Refraction of sound or waves involves a change towards sound or waves as they pass in one medium to an additional. Refraction of the path of the sound, is plus a change in velocity and wavelength of the sound.
This is explained with a rubber bag filled with carbon-di-oxide as shown in Figure. The velocity of sound in carbon-di-oxide is less than that in air and hence the bag acts as a lens. If a whistle is used as a source S, the sound passes through the lens and converges at 0 which is located with the help of flame. The flame will be disturbed only at the point 0.
When sound travels from one medium to another, it undergoes refraction.