College Film and Media Studies blog is a useful reference guide to film terms. We have been talking about the use of sound and in this post the writers explain why sound, voice and music are integral to most films and/or film viewing experiences. Here they explain the difference between diegetic and non-diegetic sound:
Diegetic sound is any sound that the character or characters on screen can hear. So for example the sound of one character talking to another would be diegetic. Non-diegetic sound is any sound that the audience can hear but the characters on screen cannot. Any appearance of background music is a prime example of non-diegetic sound.
This clip from Edgar Wright’s Shaun of the Dead simultaneously depicts both diegetic and non-diegetic sound. The sounds of the characters speaking, the records flying, and the zombies are all diegetic; the characters can hear them. Meanwhile, the beats and riffs of the background music serves as an example of non-diegetic sound that goes unheard by Shaun, Pete, and the menacing zombies.