Digetic and Non-Digetic Sound

Diegetic Sound

Sound whose source is visible on the screen or whose source is implied to be present by the action of the film: 

• voices of characters 
• sounds made by objects in the story 
• music represented as coming from instruments in the story space ( = source music). In final product, we will use a soundtrack similar to the BBC programme Luther. 

Diegetic sound is any sound presented as originated from source within the film's world 

Digetic sound can be either on screen or off screen depending on whatever its source is within the frame or outside the frame. 

Another term for diegetic sound is actual sound.

Non-diegetic

Sound whose source is neither visible on the screen nor has been implied to be present in the action: 

• narrator's commentary
• sound effects which is added for the dramatic effect
• mood music

Non-diegetic sound is represented as coming from the a source outside story space. 

The distinction between diegetic or non-diegetic sound depends on our understanding of the conventions of film viewing and listening.  A play with diegetic and non-diegetic conventions can be used to create ambiguity (horror), or to surprise the audience (comedy). 

Another term for non-diegetic sound is commentary sound.