聲波從空氣進入二氧化碳的折射

Refraction occurs when there is a difference in velocities of the wave in the two concerned media.
For example, when light travels from air to glass, the velocity of light in glass is smaller than that in air. This causes the light ray to bend, which is refraction.
If a piece of glass is made to form a spherical shape, then the glass ball will focus light rays similar to the effect of a convex (converging) lens.
Similar phenomenon can also happen in sound wave. Velocity of sound wave in a gas decreases as the density of the gas increases (if other conditions remain the same). Because carbon dioxide is heavier than air, and has a density larger than that of air. Thus, velocity of sound in carbon dioxide is smaller than that of air. Refraction would occur when sound wave travels from air to carbon dioxide contained inside a balloon. The spherical shape of the balloon, hence the shape of carbon dioxide, focuses the sound wave in a similar manner as a glass ball focuses light rays.