Below are 5 Categories which sound within a film can be identified. (Sorry Its slightly LOTR themed, it was purely accidental as these where some of the best examples I could think off!)
Acousmatic Sound or Music in relation to film:
Acousmatic Sound is sound which can be heard, but with source of the sound not being visually present, however the sound source can be subsequently revealed later on.
As a result this means the sound could start off Diegetic and then become Non-Diegetic, or visa versa.
This creates a sense of mystery and depth to a scene as it makes it seem like there is more going on outside the viewpoint the scene is playing from.
Example: Lord of the Rings: The Fellowship of the Ring (2001) They have a Cave Troll
At the start of this scene Pippin accidentally knocks a bucket down an old well alerting the orcs that they are trespassing, it cuts to a scene of the empty mine works with the sound of orcs and drums, which fits the mood of the scene since none of the fellowship know where the sound is coming from.
Diegetic Sound is anything that both the audience and the characters can hear, such as environmental noises, dialogue between characters and sound coming from objects in the scene.
This is also known as “literal” sound as it would be only what the characters would be hearing if it were a real setting.
Example: Saving Private Ryan (1998) Beach Scene:
The opening of this scene is purely diagetic sound, as there is not score over the top or any narration.
The opposite of Diegetic Sound, can only be heard by the audience and is a reprehensive of coming from outside the story space. This includes film scores, exaggerated sound effects and commentary.
Non-Diegetic Sound is crucial for setting the mood and creating an atmosphere.
Example: Lord of the Rings: The Fellowship of the Ring (2001) Prologue: …One Ring to rule them All.
In this scene we have both types of Diegetic Sound: as you have Galadriel’s commentary, (Non Diegetic) and the score swells over the top of it, in addition to the sounds of arrows flying and swords clashing. (Diegetic Sound)
Empathetic Sound is Sound within a certain scene which reflects the overall mood of the scene. Overly Non-Diegetic in nature, but can be both, this could be represented by a sad song or overture being played as a character is dying, or a happy song such as in a wedding scene.
Example: Lord of the Rings: The Return of the King (2003) Faramirs sacrifice.
In this scene Denathor asks Pippin to sing a song while we know from earlier in the film that he doesn’t agree Denathor sending his son Faramir on a suicide mission to retake Osgiliath, Denathor is blinded by grief as he has lost his favoured son Boromir, and in order to try and earn his father’s love, Faramir decides to go anyway.
This is a very powerful scene as the sounds in the scene are limited to the song, and some occasional over emphasised Foley. These consist of the chicken bones cracking to represent the impending doom of Faramir and his men, and the arrows being pulled back in the orcs bows which when they fire cut back to Denathor and Pippin, leaving us not knowing how the attack went.
In Contrast to Empathetic Sound, empathetic is sound which seems indifferent to the current mood of the scene, and at some points can be jarring or a juxtaposition.
Again overly Non-Diegetic in nature, this could be seen as a way of a director trying to reflect a particular event within the story which happens when other characters are un-aware, letting us in on a plot point which the main characters might be privy to.
Example: Psycho: (1960) Shower Scene:
Even though we have just seen Marion Crane brutally butchered by the killer, the shower is still running and acts a foreboding message later on, as they don’t find out who the murderer is till the end of the film.
Hopefully this helps when you decide to add sound to moving image 🙂