from Audio-Vision: Sound on Screen
Chapter 4: The Audiovisual Scene
Chion makes, at the beginning, a very interesting observation regarding sound. He posits that sound has no "container" like the image does (contained within the frame) and that subsequently sound must be treated differently. The combination of sound and the image, as in cinema, result in what Chion calls a "spatial magnetization" of sound by the image. That is, when sound that corresponds to the image exists in synchrony the image becomes the perceived source of the sound, even in cases where this is illusion is physically more difficult to create aurally, as is the case with mono sound.
Borrowing from Pierre Schaeffer, Chion utilizes the Greek word "acousmatic" to describe "sounds one hears without seeing their originating cause". This holds a particular critical value in the consideration of film sound when compared with what Chion calls "visualized sound" - sounds taking place onscreen and off. Chion also identifies a third category of sound which he calls nondiegetic. Nondiegetic sounds do not have an originating source present in the visual image and exist external to the world of the story (i.e. a laser sound in a Civil War story).