The art of sound pictures (1930)

234 the art of sound PICTURES since the advent of radio. We are also familiar with the vacuum tube, more properly called an audion tube, which takes the minute radio waves coming through the ether and amplifies the tiny electric currents caused by these waves into larger electric currents capable of mechanical movement in the diaphragm of a loud speaker. Precisely the same sort of audion and vacuum tubes are used in amplifying the photo-electric current to the strength necessary to move the diaphragms in the loud speakers behind the screen of the moving picture theaters. The diaphragm, when it moves back and forth in response to these changes of electric current, sets up air waves which register upon our ears as sound. The entire level of the intensity or volume of the sound from behind the screen may be regulated by using a greater or lesser number of amplifying audion tubes in the circuit between the photo-electric cell and the loud speaker. Of course, there are many refinements of electrical apparatus which enable the operator to regulate the sound effects. But the essential principle of the translation of rays of light, coming from the sound track of the film, into photo-electric currents, and finally into air waves of sound emanating from the loud speaker, remains the same throughout all the complex devices used to give the requisite refinements to the sound reproduction process. We now have before us the entire process of sound photography and sound reproduction. Starting with the original sound waves of the air caused by the speakers’ voices, we know that these sound waves are transformed into electric waves or currents by means of the microphone apparatus. The tiny microphone currents are then transformed into varying intensities of light by super