The international photographer (Jan-Dec 1939)

)ROJECTION SyMpOSIUM, pART VI. atest technical data on theatre projection sound systems features speaker system and general theatre acoustics, escribed in series of articles by outstanding engineers of RCA Manufacturing Co. Inc., Hollywood service staff. iE problem of installing a given speaker sysm in theatres of different design and acoustic operties with the idea of having a similar qualf of reproduction in all theatres requires conJerable work of one nature or another. There e many items causing variation in reproduction theatres having identical reproducing systems. )me of these are: Theatre acoustics, stage oustics, size and shape of auditorium and stage, cation of speaker system on stage and level of e reproduced sound. These items will be exained more in detail in the following paraaphs. During the early days of sound motion pictures ry little was known of the requirements necesry for a theatre type loudspeaker. It was genally assumed that a speaker system that gave >od music reproduction in a theatre was satisctory. However, this was not necessarily true hen dialogue reproduction was taken into ac>unt. The first RCA Photophone speaker sysms consisted of groups of 12-inch cone units ounted around a non-porous picture screen, his was found to give good music reproduction, it the speech intelligibility and illustion were eking. A further improvement was made by ng several cone type units mounted in flat jffles and placed back of a porous screen. It ight be mentioned here that the early type of >und screens would be found very unsatisfac>ry today. This improved speaker system did ive some better results, but there was a certain ement lacking which indicated itself by the ict that a given sound system reproducing a iven film would sound satisfactory in one there and not in another. This was largely due to le difference in acoustic properties of the thetres and the fact that the speaker systems did ot have a satisfactory directional characteristic, urther improvements were made by adding a irectional baffle to the speaker units and this nproved the distribution and speech reproduc. on to a great degree. By this time theatre acouscs began to receive some recognition and acousc surveys were made which indicated that a high ercentage of theatres needed treatment. After tie use of the directional baffle speaker for sevral years the requisites for a satisfactory speakr system for theatre work became more apparnt. The major requirements were as follows: (1) Suitable frequency response. (2) Adequate power handling capacity. <3) Units with different angular coverages for various size theatres. (4) Speaker system to have such dimensions that will allow its use in the average heatre. (5) Efficiency. With these ideas in mind the present two-way peaker systems were developed. RCA Tivo-Way Speaker System The standard RCA Two-Way Speaker System onsists of a high frequency baffle with two high requency units, one or two low frequency baffles ind units, a flat baffle extension and a dividing letwork and matching transformers. Figure 1 hows the assembly of such a speaker system. The high frequency baffle is a metal cellular ;orn having from nine to eighteen cells, derending upon the angular coverage desired. Each laffle is powered by two high frequency units. Msure 2 shows two views of the Ml-1464 nineell baffle. This baffle has a coverage of 52 legrees in the vertical and horizontal plane. The iccompanying table gives the number of cells md angular coverage of the higher frequency >affles. The RCA high frequency unit is a non-metallic By WATSON JONES cone type speaker capable of handling high power and not susceptible to damage due to impact type of sounds having peaks of high order. Due to the rugged type of construction of this unit it is possible for the field service engineers to make repairs in the field where necessary. The low frequency baffle known as MI-1456 is a folded type of horn capable of reproducing the lowest frequency necessary for theatre work. This baffle is so constructed that its depth is only 30 inches, a desirable feature where a minimum of space is available back of the picture screen, and is powered by two low frequency units. Generally the two-way speaker system is installed with two MI-1456 low frequency baffles and an MI-1459 flat baffle extension which provides a mounting for the two baffles and also extends the extreme low frequency response of the speaker system. A block schematic of a standard two-way speaker setup is shown in Figure 3. In any twoway speaker system the frequencies below a certain value are fed to one set of speakers and frequencies above that value are fed to another set of speakers; this is accomplished by means of a dividing network. In the RCA system the MI-1483A network is generally set for a 300 cycle crossover. The matching transformers shown in Figure 3 are for matching the 15 ohm network output to any desired number of speakers. In setting up a standard theatre speaker system there are certain general rules which should be followed. i.4 very important factor in this connection is to have an approved sound screen so that the loss of high frequencies and definition is as low as possible. The speaker should generally be mounted as close to the screen as possible. The speaker assembly should be mounted in such a way that the center of the high frequency baffle is two thirds of the picture height. Due to differences in acoustic properties between theatres, some change in the electrical characteristic of the amplifying system supplying power to the speakers is sometimes necessary. ISoise Level in Theatre Auditoriums The noise level in theatre auditoriums has much to do with the resultant sound obtained in theatres. Volume setting of the reproducing system in the theatre is generally adjusted to give satisfactory loudness on normal dialogue. When a recording which has an excessive dialogue volume range is played under such a condition the low level passages will be below the noise level in the theatre and therefore unintelligible. If the reproducing system volume setting is increased to get intelligibility on the low level passages, the high level passages become too loud and theatre managements invariably object to such a condition. This latter condition also tends to decrease intelligibility in theatres with poor acoustic properties due to increased echo and reverberation effects. There are a number of sources of noise in theatre auditoriums, such as ventilating and heating systems, audience noise, outside noises such as street traffic, and projection room noise. It is well worthwhile to eliminate as much noise in the theatre as possible since there is a certain amount of noise which cannot be controlled and these uncontrollable sources of noise alone generally keep the noise level up to the maximum point for satisfactory reproduction of high volume range recordings. Ventilating and heating system noises are usually of a low frequency nature and are caused by the fan and motor vibrations being transmitted to the air ducts and building proper. This can be corrected by proper mounting of motor and fan units on cushioned mountings, balancing motor armature and fan propellers and using a flexible type of connection between fan and air ducts. In some cases it may be necessary to line the inside of the air ducts nearest the fan with absorbent material. Audience noise is something that goes with theatre operation and although its effect is somewhat limited by the amount of absorption in the thearte, it is still a major item in the overall theatre noise level. Street traffic noises are generally of a low frequency nature or rumble and are due to vibration of the building or sidewalks and streets. The traffic noise entering through doors and other openings can be almost entirely eliminated by proper acoustic treatment and type of construction. Traffic noise is generally much higher in theatres during the summer season due to open doors and windows. This type of noise is very objectionable due to its erratic nature. By the use of suitable double doors and heavy velour draperies in auditorium openings the traffic noise can be decreased to a satisfactory level. This of course requires some type of ventilating and cooling system for summer use. Projection room noise in the auditorium is something that the majority of theatres seem to put up with, and it is something that can be corrected at a small cost. The reasons we may have put up with this condition for so long are that the projectionist is always in the projection room and seldom gets out to check the sound in the auditorium, and the managers on the other hand become accustomed to the noise and do not notice it. By the use of approved cushioned mountings for projectors, double-pane glass in projection and observation ports, and acoustic insulation of the front projection wall, the noise can be reduced to a satisfactory level. Acoustics The study of acoustics and its application to rooms or auditoriums received little attention until recent years when radio broadcasting and sound motion pictures became of importance. Since that time many methods and devices have been developed to further the study of acoustics and make measurements a relatively simple matter. When sound motion picture reproducing equipment was first used in theatres it was noticed that some theatres had good sound and others poor sound. This can now be largely accounted for by a proper acoustic analysis of the theatres in question. Such a condition can be corrected by proper application of acoustic materials. This procedure is generally costly and theatre managements usually object to an ex. MI-1464 9-Cell 52% Degrees Horizontal 52% Degrees Vertical MI-1465 12-Cell 70 Degrees Horizontal 52% Degrees Vertical MI-1466 15-Cell 87% Degrees Horizontal 52% Degrees Vertical MI-1467 18-Cell 105 Degrees Horizontal 52% Degrees Vertical MI-1471 10-Cell 87' •'. Degrees Horizontal 35 Degrees Vertical MI-1472 12-Cell 105 Degrees Horizontal 35 Degrees Vertical International Photographer for April, 1939 13