A word from the ancient Greek for “recounted story,” diegesis is a term used in film studies to refer to the universe of the story. It can be any kind of story, from a news clip or commercial to a dramatic feature. Diegetic sound comes from this concept and can include the voices of characters, sounds made by objects in the story and music represented as coming from instruments or devices in the story space.

Diegetic sound, also called "actual sound" by some, is any sound that originates from a source within the video or film's world. It can be either on-screen or off-screen, depending on its source of the sound.

Non-diegetic sound, also known as "commentary sound," is audio whose source is neither visible on the screen nor has been implied in the action. This can include a narrator's commentary, added sound effects or mood music in the background.

Both diegetic and non-diegetic sounds can be used in the same project. This is called trans-diegetic sound. Certain sounds may be represented as coming from the story world, while others may be represented as coming from outside the story space. A story with diegetic and non-diegetic conventions can be used to create ambiguity (as in horror) or to surprise the audience in a comedy.

A good example of trans-diegetic sound is when a character hums a tune (diegetic sound) and the humming sound turns into an orchestral version of the same tune (non-diegetic sound), which carries over into the next scene.

Another is when music plays over the opening credits of a film (non-diegetic sound), but once the title sequence ends, that same music becomes a song heard on someone’s radio in the opening scene (diegetic sound). This example links the credit sequence with the opening scene to ease the audience into the viewing experience.

Diegetic, non-diegetic and trans-diegetic sounds are important tools to aid in telling a powerful story. Great directors including Alfred Hitchcock, Orson Welles, Stanley Kubrick and Woody Allen all used them effectively, and have used narration as a tool to mislead the viewer in their stories.

Just because a diegetic sound comes from the world of story doesn’t mean it was recorded as the action happened. This is where post-production and sound design comes in.

In some scenes, the genuine diegetic sound may need to be punched up to create more excitement. Other times a line of dialog might be muffled when recorded and needs to be re-recorded in post-production. This is called ADR, for automatic dialog replacement.

Sound techniques like these are very broad. However, to tell a story and make viewers care, diegetic sound techniques are important. No matter whether you produce a simple news story, documentary or commercial, sound is as important as the image. Good audio can sell any type of program and creators should study the masters to learn to offer compelling sound.