It was a double system, that is, the sound was on a different piece of film from the picture... In essence, the sound was captured by a microphone and translated into light waves via a light valve, a thin ribbon of sensitive metal over a tiny slit. The sound reaching this ribbon would be converted into light by the shivering of the diaphragm, focusing the resulting light waves through the slit, where it would be photographed on the side of the film, on a strip about a tenth of an inch wide.