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June, 1930] VARIABLE AREA SOUND FlLMS 641
The solid lines of Fig. 4 are, of course, theoretical, except in so far as they are based on Fig. 3, which is an experimental curve. The dotted curves will serve to show how well this theory is borne out by experiment. To obtain these experimental curves two negatives were used, one weak and heavily fogged, the other about average in density and fog. The records were of the frequencies 500, 1000, 3000, 5000, and 6000 cycles, at constant amplitude. Prints were made at enough different printer light settings to obtain the form of the curve of output versus density. These prints were all developed for the same length of time, to a gamma of approximately 2. The output measurements were made by running the films on a standard projector and reading with a 1000 ohm thermal voltmeter the a.c. voltages impressed across the loudspeaker coils. The amplifier used in these tests was calibrated over the entire range of frequencies, since the tests were for the purpose of obtaining data on the frequency response of the films as well as on the general output level. The points plotted here are proportional to the averages of the outputs at 500 and 1000 cycles. Since the method of measurement does not give any absolute level for the curves, the level has been fixed by measuring the track density and fog density of the print which gave the highest point on the right hand curve and giving to that point the level thus determined from the curves of Fig. 2. It will be seen that when this is done the other points all fall into place and the curves fit well into the family of theoretical curves.
Fig. 4 tells us all that we need to know about the upper limit of the volume range. The lower limit is determined by the ground noise level. This latter is a much more difficult matter to discuss, but it is possible to draw certain conclusions. If both negative and print have received careful handling, experience shows that the ground noise on a new print is inappreciable. The noise that is important in the theater is due to dust and scratch marks which have accumulated on the film in the course of repeated projectings and rewindings. Now this will be at a maximum if the clear side of the sound track is entirely transparent. If there is a layer of fog over this side of the track it will reduce the level of ground noise in the same way that a neutral density filter placed over the slit would reduce it. The general level of sound is reduced at the same time, of course, so that if we are printing from a fairly dense negative the ratio of ground noise to maximum sound is not changed by overprinting which fogs the clear side of the track. But if the negative is weak the case is somewhat