If a person is in front of a smooth surface from which a sound is reflected, the person would hear a sound that seems to come from behind the surface (B). In theory, the sound waves hit the smooth surface and bounce off the surface. Â The sound also repeats itself. This is known as an echo. There needs to be a time gap between the sound and the echo otherwise the listener will not hear the echo. It will still be present but form part of the original sound.