"If it can be written, or thought, it can be filmed..." STANLEY KUBRIC
Asynchronous Sound Effects
From Encyclopedia PRO
Asynchronous Sound Effects are not matched with a visible source of the sound on screen. Such sounds are included so as to provide an appropriate emotional nuance, and they may also add to the realism of the film.
For example: A film maker might opt to include the background sound of an ambulance's siren while the foreground sound and image portrays an arguing couple. The asynchronous ambulance siren underscores the psychic injury incurred in the argument; at the same time the noise of the siren adds to the realism of the film by acknowledging the film's (avowed) city setting.