International projectionist (Jan-Dec 1947)

instrument with the higher frequency response. The same requirement applies to amplifiers because they also handle all types of reprodution, making a wide frequency range desirable. P.A. Work Requirements For conversational speech and public address work experience has shown that loudspeakers must be able to transmit frequencies within a range of approximately 200 to 4500 c. p. s. to insure intelligibility and clarity. Although frequencies above and below this range are not generally needed for intelligibility in speech work, frequencies below 200 cycles do help to preserve balance and naturalness of tone and may be included to advantage in the loudspeaker range. For orchestral or vocal groups a loudspeaker which can transmit frequencies within a range of 75 to 7000 c. p. s. is needed. This range is only average, however, and applies to the loudspeaker which uses only one loudspeaker unit or voice coil. A more elaborate loudspeaker, such as the RCA Twin Power unit, employs two loudspeaker units or voice coils — one for lowand the other for high frequencies. It has a frequency response ranging from 50 to 11,000 c. p. s., and as a result offers fuller and truer reproduction. In the final analysis the range of frequencies reproduced for the listening audience depends upon the quality and adequacy of the entire chain of sound equipment. To substitute an inferior unit anywhere in the chain merely reduces the effectiveness of the others, thereby impairing the listening results of the entire system. Reverberation Data When sound is generated within an area the listener receives a first impulse of this sound directly from its source, but he also hears successive impulses reflected from the walls, ceiling and other surfaces. This means that the total sound reaching the ear is a combination of both direct and reflected sound. This continuation or persistence of sound within a room after the source has ceased to emit sound is defined as '"reverberation." The time necessary for this residual sound to die away to one-millionth of its original value, or 0 level db, is called the "reverberation time." When of a satisfactory value the reverberant sound reenforces the direct sound, thereby enhancing it and improving the spatial effects as well as increasing the loudness. This is a desirable condition under which to operate sound reproducing and reenforcing equipment. On the other hand, if the tones of a speaker, vocal group or orchestra are too prolonged because of reverberation, successive sound impulses have a tendency to run into each other with a loss of intelligibility and clarity to the listeners. Under these circumstances an unsatisfactory condition is merely exaggerated by the reenforcing equipment. It then becomes necessary to reduce the reverberation by increasing the amount of sound that is absorbed at the point of each reflection. This is commonly accomplished by the use of ab sorbent acoustical materials on the ceilings and walls which, by increasing the absorption, diminish the reverberation time. Because of the widely varying characteristics of rooms and their uses no simple formula can be offered in determining desirable reverberation times. The size and shape of the room, the kind of program it must accommodate, and the size of the audience are all considerations. The frequency 512 cycles per second is generally used as a base. New Cadmium-Mercury Lamp Bows in Studios INVOLVING a "wedding" of cadmium and mercury vapor in a slim quartz tube, a new electronic lamp developed by Westinghouse Electric Corp. is considered by some eminent engineers as holding the promise of a revolutionary change in motion picture set lighting technique as well as eventually finding other important uses. Under development for several years, a 1000-watt lamp has successfully passed numerous test shootings during the past year in the studios of Samuel Goldwyn, Technicolor and others. Gregg Toland, noted cameraman, conducted many of the experiments. Additional studies were made at Technicolor and others. In large measure, data yielded by these tests provided the key to the proper combination of metallic vapors for the cadmium-mercury lamp. The new lamp was developed to afford the film industry a better, cooler and smaller high-intensity light source. Designed to simplify the problem of lighting movie sets while minimizing excessive heat, the experimental lamp was 14 inches long, Q/2 inches in diameter and tubular in shape. Numerous mixtures of cadmium and mercury were tried during the tests. Shorter Arc Types Coming Cadmium-mercury lamps of much greater power now are being developed to incorporate lessons learned from the test lamps. When they go into production, these lamps will boast shorter arcs which will provide the controlled, narrow light beams needed for scene highlights and spotlights. High-intensity mercury vapor lamps as developed by Westinghouse provide the most brilliant, long-burning light source of their type known. The only hitch in the lamp containing mercury alone is that its deficiency in the red hues makes it unsuitable for movies. Under mercury light, red lips and flesh assume an unnatural appearance, even in black-and-white photography. By adding an undisclosed amount of metallic cadmium to the mercury in the quartz inner bulb of the new lamp, red, more yellow and other colors supplement the blues and greens predominant in the mercury light. This "wedding of the metals" provides the numerous wavelengths of light to which color film emulsions are sensitive and results in highquality color as well as black-and-white pictures. Spectral Deficiency Corrected "Ever since the advent of color motion pictures," said Charles R. Long, Westinghouse development engineer, "the studios have sought lamps smaller, cooler and simpler than the widely used sources for set lighting. We expect that the new cadmium-mercury lamp will influence greatly movie set lighting practices. More powerful sizes to come will vastly increase set lighting brightness and should prove another step forward in. the constant search for better color and black-and-white cinematography." New S.M.P.E. New York Offices Headquarters of the Society of Motion Picture Engineers in N. Y. City have been moved from the Hotel Pennsylvania to the Canadian Pacific Building, 342 Madison Avenue (44th St.) where larger quarters have been leased on the ninth floor to accommodate the rapid growth of the Society and recent additions to the office staff under Executive Secretary Boyce Nemec. The new telephone number is MUrray Hill 2-2185. Engineering and editorial work of the Society, conducted in separate offices away from headquarters during the past several months, will now be reassembled in one location where it will be possible to give greatly improved service to the membership and the industry at large. Kodak Silver Usage Rivals Mint More than 12,000,000 ounces of the world's purest silver are used each year by the Eastman Kodak Co. for photographic purposes. Outside of the U. S. Mint, Kodak uses more silver than any other company or organization. INTERNATIONAL PROJECTIONIST • October 1947 73