Projection engineering (Jan-Dec 1931)

OCTOBER, 1931 Page 9 all times. Thus a permanent installation makes for better projection than would be possible were the system moved from classroom to classroom as needed. Finally, best operation of the equipment will be obtained if it is permanently housed in a suitable booth. Sound Systems The sound system as used in a schoolroom motion picture projection system will differ little from the systems as described in articles which have appeared from time to time in Projection Engineering. In fact, with proper design on control equipment the amplifier and horn reproducers as used in a schoolroom sound amplification and distribution system might be used for the projection of sound for motion pictures. Points of installation and precautions as brought out in previous articles on public-address equipment will apply equally to sound systems as designed for the projection of audible motion pictures. The quality of reproduction depends primarily upon the signal response of the entire circuit from photoelectric cell or disc pickup to the loudspeaker. If no distortion is introduced along the line and if output amplifiers and reproducer horns have sufficient capacity to handle the power, the quality will be good. It should be borne in mind, however, that the overall quality of the system may be marred by one unit which has a bad frequency response. Consequently it is of importance that units as selected for building up the system into an operating whole should have adequate response over the entire audio spectrum. Quality may be marred also by a mismatching of units. Impedances should be matched either by proper selection or by the use in impedance matching transformers. It has become common practice with some sound installation engineers to compensate for deficiencies of frequency response in the system by a deliberate mismatching of impedances, but in general better results will be obtained by the use of a tone control device. There are several excellent ones now on the market. All input and output lines should be twisted and shielded and kept far enough apart to eliminate the possibility of crosstalk. Spare Parts In all but the simplest of systems it will be well to furnish an auxiliary amplification system which may be held in reserve ready to be placed in service in case of failure of the main amplifier. The reserve amplifier should have the same frequency response and same power output, although it need not be of such serviceable construction. However, in systems where financial outlay (Courtesy, Amplex Instrument Laboratories, New York) Rack and panel used, for school sound installation. is of paramount consideration, the system may be built initially with only one set of amplifiers, space provision being made in the installation for the addition of a reserve system at a later date. For the ordinary school auditorium such as might be used for the projection of educational motion pictures, one loudspeaker will usually be sufficient. Due to its directional properties, a horn type speaker will no doubt best fulfill requirements to the best advantage. It might be brought out at this point that, no matter how excellent the quality of the reproduced sound as brought from the system, if the horn be placed behind a heavy, closely woven screen quality will be bad as heard by the audience. The screen, in addition to its importance to the visual portion of the program, is as important a link as any in the sound system. Much of the difficulty noted in understanding projected sound in many systems is due to interference introduced by the screen placed directly in front of the horns. A "sound screen" which will introduce no interference to the projected sound should be installed. In case of vibrational noise passing through the system, rubber pads should be placed underneath the feet of the projection machine. Vibration will oftentimes be transmitted from a source external to the building through the floor and hence to the amplifiers. In schools of high school grade and above, there will always be some students who are sufficiently interested in motion picture projection to wish to operate the machine. These students may be trained in the principles of operation and care of the equipment and eminently satisfactory results will be obtained by the use of their services. In conclusion it might be stated that while at the present time the production of educational films has not reached the proportions that educational exchanges are necessary, the time will no doubt arrive when such exchanges will function in much the same manner as exchanges now handling the regular entertainment films. Thus the building up of extensive individual film libraries will be unnecessary and even undesirable. Underwriters' regulations allow the keeping of 6,000 feet of film in the booth at one time. If more film is kept on hand, a specially ventilated, fireproof vault will be necessary, the specifications of which may be obtained from the Board of Fire Underwriters. The instructive sound motion picture, with its peculiar problems of production, distribution and exhibition is bound to take its place in the educational program of the future. Its coming should be anticipated and adequate preparation made by educational authorities, motion picture producers and exhibitors and sound and projection equipment installation engineers. An entirely new and vast field is about to open up, and it is one where there are possibilities of betterment in instructural methods and at the same time material savings in costs as compared with the present antiquated teach ■ ing methods, relics of a bygone age. SOUND MOVIES IN EDUCATION I N the November issue of Projection * Engineering will appear an article on the subject of sound-pictures in education, by Dr. Alfred N. Goldsmith, vicepresident and general engineer, Radio Corporation of America. Dr. Goldsmith is a widely recognized authority on this important subject.