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SOURCES OF SOUND CURRENT 515
117 to the slit. The objective lens focusses an image of the slit upon the film.
How the Photo-Cell "Sees" Volume
(7) Let us consider two musical notes that are identical in frequency, but differ in volume. Then in each case the photo-cell will see the same number of changes of light per second. But there will be a more drastic change in the case of the louder note, in consequence of which the cell will create a stronger ripple in the direct current flowing through it. The explanation of this cell action will follow in a moment.
(8) Reference should first be made to Figures 101 and 102. These show that in the "variable density" type of sound track the dark areas are darker, and the light areas lighter, in some portions of the sound track than in others. Those parts of the track in which the contrast between light and dark is greatest are the parts in which loud sound has been recorded — that is, those parts in which the action of the light valve in the recording studio has been strongest. Those parts of the variable density track where the light valve moved very little, and the contrast between light and dark areas is not great, represent low volume.
(9) With a track of the "variable area" type (Figure 101) the amount of light reaching the photo-cell is varied by altering the areas of the light and dark portions of the track.
Noiseless Recording
(10) The earliest sound tracks were based on the principle of recording dark areas on an otherwise transparent track. The result was that during times of low volume or no sound at all light passed through the film to the photo-cell, and caused a noise called the photo-cell "hiss." The grain of the film also created "ground noise." (11) Noiseless recording involved no apparatus changes in the projection room, but modifications in the studio that kept the sound track practically dark when there was no sound, and illuminated it only in proportion to the