Showmen's Trade Review (Jan-Mar 1947)

E 24 SHOWMEN'S TRADE REVIEW, February 1, 1947 A Neat FOREST ELECTRONIC CONTROL PROJECTION LAMP A new and highly acclaimed development' — ELECTRONIC CONTROL — makes present mechanical systems of feeding the positive and negative carbons in projection arc lamps oldfashioned and obsolete; assures faultless arc control of precision accuracy, maintaining the proper focal point necessary for producing maximum light intensity without the necessity of constant watching and adjusting. Eliminates cams, gears, clutches and associated gadgets. MFG. CORP. 60 Park PI., Newark 2, N. J. The Modern Means of Converting A. C. to D. C. as a stable power supply for projection arc lamps STRONG COPPER OXIDE AND TUBE-TYPE RECTIFIERS Low original, operating and main> tenance cost. Quiet operation. Distributed through leading Independent Theatre Supply Dealers. The Strong Electric Corp. 87 City Park Avenue TOLEDO 2, OHIO The World's Largest Manufacturers of Projection Arc Lamps The NEW PROJECTOR Model "A" I PROJECTOR |""Jl^gftar^iau,.... n — DISASTROUS FILM FIRES — DON'T JUST HAPPEN! REEL BAND O PREVENTS FILM DAMAGE O PREVENTS FILM FIRE PRO-TEX REEL BAND CO., 200 Film Bldg., Cleveland, 0. EVERY WAY The New 1947 35mm Projector is Here Today! An achievement of the utmost in modern scientific engineering! The very finest in projection efficiency. Write, wire or cable for full particulars BLUE SEAL CINE DEVICES, Inc. 137-74 Northern Blvd., Flushing, L. I., N. Y. Cable: "SOUNDFILM" SOUND REINFORCEMENT (Continued from Page E-12) ters" — the one at top center is the final master that governs the overall volume of the mixed sound just before it goes on to the amplifier. The mixer input inpedances must match those of the microphones or other sound sources connected to it; its output impedance must match the input impedance of the amplifier. All components of a sound reinforcing system must be suitably matched as to impedance, power levels and frequency characteristics. Equipment components cannot be thrown together at random. A typical Altec Lansing amplifier is a four stage unit with 90 decibels of gain, 15 watts output, frequency response flat within plus or minus one db from 20 to 20,000 cycles, and with not more than two per cent total harmonics at full power output. Another typical amplifier is a onestage, push-pull high power unit (75 watts output) with a gain of 15 db, response flat within one db from 20 to 20,000 cycles and with less than two per cent total harmonics at full power output. Both may be considered specific examples of audio amplifiers designed and built from the beginning for sound reinforcement of the highest available quality. Altec reinforcement speakers are of the two-way type, having both high frequency and low frequency units built into a single reproducing assembly. Crossover — the frequency at which highs and lows are separated electrically by a network — is 2,000 cycles. This type of speaker again exemplifies the importance of the highest possible sound quality, and the lengths to which manufacturers go to provide it. No single diaphragm can handle both high and low frequencies with complete fidelity. When all frequencies are reproduced by the same vibrating cone, intermodulation distortion results. This fault is minimized by using two separate cones or diaphragms, each operating only in its own range of frequencies. Effective distribution of the higher frequencies is aided by a diffusing multicellular horn mounted in front of the hf diaphragm. In many cases, the large theatre type multicellular horns equipped with speakers are used to supplement this equipment. In some theatres the cost of a public address installation is minimized by utilizing the regular sound amplifier. The public address equipment is thereby reduced in number of items needed, and consists only of microphones or record players, mixer if needed, and speakers. This arrangement is economical, but it has certain disadvantages, and it is noteworthy that Showmen's Trade Review's Projection Advisory Council (see STR for August, 1946, Page E-26) decided against that practice. As can be seen from the preceding paragraphs, the problems involved in engineering and installing sound reinforcement systems are somewhat complex and cannot possibly be covered in a single article of this nature. However, in a series of articles to follow, dealing with certain specific types of installations, it is hoped to cover all the various angles of this very important subject. (#5).