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Suppose a sound wave is emitted by an object in a medium like a gas so it travels in some direction. If the wave meets a rigid object, for example a wall, it reflects back as one should expect; and if it encounters another medium, like a denser one, it will be transmitted to the other side. In those cases the sound wave keeps traveling, and of course it can loose energy in its path and even be absorbed by some object, so it can be converted into another form of energy.

Now, I'm aware that sound waves can't travel in vacuum, so my question is: What happens to a sound wave that is traveling in some medium and encounters vacuum ?. Here's a drawing of the situation:

Situation

I'm thinking about the usual wave phenomenoma and reflection is not a logical option since there is not a defined object that can work like a wall, and transmission doesn't make any sense at all since sound waves can't travel through vacuum. So what happens with that wave ? Where does its energy go ?.

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  • $\begingroup$ How do you enforce the separation of the medium and the vacuum? If the medium is a gas, it simply flows into the vacuum, making the question moot. If the medium is a liquid with a vacuum above it, it boils until there's some vapor above it, making the question moot. If the medium is a solid (or if you use some other force to forcibly confine a liquid or gas), the forces at the interface are different than the forces in the bulk, making it very similar to hitting a wall. $\endgroup$
    – probably_someone
    Commented Aug 14, 2019 at 13:56
  • $\begingroup$ I'm not sure if you can do this at all, but I can think of a wall separating the medium and the vacuum, so when the sound wave is close to hitting it, the wall is removed. $\endgroup$
    – Pablo Navarrete
    Commented Aug 14, 2019 at 14:03
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    $\begingroup$ An actual example is an atmosphere, which uses gravity to bound the medium. Here the density declines until it reaches the exosphere where molecules behave ballistic rather than as a fluid. So the sound wave speeds up as the density declines, and then briefly becomes a fountain of molecules flying along parabolic trajectories. $\endgroup$
    – Anders Sandberg
    Commented Aug 14, 2019 at 15:52
  • $\begingroup$ @probably_someone Sound travels perfectly well in solids. There is no problem enforcing the separation of a solid and a vacuum. $\endgroup$
    – alephzero
    Commented Aug 14, 2019 at 16:09
  • $\begingroup$ @alephzero That's why the question is only moot for unconfined gases and liquids. My last sentence of my comment addresses solids: sound reflects off the surface of the solid, which is also the solid-vacuum interface and also very similar to hitting a wall. $\endgroup$
    – probably_someone
    Commented Aug 14, 2019 at 16:12

2 Answers 2

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It would get reflected due to the impedance transition from free air to vacuum. The exact nature of reflection will depend on the exact way how the air is separated from the vacuum. Whatever means this may be, it would have to prevent air molecules to enter the vacuum, i.e the normal velocity component at the boundary must be zero.

In other words: you need a some sort of a wall and the boundary would behave acoustically like a wall does.

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  • $\begingroup$ You don't need an explicit wall if the medium is a solid body, but of course the surface of the body still acts like a wall in any case. $\endgroup$
    – Solomon Slow
    Commented Aug 14, 2019 at 14:06
  • $\begingroup$ A solid body between an air volume and vacuum? That sounds a bit like a wall to me :-) $\endgroup$
    – Hilmar
    Commented Aug 14, 2019 at 14:09
  • $\begingroup$ The medium could be a solid body. $\endgroup$
    – Solomon Slow
    Commented Aug 14, 2019 at 14:47
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    $\begingroup$ Actually, the effect of the wall can disappear if its thickness is short compared to the wavelength and its characteristic impedance is low enough. From page 154 of the 4th ed. of Fundamentals of Acoustics by Kinsler et al., "Thus, a thin membrane of solid material of appropriate characteristic impedance may be used in preventing two gases or two liquids from mixing and yet not interfere with sound transmision between them." You'd get total reflection from the vacuum with an inverted waveform in this case. $\endgroup$
    – Erlend Magnus Viggen
    Commented Aug 16, 2019 at 7:36
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The Sound wave would continue to propagate outward until it cannot continue, even while in a vacuum, and like many of us know, it would not be considered sound unless there are particles to be pushed against it and received by us through our sensation of hearing, and a Vacuum is a place devoid of said particles to carry sound.

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  • $\begingroup$ The wave would not continue into the vacuum. You say it would not be considered sound, but there would simply be no wave. With no particles there can be no wave. Say you have to pieces of metal separated by vacuum - a sound wave travelling through one of them would not travel through the vacuum and into the other one. $\endgroup$
    – Codename 47
    Commented Aug 14, 2019 at 16:25
  • $\begingroup$ I did not say it would go into the vacuum, I said the wave would continue because there is particles to carry the sound, in a vacuum there would be no sound at all $\endgroup$
    – C. Jordan
    Commented Aug 15, 2019 at 2:11

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