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74
BRITISH KINEMATOGRAFHY
Vol. 20, No. 3
and by sprocket teeth engaging the perforations.
A frequently used construction is that of
Fig. II Path of the light rays for normal sound reproduction.
one or more guide rollers on a pivoted mount which is often air or oil damped. The task of this mechanism is to prevent the vibrations
from being propagated to the sound-scanning spot.
This construction, however, is only partially satisfactory and is unnecessarily complicated, because the film itself can be used as a perfect mechanical filter. When a film is stretched with a very small force over a roller, slack and resilient bends will be formed on either side of the roller. These bends eliminate vibrations of any frequency — provided that the amplitude is not too large — since in the first place the film material has very small mass and in the second place the bends act as frictionless springs. It is well known that the lower the tension on the film, the better the elimination of longitudinal vibrations.
Now, if both the sound drum and its pressure roller run in ball bearings, and if the bearings are lubricated with oil instead of grease, the tension on the film can be kept as low as 40 grams, about 1J ounces.
In order to eliminate even the largest amplitudes, it is necessary to increase the number of bends, which can easily be done by inserting one or more rollers in the film
path between the sound
drum and the take-up
sprocket. However
very low
film is no
ciently in
the sound
special
, due to the tension, the longer sufficontact with
drum. If no measures were
l^f
taken, the film would make very small and rapid movements perpendicular to the surface of the drum, giving the effect of a drum of varying diameter. These movements would hardly be visible with the naked eye, but would cause scanning faults due to
Fig. 12 The position of the sound track