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Let's say you are in a space ship.
The inside is filled with air, a medium in which sound waves can exist and travel.
Then there's a wall, still a medium in which sound waves can exist and travel.
And outside, there is a vacuum, sound waves cannot exist.

As sound waves have energy, they can't just disappear, because their energy can't just disappear.
My question is : When there is a sound in the spaceship, what happens just where the wall ends and the vacuum begins ? Where do the waves go ? Where does the energy go ?

sound waves going to a vacuum

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  • $\begingroup$ They reflect back - there is no other choice given the boundary conditions. $\endgroup$
    – Jon Custer
    Commented Jan 5, 2016 at 0:43
  • $\begingroup$ An equivalent question is "what happens to a wave traveling on a string if the string ends?" It looks like this. As Jon said, it just reflects. $\endgroup$
    – knzhou
    Commented Jan 5, 2016 at 0:45
  • $\begingroup$ However, if the transition to vacuum is not sudden (i.e. the air pressure just gradually decreases), you get impedance matching and so the wave will keep propagating. It'll just get quieter and quieter as it approaches vacuum. $\endgroup$
    – knzhou
    Commented Jan 5, 2016 at 0:47

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Sound waves inside the ship hit the wall and are reflected but also somewhat absorbed by the wall, which makes the wall oscillate.

If there was air at the other side of the wall, the oscillating wall would cause sound waves to occur but if there a vacuum there then there's no medium to form sound waves with. So no sound can be transmitted through the wall if the other side is vacuum.

Where do the waves go ? Where does the energy go ?

The energy of the sound waves hitting the wall is partly reflected by it, partly absorbed. The absorbed energy is gradually dissipated due to the fact that no material is 100 % elastic, so some hysteresis (friction} will always occur.

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