> > > >

1954-55 Theatre Catalog, 12th Edition, Page 265 (229)

1954-55 Theatre Catalog, 12th Edition
1954-55 Theatre Catalog
1954-55 Theatre Catalog, 12th Edition, Page 265
Page 265

1954-55 Theatre Catalog, 12th Edition, Page 265

ter line of the high frequency unit should be located between one-half and two-thirds of the picture height. In some cases, in order to obtain proper spacing, it has been expedient to leave off the outer wings, or baffles, of some of the current models of loudspeakers. Also, if it is necessary to have the low frequency sections a little closer than the specifica< tion, the high frequency units can usually be moved outward from the center line of the low frequency units. In the use of these expedients good judgment must prevail since, obviously, it is possible to overdo these compromises. Generally speaking, loudspeaker installations for certain recent releases have placed the speakers further apart than specified above and if this difference does not exceed 10 per cent between the ideal and the actual conditions, then no relocation of the speakers need be made for exhibiting the various products. The reader may wonder how sounds can be made to localize at or beyond the edge of the screen when there is no loudspeaker located in these regions but with true stereophonic sound this is readily done. Loudspeakers kept within the boundaries of the screen permit the recording engineer to provide close-up quality of sound whenever required. OEstage sounds are achieved by the natural perspective effects and the manner in which the relative loudnesses of the channels vary as the sound source moves laterally away from the picture scene.

For good performance of the high frequency unit, the horn mouth should be as close to the screen as possible and since there is a definite relationship between the high frequency and low frequency units for correct phasing at the crossover frequency, this usually demands that the low frequency unit also be placed close to the screen. Hence, with curved screens as recommended for CinemaScope, the low frequency units will usually be pointing slightly toward the center of the audience. Ideally, the high frequence units should be twisted around to point directly out rather than be fftoed in? and insofar as it is possible, covering the seats at the far side front and one the near side, but favoring the latter if a compromise must be made.

The customary check for speaker unit phasing should be performed.

After the initial adjustment according to the specifications, the sound distribution from each of the loudspeakers should be checked throughout the house and, if necessary, the high frequency unit twisted or tilted to obtain a suitable distribution. The final test on the loudspeaker placement consists of reproducing material from each of the loudspeakers in turn and checking throughout the house to see that the sound directivity is not affected by some peculiar acoustic situation. A circumstance which must be avoided is sometimes caused by peculiar acoustics whereby reflected sound occurs from the opposite wall, the result being, insofar as the audience is concerned, as though the sound originated from the opposite side of the screen than that which was intended. In general, it is better to use high frequency horns whose distribution just fits the house rather than have ex 1 954-55 THEATRE CATALOG



SHOWN against a blackboard diagram of a new 40mm CinemaScope lens is John D. Hayes of the Bausch 6. Lomb Company. He holds the first of the new lenses recently shipped to ZOth-Fooc. The 40mm lens is the first of a complete range of focal length up to 152mm, now under development.

cessive spread. If the loudspeakers must be occasionally moved, the proper locations should be marked or some other provision made so that they can be readily returned to the desired placement.

Loudspeaker Sys'l'ems-Surround (Audiforium)

When loudspeakers are installed upon the walls of a theatre to reproduce effects which are intended to surround, or encompass, the audience, great care must be exercised to assure that uniform distribution of signal is obtained, that no large part of the audience hears the effects from a definite source unless such intent is deliberate in the production, and that loudspeaker units are selected which will reproduce the effects with some reasonable fidelity.

Generally, and particularly in theatres with mezzanines or balconies, the surround loudspeakers must be placed close to the audience seated at the sides, and, because of this closeness, reproduce the audio signals as discrete sources rather than as a surround. This can only be corrected by using sufficient numbers of loudspeakers each having proportionately low input but having a total sound output power sufficient to fill the auditorium. This philosophy does not forbid the use of any particular group of side, or rear, speakers for extreme directional effects; each kind of effect merely determines the circuit design and amplifier requirements. It is suggested that the left side, right side and rear speakers be wired as separate groups, and each group brought to the amplifier rack as separate circuits. With this cire cuit arrangement all speakers can be driven simultaneously or changes readily made to drive them as independent groups with their own amplifiers. The events of the future will finally dictate the best arrangement and wiring as suggested herein will provide flexibility. With CinemaScope, as now foreseen, the

single effects signal will be fed to all the surround speakers simultaneously. It is quite feasible to uitilize certain control signals in the future to switch the effects to any particular group of surround speakers.

The reproduction of effects requires

a good frequency response and surround

speakers should be chosen which reproduce all significant frequencies reasonably well. Many excellent effects contain considerable amounts of low frequencies and if these are not reproduced the whole reason for the effect is lost. In many instances, loudspeakers have been selected for the surround which have too little low frequency response.

Ampliner System

The amplifiers used in each channel should be identical units and each power amplifier should have a power capacity equal to that demanded by the theatre from a single system such as now being used. This means of course that the total power available in the theatre will be three times, or 5 db, greater for stereophonic than for single channel. Since the trend is toward greater power in the theatre to enhance the reproduction of spectacular effects, this added power is gained automatically by the installation of stereophonic sound equipment. The reader may wonder why the power capacity of the individual channels should be capable of filling the house but will recognize that this is true if he will consider that there are times when effects are principally reproduced by only one of the three channels.

Signal and Control for

Surround Effects

Track No. 4 (Effects and/or Control) of the CinemaScope composite release film is intended to furnish the signals for the surround effects. Whenever the theatre is showing a picture which provides surround effects, the track and
1954-55 Theatre Catalog, 12th Edition, Page 265