How Does Sound Get on Movie Film?

If you were making a movie and wanted to have a sound track to go along with it, you’d probably use a tape recorder especially made to work along with movie cameras.

You could simply tape the sounds while you filmed the action. Or you could connect the movie camera with the tape recorder by means of a cable, so that when the movie camera started and stopped, it would start and stop the tape recorder. This method is called synchronized sound.

If you’ve ever seen a movie being made, you may have noticed that before a scene begins, just when the camera starts rolling, someone places a small blackboard in front of the camera, with the scene number on it, and claps a stick against the top of the board.

Later, when the sound track and film are being put together, the editor can match up the click of the stick on the sound tape with the frame of film in which the stick meets the board, so that the film and tape will be synchronized.

After the film is finished, you’ll have a separate sound track. There are two ways to put this sound track on the film. With one system, called optical sound, a laboratory makes a photographic image of the sound waves. This sound track looks something like the bumps and valleys in the groove of a record. The optical sound track is placed in a thin strip that runs along the edge of the film.

The other way to put sound on film is with a magnetic strip. A strip of blank magnetic tape, the same kind you use in an ordinary tape recorder, is placed along the edge of the film. A specially designed movie projector or tape recorder transfers the sound track from the original tape to the magnetic strip along the film.

Some projectors can play back only optical sound, but others can play back both kinds. When the projector is showing a sound film, a separate sound system picks up the sound track from the strip along the edge of the film and plays it back through a speaker.