The motion picture projectionist (Nov 1931-Jan 1933)

November, 1931 Motion Picture Projectionist 35 cidence. Data collected by this Committee show that the average projection angle is approximately 15 degrees, measured to the perpendicular to the screen. Therefore, we believe that measurements with light incident on the test sample 15 degrees above the normal will give information more in keeping with conditions of actual practice. The reflected light would be measured in a horizontal plane and in a vertical plane containing the light beam, both normal to the screen sample. A more complete discussion of this question will appear in our later report. The accompanying curves, Figs. 1-6, illustrate the variation in distribution of light at three angles of incidence, 0, 15, and 30 degrees from normal. It will be noted that with a smooth diffusing type of screen the difference between measurements at zero and the other angles is appreciable but that the distribution is relatively uniform. In the horizontal plane, there is considerable diminution of reflected light and some equalization in distribution for both beaded and metallic types. In the vertical plane for a beaded screen it is a distinguishing feature that the axis moves to follow the incident light beam so that a good portion of the light is reflected back upon itself. With a metallic screen, the axis is at the specular angle. It its planned to make recommendations for the types of screen to be employed in theatres of different architectural design, as is now being done to some extent, but as yet the Committee is not ready to go on record with definite suggestions. Variation across Screens. — Because of non-uniform light incidence over the total area of the screen and because of the non-uniform reflection characteristic, there will usually be variations of brightness in a projected picture. AH theatres are subject to this effect at the front of the house but, of course, the continuous change of intensity in the pictures reduces its noticeability. We shall propose limits for allowable brightness differences. Sound Transmission Theory. — The design of screens from the standpoint of sound transmission presents problems which are simple in comparison with optical considerations. The great importance of good sound transmission characteristics should, however, be recognized. An analysis of the general problem of transmitting sound through a material such as a screen indicates several possible methods; certain practical considerations, however, limit the designer to the use of two. A screen may be expected to radiate sound as a result of being set into vibration by sound impulses emanating from the loud speaker immediately behind it, or the sound impulses may be transmitted through the air spaces in the screen material. Types of Air Spaces These air spaces may simply be those due to the porosity of the material itself, but better control of the transmission characteristic may be effected by deliberately providing air passages of the proper size and number. This may be accomplished by careful weaving, punching, or other means. All commercial types of screen depend largely upon this method of transmission although many depend upon the diaphragm action of the screen to overcome a loss which may occur at low frequencies due to a decrease in the radiation resistance of the air passages in this part of the frequency range. Because of the desirability of affecting the optical characteristics of the screen to as small an extent as possible, the perforations or air spaces in the screen are made as small as is practicable and their number is limited to the absolute minimum. Fortunately, it is possible to obtain quite satisfactory sound transmission by using rather small, widely spaced openings which, in the aggregate, offer a comparatively small total open area in the screen. It is felt that an aggregate open area amounting to 5 per cent of the total screen area may be considered tolerable from the light reflection standpoint. On this basis it is found that the sound reauirements may be met without impairing the detail of the picture. The relations between the screen thickness and the size and number of the holes may be worked out rather easily by applying the known acoustical theory; an approximation wul serve, however, lor the practical designer. For perioraied screens it has been found, in general, that if the diameter of the perforations is equal to three or four times the thickness, the aggregate area of the openings being 5 per cent of the total screen area, satisiactory results may be obtained. This applies to the usually used materials and, of course, must at present be considered true only for them. Furthermore, it applies only over a limited range of screen thickness. This relation shows that it is desirable to keep the screen thickness at as low a value as is mechanically and optically practical. Test. — It is the present practice to measure the sound transmission characteristics or response characteristics of each type of screen before approving it for use with sound projecting equipment. Although there are various methods by which these acoustical measurements may be made, the commonly used method involves response-frequency measurements of the output of a loud speaker with the screen placed before the speaker in its normal position and with the screen removed. In order to adhere as closely as possible to actual field conditions in making these measurements, a loud speaker of the type used in the field should be employed. Since this method of test approximates closely the theatre conditions and since it includes the effect of the diaphragm action of the screen, if present, it is probably the most desirable method of making the measurements. The response measuring technic should conform with accepted loud speaker response measuring methods. Tolerances. — There are three factors which must be determined before a proper judgment of screen performance may be made. The general loudness attenuation effect, the frequency range for sound transmission, and the regularity of frequency response all enter into the determination of the suitability of a screen from the acoustical standpoint. In general, little trouble is experienced in obtaining efficient low frequency 5. Distribution oi Brightness in Horizontal Plane for Metallic Screen Fig. 6. Distribution of Brightness in Vertical Plane for Metallic Screen