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A Study of Ground Noise — Sandvik 797
Going back to Dr. MacKenzie's comments, I think the experiments were made in a way strictly comparable to practice. The projector with the sound recording elements was set up the same as in practice, and this condition was maintained, so that the first three films had a certain illumination incident on the photo-electric cell. In the case of the developed films, the setting was not changed, so that the actual illumination on the cell was lower, as it will be in practice. We were trying to find how much actual noise was attributable to the film, and I think the conditions were strictly comparable and correct.
Mr. Crabtree: You spoke of wiping filmmanually as compared with machine wiping. Do you mean squeegeeing with chamois or pneumatically?
Mr. Hall: Pneumatically.
Mr. Crabtree: That emphasizes the importance of using water which does not have a large saline content. Drops of water containing dissolved matter on the film make spots, and they contribute to the ground noise.
Mr. Jones mentioned the matter of cleaning film. Machines are available for satisfactorily cleaning film which has accumulated oil and dirt. The cleaning process consists in immersing the entire film in an oil solvent and then squeegeeing and drying. I think such a machine should be in every theatre devoted to the projection of sound movies. Laboratories are devoting attention to suitable lubrication of film intended for sound work and to methods for reducing the propensity of the film to accumulate dirt and scratches. In order to suitably lubricate the film, it is usual to apply a thin line of wax to the edges but if an excessive quantity is applied, this encroaches on the sound track area, and gives ground noise. Satisfactory experiments have been made by applying a solution of wax in a solvent with the edge waxing machine whereby only a small quantity of wax is applied. Only. a very minute quatity of wax is necessary to eliminate troubles which arise if the film is not waxed. In the past, we have been putting on too much lubricant. Experiments are in progress on the effect of varnishing the film but they have not advanced to such a stage that we can say anything about it.
Mr. Edwards: Was the trial made with a loop, so that the number of times it passed the light aperture could be checked? Have