Cinema is classically described as a visual medium. But turn off the audio as you watch a movie, and you will grasp the centrality of sound—speech, sound effects (all nonvocal noises), and music—to the telling of stories on film. It is the interaction of sound with image that gives films much of their depth and solidity, emotion and meaning. Yet sound tends to be unnoticed, "invisible," when it stays within the norms and conventions of Hollywood filmmaking. The paradox of film sound is that it takes great artifice to produce the sounds that apparently emanate from sources onscreen, seeming so natural that we take them for granted.
"Illusionism" describes the dominant aesthetic of mainstream film: technique is hidden, made invisible, so as to give the impression that we are looking into a real world and do not have to be conscious of camera operators, flubbed lines, editors—all the work that constitutes the production of this illusion. To be sure, sound is not the only arena of classical filmmaking technique that subordinates its presence so as not to distract us from immersion in the narrative. There is a vital difference between sound and image in regard to transparency, however, because filmgoers are more conscious as viewers than as listeners. Whereas we notice most everything in the frame, we rarely notice most sounds (in life or in film). As a result, film sounds can be manipulated to depart from realistic standards to a much greater extent than images.