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June, 1930]
VARIABLE AREA SOUND FILMS
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power, and differ only in speed and, slightly, in inherent contrast. The DuPont "V.A." film is much the fastest of the three, but also has the lowest contrast.
The makers of film have been for some time experimenting with new emulsions for sound recording. As far as the variable area process is concerned I believe the line of attack is fairly well indicated by the results which have been given. Improvement in resolving power without increase in speed will result in only slight improvement of sound reproduction. What is needed is increase of speed without loss of resolving power to a point which will permit a considerable reduction in the width of the recording slit. But even this will be of
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FIG. 9. Comparison of losses on three different film stocks. The curve drawn is the same in all three cases.
importance only when other parts of the system, in particular the loudspeakers, have been developed to a point which will permit the reproduction of frequencies in the range between six and ten thousand cycles per second.
Before concluding, let me summarize the properties of variable area recording as they are shown by this study:
If the recording is done on any of the present "positive" film stocks the best results will be obtained by working for a density of 1.3 in both negative and positive, with development to a gamma of approximately 2 in each case.
But on the basis of the ability of the ear to distinguish differences