Sound operates on the audience below conscious awareness. Low-frequency rumbles (called infrasound) create physical unease without the audience identifying the source. The absence of ambient sound in a scene creates tension more effectively than any visual. The brain is wired to treat silence as a sign of danger.

Walter Murch, who coined the term "sound designer" for his work on Apocalypse Now, layered helicopter sounds with Wagner's "Ride of the Valkyries" to create an audio nightmare that captured the madness of Vietnam. The sound design is as much of a character in that film as any actor.