Stereophonic sound used four or more analog magnetic audio tracks around the edges of the film. Magnetic tracks could not produce as clear a sound as the conventional optical audio tracks, and they tended to fade over time, but they took up a lot less space on the film. The standard film format did not have enough room for more than two optical tracks, but it was possible to squeeze as many as six magnetic tracks around the film frame. (See How Movie Sound Works to find out how optical and magnetic soundtracks work.)
In the stereophonic system, three to five channels drove speakers behind the movie screen. The popular four-channel system included one channel driving a speaker on the left, one channel driving a speaker on the right, one channel driving a center speaker and one channel driving surround speakers along the sides and back of the theater. Some systems boasted five separate channels behind the screen and one surround channel.
In these movies, most of the sound is recorded on the front channels so that the words seem to come from the screen. When an actor speaks on the left side of the screen, the dialogue sound comes from the left speakers. When an actor speaks on the right side, the sound comes from the right speakers. Most dialogue is also channeled to the center speakers, which serves to anchor, or focus, the sound on the screen. The rear track (or tracks) are typically reserved for "effect sounds," such as ambient background noise or a voice coming from off-screen.
In the 1970s, Dolby Laboratories introduced a new sound format based on this same configuration. In the next section, we'll see what made this system the new standard for theater sound.