed' liquid

1. Aug 16, 2016

### Electrino

Hi,

I have a general question I was thinking about...

So, in a liquid the particles are already very close together and as a result they are, in effect, not compressible. That being said, if we take water, for example, it is possible to compress is slightly. It requires, however, a great deal of pressure to achieve very little volume reduction. For example the water at the bottom of the ocean is compressed by the weight of the water above it all the way to the surface, and is more dense than the water at the surface. So even though liquids can compress a tiny bit we cannot reduce the inter-molecular distance beyond a certain limit without changing its phase.
So my question is.... If we had a container, filled with liquid, and we placed a small sound emitting speaker and noise detector into the liquid and then compressed the liquid as much as possible using a piston... would a sound wave (which is a compression of the medium it is in) be able to travel from the emitter to the detector?
Or would the liquid (which is already compressed to its max value) act almost like a perfectly rigid body and instantly dampen the sound wave?

2. Aug 16, 2016

### Staff: Mentor

How much pressure do you think you would have to apply to reach the state you are referring to?

3. Aug 16, 2016

### Twigg

Every fluid is going to have an absorbance, which probably depends on pressure as you say. But I highly doubt there are any "perfectly" damping fluids. At some length scale, you will see oscillation. In short, for the thought experiment you posed, the answer depends on how far the detector is from the emitter, how strong the initial sound wave is, how sensitive the detector is, and what the detector's noise floor is.

Ignoring the issue of phase, whether you're in liquid or solid phase, any body with restoring force and finite damping will carry sound waves, to some extent. At the extremely high pressure limit, the Pauli exclusion principle puts a hard limit on how much you can compress atoms, so there's always going to be some restoring force, and I don't think you'll ever be able to achieve infinite damping. Moreover, what matters isn't the damping coefficient itself so much as the ratio of restoring force to damping force. And since the exclusion interaction is pretty darn stiff, I think you'd actually have a pretty lossless medium in this limit. Just a guess though.

4. Aug 16, 2016

Sound still travels through solids, and generally better than in liquids and gases. In such a "maximally compressed" state, if expect that not only would the sound transmit just fine, but it would also do so with comparatively little attenuation or distortion compared to "normal" water.

5. Aug 16, 2016

### Electrino

Interesting answers... So the general consensus is that the sound wave would travel just fine through the compressed liquid!

@Chestermiller: taking a quick look at the phase diagram for water, to turn liquid water to solid at around 0C you need around 620 MPa. Or at the max temperature value for liquid water at 400C you need around 10GPa. So somewhere along this limit.

6. Aug 17, 2016

### snorkack

Sure we can. It´s just that freezing point depends on pressure.
A perfectly rigid body would transmit sound instantly.
Which is why all bodies are compressible: due to relativity it is not possible to transmit sound faster than light. (But there is not such a theoretical objection to transmitting sound at exactly the speed of light).

7. Aug 17, 2016

### Khashishi

There's no such thing as a totally incompressible material. So sound can always travel. If you compress the liquid enough, you'll turn it into neutron matter (as in a neutron star). You have to compress a liquid into a black hole before you can't compress it any further. At that point, yes, black holes will totally absorb your sound.