International projectionist (Jan-Dec 1940)

Record Details:

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obtained by the ever-present odor emanating from the sound screen surface. In many territories certain gases in the air tend to speed up disintegration or evaporation of the chemicals and thus contribute to discoloration. One of the most commonly observed -effects is yellowing, but others which may not be as readily apparent also appear. The rough pigment surface collects dust which may tend to cause detrimental chemical reactions as well as darkening of the surface, commonly referred to as "greying". Dust also collects in the perforations, thereby reducing their diameter and seriously affecting the sound transmission characteristics. • Continuing Light Loss Technical data gathered by one reliable investigating body indicates that the light-reflecting properties of sound screens deteriorate at an average of approximately 4% per month. At this rate, at the end of one year, the average sound screen is only about onehalf as efficient as a light reflector as when it was first made. Likewise, the sound transmission characteristics also deteriorate unless the perforations are kept clean. At this screen age, either or both of two somewhat inefficient and sometimes costly remedies are commonly applied. In some instances, the projectionist increases the arc current, and while this may restore the reflected light to its original value, generally it is costly in the long run and, furthermore, it may contribute to uncomfortable viewing because the color and wave-length balance are not the kind of light quality that contributes to good seeing by the human eye. Also, by this time the sound quality undoubtedly has suffered, and the sound engineer sometimes resorts to complimentary recompensation of the sound system amplifiers to accentuate the electrical output of the frequencies that are being attenuated unduly by the dust-filled perforations, thus blasting the sound through the screen by brute force. While this may help to make the sound more intelligible, the poorer sound transmission qualities of the screen cause a ragged response which detracts from the smoothness and natural characteristics essential to comfortable hearing. In other instances, we find that resurfacing of the sound screen is resorted to when its light-reflecting properties become impaired. If this is skillfully done by one of the few experts specializing in this kind of work, an apparently good resurfacing may be had. However, it is generally agreed that a first-class job involves a multiplicity of coatings and that, regardless of how skillfully applied, the additional coats of resurfacing material make the sound screen thicker and reduces the diameter of the perforations. In view of the fact that the thickness of the sound screen and the diameter of the perforations (for a given number of perforations per unit area) are the factors governing sound screen transmission characteristics, even a single resurfacing leaves it appreciably worse from a sound-transmission standpoint than when it was new. Likewise, the reflection characteristics, although they may appear to be as good as when the screen was new. never quite approach the original. Nevertheless, many theatres resurface a screen approximately once a year for several years or more and replace it with a new one only when the perforations have become almost completely clogged. This particular method of coping with this difficult condition appears to be logical from a purely economic point of view only so long as all factors pertaining to this problem are not given their proper importance. Regardless of what is done to the average sound screen, once it has deteriorated it is apt to be a relatively poor compromise from the standpoint of true efficiency. The projectionist may still be pouring good light Sound Screen Brevities All types of sound screens undergo a steady deterioration from the day they are made. . . In many territories certain gases in the air tend to speed up disintegration or evaporation of the chemicals and fhus contribute to discoloration. . . Light-reflecting properties of sound screens deteriorate at an average of 4°/c per month. . . . Even a single resurfacing leaves a sound screen appreciably worse from a sound-transmission standpoint than when it was new. Likewise, the reflection characteristics never approach the original. through the projection port and good sound may be emanating from the stage loudspeakers, but the good work on both points may be nullified by a sound screen that has ceased to function within tolerable efficiency limits. • Suitable Test Apparatus • Part of the lack of appreciation of the important qualities of a really good sound screen has been due to a lack of test apparatus capable of making comparatively rapid and accurate checks directly in the theatres where the sound screens are installed. While a few instruments have been available with which to measure incident light to the screen, it was not until very recently that a practical portable instrument that would reliably measure the reflection characteristics of sound screen surfaces was developed. To be truly valuable it must react to color values in the same manner as the human eye. Properly coordinated use of such instruments of recognized reliability may aid materially in determining the true efficiency of the light transmission characteristics of sound screens. The sound transmission properties of sound screens depend upon a number of important details that seem to vary considerably with various types and make. As an example, one of these details is the varying weight or thickness. Another is the size and number of perforations per square inch. We find some sound screens with forty holes per square inch .050" in diameter, some with twenty-four holes per square in .060" in diameter, etc. Frequently one sees sound screens that from the standpoint of appearance would be expected to have adequate perforation but are too thick and consequently possess too high an acoustic impedance' to permit good sound transmission qualities. The S. M. P. E. standard provides that screen attentuation at 6,000 cycles shall not be greater than 25%, and at 10.000 cycles not greater than 37% relative to the response at 1,000 cycles. It should not be overlooked that this standard pertains to the maximum loss permissible and merely identifies a tolerable sound screen, which may not be an ideal one. From this it is apparent that there is considerable margin for improvement above the standard. In view of the present characteristics of most types of sound screens in use. it may be well to remember that the best results from $10,000 worth of sound and projection equipment are obtained from the $100 sound screen by replacing it regularly at least once a year. 8 INTERNATIONAL PROJECTIONIST