Optical sound
![Edge of a 35mm film print showing the soundtracks. The stereo optical sound strip is located on the right, with waveforms for left and right channels.
To the far left is the SDDS digital track (blue area to the left of the sprocket holes), then the Dolby Digital (grey area between the sprocket holes labelled with the Dolby](/Images/godic/202501/12/35mm_film_audio_macro3757.jpg")
![A transparent program disc imprinted with concentric optical sound tracks, used for the Optigan musical organ](/Images/godic/202501/12/OptiganProgramDisc3757.jpg")
![Example of a variable-area sound track on the right side of the frames on this 16mm filmstrip. The width of the white area is proportional to the amplitude of the audio signal at each instant.](/Images/godic/202501/12/USN16mmSoundtrack3757.jpg")
![Newspaper ad for a 1925 presentation of De Forest Phonofilms shorts, touting their technological distinction: no phonograph.](/Images/godic/202501/12/Phonofilm13757.jpg")
Optical sound is a means of storing sound recordings on transparent film. Originally developed for military purposes, the technology first saw widespread use in the 1920s as a sound-on-film format for motion pictures. Optical sound eventually superseded all of other sound film technologies until the advent of digital sound became the standard in cinema projection booths. Optical sound has also been used for multitrack recording and for creating effects in some musical synthesizers.