> > > >

1945 Theatre Catalog, 4th Edition, Page 346 (322)

1945 Theatre Catalog, 4th Edition
1945 Theatre Catalog
1945 Theatre Catalog, 4th Edition, Page 346
Page 346

1945 Theatre Catalog, 4th Edition, Page 346

bath, to insure smooth operation and long life.

The medium used to carry the sound signal from the soundhead to the amplifier is also important. Either low-capacity cable coupling or transformer link coupling, both standard methods, may be used. When low-capacity cable is used, its capacity must be kept at an extremely low value, which is done by making it short and of a length calculated to provide the maximum transfer of power from photocell to the amplifier. If transformer link coupling is used, the photo cell transformer in the sound-head and the input transformer in the amplifier must be matched to each other very carefully, or distortion and loss of sound power will result.


The most flexible components in the whole sound system are the amplifier, or amplifiers, yet they are nonetheless critical in design. Their flexibility allows easy adjustment of sound quality to suit the particular auditorium in which the sound is delivered, and enables the designer to compensate for the fact that no known loudspeaker system can reproduce all sound frequencies with equal loudness when receiving a constant amount of power.

RCA amplifiers are matched to the loudspeakers with which they are used, and by designing the amplifier to give increased output at frequencies at which speaker efiiciency is low, sound of approximately equal loudness at all frequencies is reproduced. Proved in wartime equipment, and incorporated in all RCA theatre sound systems now being manufactured, are high-fidelity amplifiers using inverse-feedback circuits which stabilize their amplification and virtually eliminate amplifier distortion.

T0 permit high output from an amplifier, special types of tubes were formerly


necessary, but because of the great variety of highly efiicient tubes available today, that condition no longer prevails. Todayls amplifier gives better quality and higher power, while using tubes which are obtainable from any good radio store. Tube replacement is thereby greatly facilitated, and at nominal cost.


Loudspeakers, the remaining portion of a sound system, have been the subject of many major changes since the advent of sound in theatres. The latest improvement is the replacement of the heavy, bulky, power-consuming loudspeaker field magnet by a permanent magnet made of an alloy of aluminum, nickel, cobalt, and iron. This material, developed just before the War, and improved during it, has proved itself sufficiently strong and permanent to replace the wire-wound fields previously used, while at the same time making possible increased speaker efiiciency.

Two resulting features, which are attractive because of their cost-saving nature, are the continual saving of electric power, and lower installation expense. This double saving arises from fact that it will no longer be necessary to install the two or more wires required from the booth to the stage for the older type speaker fields. In addition two possible sources of trouble are avoided by the elimination of the speaker field winding and its associated power supply unit.

Two-way speaker systems have been standard equipment in all high-quality motion picture sound reproducing systems for some years. RCA two-way systems are matched to provide smooth, balanced sound. No power-wasting resistor networks are used.

Background noise is minimized by careful design of the high-frequency units,

which incorporate a mechanical ffcutoff filter" to eliminate film scratch, amplifier hiss, and other very high-frequency noises above the recorded sound range. Their inherent full, yet mellow, tone brings out the richness in any recording without emphasizing any defects of pos sible poor recordings. Their 400-cycle crossover frequency permits crisp, clean speech reproduction without noticeable shifting between the high- and low-frequency speakers.

The speaker units themselves are designed to permit field replacement of cones: they do not need to be returned to the factory for repair. Clearances between cone supports, cones, voice coils, and air caps are ample to permit longcontinued operation in the presence of inevitable backstage dust. Furthermore, large amounts of power, such as occur in battle scenes, explocions, etc., cannot cause the voice coil or cones to rub or distort. I

The speakers are fed from the amplie fier by a 250-0hm line. The crossover network, and the line itself is inherently low-loss.

Three highafrequency horns are available, permitting a proper match of sound distribution angle in any theatre, whether wide or narrow. Thus, all parts of the theatre are adequately covered with sound, without loss to the side Walls.

The low-frequency baffle, with its scientifically correct, slow rate of taper, and its enormous effective mouth area, reproduces even the lowest tones with remarkable clarity and efficiency.

These various units are assembled into six different sizes of speaker assemblies, to match the auditorium of any theatre, whether large, small, wide, or narrow.

The position of the high-frequency and low-frequency speakers in relation to each other must be such that the sounds which come from both will be ffin step" and free from overlap and fuzziness. To minimize loss of sound, the high-fre THEATRE CATALOG-1945

Amps vru . ,.
1945 Theatre Catalog, 4th Edition, Page 346