Movie Makers (Jan-Dec 1948)

201 Choosing a sound projector Amplifier? Loudspeaker? Tone quality? A sound research engineer discusses these and other important features JOHN R. HEFELE, ACL IN THE survey of sound projectors, there are specifications of amplifier power output, loudspeakers, frequency range, etc., which are expressed in terms which may or may not be strange to the average movie maker. While each manufacturer has engineered and correlated his particular sound equipment to be adequate for the purpose or for the size of audience intended, an intelligent selection of the sound system by an amateur must be dictated by his requirements and of course by the size of his pocketbook. The larger audience will require a larger room, a larger sized picture, a more powerful sound system to produce an acceptable sound level. For home or living room use, where most amateur pictures are projected, sound quality rather than sound quantity may be of greater importance. FUNCTION OF SOUND PROJECTOR Essentially, the function of the sound portion of a projector is to reproduce audibly the contents of the sound track which is printed alongside the picture frames of a sound film. The operation is initiated by a separate projection system which projects a tiny beam of light through the serrations of the sound track onto a photocell. The efficiency of this projection system is conditioned by all of the same factors as the picture projection system — lamp brilliance, lens speed, accuracy of alignment, adaptability to adjustment and cleaning. The sound track must be run through this light beam at constant speed, and several different methods are used to convert the film motion from the intermittent one required to project the picture to the constant one required in the sound gate. The photocell translates the variations of light intensity created by the sound track into electrical current variations, which are then amplified by a vacuum tube amplifier and impressed onto a loudspeaker to reproduce to an audience the sound accompanying the pictures. SOUND POWER OUTPUT The volume of sound needed for satisfactory reproduction of speech or music depends upon several factors besides the size of the room. The most important of these is the amount of sound absorbing material present. A room in which walls, ceiling and floor are of hard materials, such as wood or plaster, requires comparatively little sound energy from the loudspeaker to produce a loud effect. But sound heard under these conditions does not have the clearness or the pleasing quality that is obtained in a room where there are curtains, carpets or other materials — including the clothing of the audience — that absorb sound. Beginning on page 202 of this issue, Movie Makers presents a survey of 16mm. projectors currently being offered the amateur and the nontheatrical projectionist. There are twenty seven of these instruments — eight of them silent, nineteen sound on film. For an evaluation of the 16mm. silent projectors, the new filmer should refer to Choosing Your Projector, in the April Movie Makers. Similar guidance in choosing a sound projector is presented here, by an expert in the field. — The Editors. Fortunately, the sound output can be conveniently adjusted by the volume control on the projector, so that all that is necessary is to make sure that the projector selected has adequate maximum power for good music reproduction in the room or rooms where it is to be used. In a general way, it may be said that a power output of 5 to 10 electrical watts will be sufficient for almost any normal sized living room, or a small meeting room with a maximum seating capacity of 30 to 40 persons. For larger audiences, say from 100 to 300 persons, at least 15 electrical watts will be needed — and more should be available. For auditoriums, power amplifiers with correspondingly heavy speakers will be required. These are generally available with arc projectors. THE LOUDSPEAKER Measurement of sound power by electrical watts is not an entirely satisfactory procedure, because loudspeakers differ considerably in the efficiency with which they convert electrical energy into sound energy. Unfortunately, the conversion efficiency of a loudspeaker is exceedingly difficult to measure. For this reason, all that can be done is to make certain that the projector selected, in combination with its particular loudspeaker, is capable of producing adequate loudness before it overloads; that is, before it reaches the point at which further increase of loudness causes the sound to be just noticeably distorted. The demand for small size and lightness of weight — with increased portability — has led to certain practices in the design of sound reproducing amplifier and loudspeaker that are deplored by most of the quality-conscious engineers in the industry. A loudspeaker, to be efficient and moderately free from distortion, is necessarily large and requires a moderately large magnet to produce a strong magnetic field around the coil through which the signal currents are passed. Reducing the weight of the loudspeaker necessarily means reducing the size of this magnet, thereby reducing the strength of the magnetic field. Now, if the same amount of sound is to be produced as before, the amplifier must deliver more electrical power to the loudspeaker having the weaker field. This can be done only by employing types of vacuum tubes that produce more distortion than the types in common use a few years ago when loudspeakers with massive field magnets were the rule. Thus, because of the need for high amplifier power to offset loudspeaker inefficiency, the amplifier supplies a slightlv distorted signal to the speaker which, because of its low efficiency, introduces still more distortion of its own. [Continued on page 208]