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158 RICHARDSON'S BLUEBOOK OF PROJECTION
chair backs. From point A measure along this line a distance equal to the distance from point A to point B, and tie a string* to the line. Next from point B measure the distance to the desired point, in degrees, from the center line. Call this point C. The plan is shown in Fig. 51.
We now have points A, B and C established. Next multiply two times the distance point, A to B, in inches, by 3.1416, and divide by 360. The result will be the length, in inches, of one degree of the circumference of a circle, the radius of which is the distance from point A to point B. If we now measure the distance from B to C in inches, and divide by the length of the degree, we arrive at the number of degrees point C is from the center line of the theatre. This is termed the "viewing angle. " The same calculation may be made from any point in the auditorium to any point on the screen.
Because we have measured in a straight line from B to C, our answer will not be precisely correct, but near enough for all practical purposes. If an exact calculation is desired, lay several short, wide boards on the chair backs along the arc of the circle found by swinging point C (mark on line) around from C to B. Trace the circle along the boards and measure the length of the arc exactly.
Perforated vs. Solid Screens
(43) Perforated screens are rated lower than solid screens for several reasons.
They require more frequent cleaning and refinishitig than solid surfaces because the air circulating through the hundreds of thousands of tiny holes leaves a deposit of dust in the walls and around the rims. Discolorations caused by gases soon appear, inducing the reflecting power of the screen to diminish rapidly. It is more difficult to clean a perforated screen than a solid one. Without a suitable apparatus and experience it is equally hard to refinish a perforated screen without reducing the perforation diameters or filling them in completely, thus reducing and (or) setting up uneven sound distribution.