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SOUND TECHNIQUE 233
apparatus will be activated, and the louder the sounds issuing from the loud speaker will be.
Let us see now how this photo-electric cell is used to translate black and white marks on a strip of film into tiny electric currents. The nature of the cell, as above described, is such that the brighter the light which is thrown upon it, the greater will be the volume of the electric currents coming out of the cell. An electric light in the projection machine is so focused that it shines directly through the sound track of the film on to a photoelectric cell. When a light area of the sound track passes in front of this light, a great deal of light will shine through it on to the photo-electric cell, and a comparatively large volume of electric current will flow out of the cell as a result. When a dark bar appears on the sound track, the light shining through it will be correspondingly interrupted. As a result, the photo-electric currents will be comparatively small.
No matter whether the sound track is recorded by the variable density method or by the variable area method, the same principle holds true with respect to its interruption of light focused upon the photo-electric cell. The principle, however, is very simple. The greater the light area on the film, the more light will fall on the photoelectric cell, and the more electric currents will be generated by the cell itself. By this process, as you can readily see, sound photographs are translated into electric currents of varying strength. It is now a comparatively simple matter to amplify or increase these currents until they are able to activate the vibrating diaphragm of a loud speaker apparatus.
The principle of the loud speaker is familiar to us all