International projectionist (Jan-Dec 1956)

not be done, two or three thicknesses of heavy velour-type material, preferably black to prevent light reflections from appearing through the perforations of the screen, may be hung in front of the rock-wool padding. The velour should be very loosely draped and separated from the rock wool by an inch or two. The exact distance from the rear wall to the speaker baffle appears to be unimportant, small changes in this distance having no appreciable effect upon sound quality. The reason is obvious : the frequency range of 50 to 10,000 cycles per second includes sound waves of from 200 feet to 1 1/5 inches in length. It is recommended, however, that the distance from the baffle to the rear wall be somewhat greater than the wavelength of sound at the crossover frequency where both high-frequency and low-frequency units emit about the same volume. For a crossover frequency of 400 cycles, this wavelength is 2% feet. A baflleto-wall separation of from 4 to 7 feet is satisfactory. Assuming that the rear stage wall has been effectively treated with soundabsorbing material, the next step, even before the speaker cabinets are draped, is to make sure that the different speakers have been correctly phased, and that each unit is positioned to minimize the acoustical defects of the auditorium while obtaining the most uniform distribution of sound. The low-frequency speakers should be connected in parallel, and all should be in phase electrically with respect to each other. That is, the diaphragms of all the LF speakers should move in and out together when energized by the fluctuating currents passing through the voice coils. If these units are PM (permanent-magnet) speakers, there are only voice-coil connections to be considered — two terminals for each speaker. EM (electromagnet) speakers have two sets of connections (four terminals), one set for the voice coil and one for the field magnets. The field-coil terminals are usually coded by -) and — symbols, making it easy to "pole" the fields of all LF speakers. Test Voice-Coil Terminals Voice-coil terminals, even when number or color-coded, should be tested to make certain that all connections have been correctly made. If the diaphragms of two speakers move in opposite directions, the sound waves they emit will be 180 degrees out of phase, and most of the volume will be lost in the area where the waves from the two speakers overlap. Testing the polarity of the voice-coil connections is easy wherever it is possible to reach the diaphragms of the LF speakers from the front of the units. Connect a sensitive DC voltmeter to the low-frequency output leads of the crossover network and depress the diaphragm of each LF speaker by hand, noting the direction in which the voltmeter needle swings. Deflection of the needle should be in the same direction for each unit. A "listening test" may, be used if it is not possible to reach the front of the speakers. Disconnect the highfrequency units and play sound over the system. (Only the bass tones will be heard, of course.) Listen to the reproduced sound while walking from one side of the auditorium to the other, preferably down front near the stage. A "dead spot" of low volume between the two LF speakers indicates an outof-phase relationship. Reverse the voice-coil connections of only one of the speakers and repeat the test to find out if the dead spot has disappeared. The high-frequency units must also be in phase with one another; and to test the polarity a voltmeter is connected to the high-frequency output of the crossover network. A sharp blow of the palm of the hand on the orifice of each HF speaker (trumpet removed) serves to depress the dia phragm sufficiently to obtain a deflection of the voltmeter needle. The phase relationship between the LF and HF units depends on the type of HF speakers used and upon their position relative to the LF units. The sound waves emitted by the HF speakers are so short — from 3 feet to about an inch in length — that any change in the distance of a HF speaker toward or away from the plane of the LFspeaker baffle affects the acoustic phasing. Incorrect phasing of the HF speakers is noticeable to the ear as a loss of volume and sound quality in the crossover range of frequencies (350550 cycles in most systems). These are the frequencies reproduced by both the LF and HF units, all lower frequencies being emitted by the LF speakers alone, and all high frequencies by the HF speakers alone. In phasing HF speakers we are concerned only with the crossover frequencies, for phase relationships have no effect on the sound of tones consisting of a low "fundamental" and a number of higher "harmonics." Instructions Differ Some manufacturers of theatre sound equipment instruct that the HF units should be in phase electrically with the LF units; others state specifically that the two units should be electrically 180 degrees out of phase. The manufacturer's instructions should PLANNERS OF INTERNATIONAL PROJECTION EQUIPMENT SHOW The boards of directors of Theatre Equipment and Supply Manufacturers Association and Theatre Owners of America lunched together at the Sheraton Astor Hotel in New York City on January 10th to discuss plans for the International Trade Show, Fair and Exposition to take place next September at the New York Coliseum. Those present, left to right (seated): Larry Davee, M. H. Stevens, TESMA; Herman Levy, TOA general counsel; Walter Reade, Jr., former TOA president; William A. Gedris, TESMA; Lee Jones, TESMA president; Myron Blank, TOA president; Merlin Lewis, TESMA executive secretary; Horace Denning, TOA vice-pres Robert Livingston, TOA secretary; Joe Alterman, TOA. Standing, left to right: Carl Anderson; Albert Pickus, Pat McGee, and Julian Brylawski, TOA vice-presidents; George Gaughan, TOA field representative; Ernest Stellings and George Kerasotes, assistants to the TOA president; Tom La Vezzi, TESMA vice-pres.; V. J. Nolan and Joe Fetherston, TESMA. 10 INTERNATIONAL PROJECTIONIST • FEBRUARY 1956