Recall of Krell Amplifiers

A theremin was not used for the soundtrack of Forbidden Planet, for which Louis and Bebe Barron built "disposable" oscillator circuits and a ring modulator to create the "electronic tonalities" for the film.[20][21]

The musical score, termed "electronic tonalities," was composed by Louis and Bebe Barron. They were a married couple, collaborators of avant-garde composer John Cage. The "bleats, burps, whirs, whines, throbs, hums and screeches" that make up the sound track contained carefully developed themes and motifs, as well as providing general atmosphere. Unlike Bernard Herrmann's score for The Day the Earth Stood Still, which used the Theremin as well as an unconventional selection of standard musical instruments, Forbidden Planet's innovative score was entirely electronic.

The groundbreaking Forbidden Planet soundtrack is credited as the first all-electronic music soundtrack for a major commercial movie and was unlike anything that audiences had heard before. In fact, during one preview of Forbidden Planet, the audience actually broke out in spontaneous applause as the sounds of the spaceship landing on Altair IV filled the theater!

Making electronic music was slow and laborious process back in the 1950s. By following the equations presented in the 1948 book, Cybernetics: Or, Control and Communication in the Animal and the Machine, by mathematician Norbert Wiener, Louis Barron built electronic circuits which he used to generate sounds. Most of the tonalities were generated with a circuit called a ring modulator. After recording the sounds, the couple further manipulated the material by adding effects such as reverb and delay and also by sometimes reversing and changing the speed of certain sounds.