Diegetic-Sound: These sounds are sounds that both the audience and character can hear. There is a multitude of ways this can be done; environmental sounds, dialogue, and sounds coming from inside the scene.
Another way to refer to this type of sound is, ‘literal sound’.
The example above is the opening scene of the movie ‘It’ (2017). The scene consists of many diegetic sounds such as rain, dialogue, and footsteps. All of these are perfect example of diegetic sound since they were shot, and recorded on scene.
Non-Diegetic Sound: Non-diegetic sound is sound that is added into a scene after it has been filmed. These sounds can include, zaps, booms, and pows. To be short, it is usually sounds thaty cannot be replicated in real life, or cheaper to add on later.
It is added in what is called ‘post-production’, or, after the filming of the scenes.
The above example is the widely known film, ‘A Quiet Place’. In this film, the physical villain is the creature. Since this creature is obviously done through CGI, it must use fake, inorganic sounds. That’s when fake, non-diegetic screeches are added in through editing.
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