Because we tend to think of film as a fundamentally visual medium, the importance of
film sound is often overlooked. Whether noticed or not, sound is a powerful film
technique. Indeed, sound can actively shape how we perceive and interpret the image.
In the context of a film, moreover, sound falls into two categories: diegetic and nondiegetic.
Again, the film’s diegesis is the world of the film’s story; this includes events
that are presumed to have occurred and actions and spaces not shown onscreen.
Diegetic sound, then, includes any voice, musical passage, or sound effect represented
as originating within the film’s world. This can include the sounds of cars and
conversations during a scene depicting a busy metropolitan street. In contrast, nondiegetic
sound would include mood music or narrator’s commentary represented as
coming from outside the space of the narrative