International projectionist (Jan-Dec 1936)

14 INTERNATIONAL PROJECTIONIST September 1936 high-grade microscope the minute sound track pattern is greatly magnified and projected through a large masking slit approximately 1 inch long and .00012 inch wide and into the photoelectric cell. Both the slit and the cell are enclosed in a compartment which Erpi asserts is dust-and-oil-free. It is apparent that this arrangement constitutes a rather long light path, its principal function apparently being to effect a reduction in ground noise. This head is adaptable to push-pull reproduction. The worth of this unusual film-scanning method cannot be evaluated by the writer at this time, only one experiment therewith having come to his attention, the results of which are unknown. Reports from the field relative to the practical application of this method to soundpicture projection will undoubtedly be received, however, and will be published in these columns. A self-aligning, grooved mounting plate couples the projector mechanism to the soundhead. Projector head replacements can be made in less than twenty minutes. Provisions are made for mounting with Simplex, Super-Simplex, Surefit, Motiograph and Powers projector Mechanisms. Both a. c. and d. c. motors start the sound and projector mechanisms gently and provide uniform vibrationless power. They are equipped with sealed ball bearings and may be interchanged in less than fifteen minutes. A self-contained oil supply provides complete automatic lubrication. I. P. readers are already familiar with the development of the cellular type loudspeaker,1 thus this exposition of the Mirrorphonic "Di-Phonic" speaker equipment (Fig. 8) will be limited to a review of the technical reasons which dictate the use of such speakers in any high-quality sound reproducing system. The Mirrorphonic speaker system departs from previous sound system design in both its electro-acoustic characteristics and physical appearance — yet it retains a time-proved feature: the principle of "exponential" horn taper. Exponential taper has been both practically and theoretically demonstrated to be the most effective horn design for radiating a "flat" or uniform field of acoustic energy from a vibrating diaphragm. The ordinary loud-speaker (either the horn or baffle type) becomes directional in its characteristics at the higher frequencies. Low-frequency sounds spread in all directions from the mouth of the horn; but the higher frequencies tend to concentrate into a beam which is projected directly ahead of the horn. The width of this beam becomes narrower and narrower as the frequency increases. Because of this fact, the audience never hears quite the proper blending of sound frequencies. Those patrons directly in front of the horn receive too great a proportion of the high frequencies; while those on the sides receive too much of the low frequencies. In addition to this separating effect, it has been found impractical to design a single loudspeaker with sufficient performance latitude to effectively respond to both extremely high and extremely low sound frequencies. FIGURE 7 Film-scanning system of the Erpi Heavy Duty soundhead which reverses the usual order. Light beam is transmitted from the celluloid side (back) to the emulsion side (front) of the film FIGURE 8 Di-Phonic speaker system, comprising one cellular h.f. unit and two l.f. units with bafflle. Overall depth 48". Speaker network in box for amplifier rack mounting The Mirrorphonic (Di-Phonic) system employs two separate "cellular" type radiators or "elements." One element, somewhat resembling a "baffle," is used for the frequencies from about 35 to 300 cycles; and another type, which has the outlines of a horn, is used for the range above 300 cycles. The new equipment is capable of yielding a power range of 100,000,000 to 1, and therefore can handle the entire volume range of a symphony orchestra. When speaking of the "range of loudness" which can be handled by an electrical system of reproduction, one is concerned with the differences between the loudest and faintest sounds which it can reproduce. There is, in addition, the problem of handling the peaks of maximum loudness. These peaks, in the case of music from a symphony orchestra, are beyond the possibilities of the ordinary loudspeaker to reproduce without distortion. Low-frequency sounds make the largest contribution to the peaks of sound power which must be handled to meet these conditions. The diaphragms of the Mirrorphonic low-frequency element are nearly 20 inches wide, and as many as four of these driving units are coupled to the low-frequency element. Both the high-frequency, or "multicellular" element, and the low-frequency element, which resembles a dish-type baffle, are divided into individual cells. Each cell of the high-frequency element is developed in accordance with the exponential taper principle; and although the low frequencies are less difficult to evenly radiate, a special taper development is applied to this element also. A special "network" of the low-loss (Continued on page 29)