# What happens to sound?

1. May 9, 2008

### Bigee

In any release of energy, it is converted to various forms. But which is given preference over which? Cuz in space if there is a huge energy release, It wont be converted into sound...or would it? Would it be seen as heat, light, sound ...and if so in what proportion?
(inspired by the flaw in star wars movies in which sound is heard in space after the explosion)

2. May 10, 2008

Sound can't travle through space (it weas still a cool effect though =p). I guess the particles hitting your ship could cause sound, but sound is basicly kenitic energy so in some ways sound can travle through space. There needs to be an actual medium for it to travle in though, because in air one particle hits the next one and so on.

3. May 10, 2008

### openbahr

is dark matter a medium?

4. May 10, 2008

### Staff: Mentor

Yes - in a sense. See - https://www.physicsforums.com/showthread.php?t=232691

DM interacts gravitationally, but not electromagnetically, at least not in a way that we can 'see' (detect) it. Perhaps DM experiences the weak interaction, as in WIMPS.

Sound involves the propagation of energy and transfer of momentum by virtue of atomic or molecular interactions - collisions or vibrations.

The speed of sound depends on the density of the material (medium). Sound does not propagate in a vacuum, i.e. in the absence of matter.

5. May 10, 2008

### xantox

Astronuc is correct, however, madmike159, space is not quite empty, and there are sounds in space. In the solar system there is the solar wind, which is a plasma with a density of about 5..10 protons per cubed cm. In our galaxy there is almost everywhere an hydrogen plasma with a lower density. And even at bigger scales there are ultra-low density structures between galaxies. In such plasmas, pressure waves may propagate exactly like a sound in air (though human ears would not be able to detect them).

Last edited: May 10, 2008
6. May 11, 2008

### billiards

I have read that it might be possible for the shockwave propagated from a supernova to instigate the collapse of a cloud of gas to form a new solar system. One thing I never quite understood is how the shockwave (which I understand is a form of sound) can travel through space, as presumably it would need to do, to cause disturbances in the galaxy. Can anyone shed any light on this for me?

7. May 11, 2008

### Monocles

Ok, this thread makes me wonder something. What if somebody is near the edge of the atmosphere and makes a very loud noise. What happens to the energy in the sound wave when it gets to the end of the atmosphere and there is just a vacuum?

8. May 11, 2008

### Staff: Mentor

This is a key point! In plasma physcis, one addresses 'acoustic waves'.

If one made a sound, assuming that the source used a gas at 1 atm of pressure, it would dissipate very quickly.

It might help to compare atomic/molecular velocities of a gas (look at a Maxwell-Boltzmann distribution) with the 'speed of sound' of a gas as a function of density of the gas.

Last edited: May 11, 2008
9. May 11, 2008

### xantox

It may seem counterintuitive, but the speed of sound in a plasma does not depend on its density, but on its temperature - which is generally rather high in the case of interstellar plasma, in the order of 10000K (and this is still considered a 'cold' plasma). This translates into a sound speed in the order of 10 km/sec or higher for all our galaxy.

10. May 11, 2008

### pixchips

Fascinating thread: just one factor ... it would be hard to MAKE a sound near the edge of the atmosphere. If something moves quickly or vibrates in a thick gas, significant energy is coupled to the medium acoustically. As air gets thinner, less energy gets coupled acoustically, its harder to make a sound. (I think this effect happens on a practical level in airlocks ... as they get pumped out, everything gets very quiet.) On the other hand, when a supernova goes off, there's a lot of total mass (though the density may not be very high once it leaves the star) and its moving REALLY fast, so lots of energy gets coupled. Not sure how to sort out how much is acoustic, and how much just kinetic, since the medium is so variable.

11. Mar 2, 2009

### Archosaur

There is sort of a natural bias toward the conversion of energy into heat energy, in the sense that you can't efficiently turn heat energy into working energy (in a steam engine, a lot of heat $$\rightarrow$$ a little less heat and a little bit of working energy) but we can efficiently turn working energy into heat (when you rub your hands together, working energy $$\rightarrow$$ heat energy and sound energy, which dissipates into more heat energy).

You should google the word "entropy".

As far as the rest of you question goes:

Sound is just a wave that is a result of particles running into each other. The particles emitted from an explosion on earth collide with particles in the air and transfer some of their kinetic energy to them, which collide with other particles in the air, and form a wave that travels to your ear. In space, free-floating particles are much more spread out than in the air, so a sound wave does not propagate after an explosion. So, where does the rest of the energy go?

The initial particles from the explosion in space don't collide with other particles, creating sound, but because of that, they don't loose the kinetic energy they receive from the explosion. The particles from an explosion in space have more kinetic energy than the particles of that same explosion in air. And the kinetic energy of particles is also called.... heat energy.

Which goes right back to your first question.