The art of sound pictures (1930)

SOUND TECHNIQUE 221 speak with the receiver held to the ear of a person perhaps several hundred miles away. Each time this electric current is made to vary by the air waves of your voice, the current, in turn, moves another diaphragm in the receiver, and this second diaphragm translates the vibrations once more into air waves, thus producing the exact sound of your voice in the ear of the person listening. In the case of the ordinary telephone, therefore, air waves of sound are translated into electric waves and then back again into sound waves. The microphone which is used to pick up sounds for radio transmission, and also for sound pictures, amounts to little more than a highly sensitized telephone transmitter. The same principle of translating air waves of sound into variations of electric current is employed in both. In the microphone apparatus, however, as used in sound pictures, tiny electric currents, generated by the microphone in response to sounds made by the voices of the players, must be translated still further into still finer waves of light, instead of being changed back again into sound waves, as in the case of the telephone. This can easily be done by connecting the microphone currents with the electric current of an electric light in such a way that the brightness of the electric light is varied in accordance with the variations in the microphone currents. When appropriate connections have been made in this way, so that the microphone currents are superimposed upon the electric lamp circuit, the lamp will flicker up and down in brightness in exact correspondence with the sound waves of the voice registered by the microphone. We now have the essential parts of our sound photog