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:
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 ?.
Best Answer
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.