International projectionist (Oct 1931-Sept 1933)

18 INTERNATIONAL PROJECTIONIST June 1933 PERFORATIONS IN SOUND PICTURE SCREENS Foreword: Attention has been directed of late to the sound picture screen as a very important link in the reproduction chain. Evidence is not lacking that the transmission of sound to a theatre audience involves something more than merely punching a given number of holes, evenly spaced, in a standard screen. Then, too, there are those who hold that neither perforated nor porous screens are essential to good sound reproduction, the conclusion reached by this group being that the horns should be positioned at either the top or sides of the screens and thus permit the use of a solid screen surface. This latter topic is not discussed in, the accompanying article, which was prepared by the Acoustic Department of Electrical Research Products Co. The article does establish, however, that the number of perforations, the frequency of the sound-wave and the character of the screen itself are inseparably linked, each of these items exerting a profound influence upon the transmitting qualities of a given screen. Table A included in this article reflects first publication anywhere of important transmission figures. — Editor. 'T' O THE audience in a motion picture theatre, the screen is the most apparent element in the projection of a picture. Before the introduction of sound, the screen's sole purpose was to serve as the source of visual stimuli. At present, however, its functions are two-fold as it must serve in an acoustic capacity as well. With loudspeakers behind it, the screen must now faithfully transmit to the audience the sound impulses accompanying a visual portrayal. The characteristics of sound motion picture screens have been discussed by H. F. Hopkins in a paper appearing in the "Journal of the Society of Motion Picture Engineers" for September, 1930. He explained that a sere can transmit sound in three ways : by motion as a diaphragm, by direct passage of acoustic energy through perforation or pores in the material, and by wave motion within the material. The last method is of comparatively minor importance and it turns out that most of the acoustic power originating on one side of the screen is transmitted to the audience by the first two methods. At low frequencies, diaphragm action induced by the acoustic power generated in the loudspeakers is the effective agency for the transfer of sound. In this case, the screen acts as a secondary radiator. At high frequencies, holes punched or woven into the screen are the effective means for the transmission of sound. Effect of Perforations In his paper, Mr, Hopkins discussed these jf actors in detail. Other work done since the time the paper was written substantiates his observations on the importance of perfora tions or holes in the screen. Some measurements made at Bell Telephone Laboratories are of interest in this connection. Tests were performed on a series of screens identical in all respects, save that the number of perforations was varied from zero to a condition whereby the perforated area comprised 9 per cent of the total screen surface. It was found that as the amount of perforation was increased, the reduction in acoustic power caused by the presence of the screen in front of the loudspeaker grew gradually less. In other words, the screen became more acceptable as a link in the sound reproducing system with increasing amounts of perforation. As was expected, the influence of the perforations was most manifest at high frequencies. Table A shows the results of the measurements with the screen losses expressed in decibels. The very real efficacy of perforations is shown by the table. At frequencies below 1,000 cycles, the improvement is slight; above this point, the contrast between the screen with no perforations and that with perforations corresponding to 9 per cent of the total surface is very marked. At 10,000 cycles, for example, there is a difference of 17.5 db. From an optical viewpoint, of course, it is essential that the perforations be maintained at a minimum and that their size be small. Present types of screens represent satisfactorycompromises between the optical and the acoustical requirements. The thickness of the screen and the diameter of the perforations or other holes are also factors requiring consideration from an acoustical viewpoint. Acoustical theory — a detailed discussion of which is beyond the scope of the present treatment — indicates that the ratio of the thickness of the screen to the area of the individual openings should be small for efficient transmission of sound energy. In designing a screen, all these factors must be considered in order to produce a product that is commercially acceptable. Brevities "The Essannay Electric Manufacturing Co., manufacturers of the Strong Change-over and other projection room accessories, is now established in new quarters at 823 South Wabash Ave., Chicago, 111. L. D. Strong, president of Essannay, is a member of Local Union 110, I. A. • Bizzelle & Company, Inc., with offices at 220 West 42nd St., New York, will handle all export sales for Pacent Engineering Corp., which recently purchased the principal assets of Pacent Reproducer Corp. and the Pacent Electric Co. • Reorganization of the Chicago Cinema Equipment Co. has been completed, the company to continue under the same name the manufacture and development of projection equipment, stage lighting equipment and other theatre accessories. Chicago Cinema has absorbed the Gallagher Orchestra Equipment Co. and will continue to serve the orchestra trade. Engineers of the company will be glad to consult with projectionists, managers and architects in the design of theatre equipment. Officers of the company are: Walter Pitann, president; Alexander Berg, Table Frequency Perforation: 0% A Tranmission Loss — db 4.4% 6.8% 9% 55 5 .5 .5 .5 .5 .5 .7 2.0 3.0 4.5 6.0 .5 .5 .5 .5 .3 .5 .7 1.0 2.3 3.8 .0 100 200 500 1,000 5 5 5 3.0 .0 .0 .0 .0 2,000 6.7 .5 4 000 11.0 .8 6,000 14.5 1.2 8,000 18.0 2.0 10,000 20.5 3.0