Which wave phenomenon makes it possible for a player to hear the sound from a referee's whistle in an open field even when standing behind the referee?

The diffraction of sound waves is the mechanism that sheds acoustical energy into acoustical shadows, which are areas that are prevented from direct incidence of sound as by an obstacle. Because of diffraction, it's possible, to hear the noise of a road even in the backyard of a building, or the sound of the referee's whistle when standing behind him.