International projectionist (Jan-Dec 1956)

always be followed. After the speakers have been connected and correctly phased electrically, sound should be played and the HF speakers moved toward and away from the screen until the best quality of sound is obtained in the auditorium. If it is found that the best position of the HF speakers is such that their trumpet mouths are more than a foot behind the plane of the LF baffles, reverse the HF voice-coil connections and move the speakers nearer the screen. A half wavelength in the crossover-frequency region ranges from 12 to about 21 inches, indicating the distance these speakers will have to be moved if the leads are reversed. If they are too far from the screen, annoying sound reflections may be produced. This procedure is absolutely necessary to bring about the most satisfactory acoustic phasing of the LF and HF units. As a final test, a recording of heavy male speech should be played. This type of sound is very rich in the frequencies included in the crossover range. If all is well, actors will seem to be speaking from the surface of the screen rather than from some point behind it, and medium-frequency sounds will be clean and clear. The HF horn-sled or cradle should then be fastened securely to the LF horn or speaker cabinet. Pick up all tools and screws, etc. which may be lying on the speaker cabinets. Loose objects will vibrate under the impetus of powerful sound waves and create rattles in the sound. Backstage Acoustics This done, attention should once again be directed to backstage acoustics. Even when the rear wall of the stage has been covered with stmndabsorbing material, draperies of black velour or heavy black velvet should be applied to the back of the screen, acoustically isolating the backstage region from the auditorium except for the horn mouths. The velour is draped loosely over the back of the speaker units, but preferably not touching them. This expedient improves the performance of the HF speakers, and any drapery at the back of the LF units is unnecessary except that it absorbs back radiation from the HF units. Lowfrequency sound waves not only penetrate many materials that effectively absorb high-frequency vibrations, but, being less directional, low-frequency waves go around corners with comparative ease. Satisfactory reproduction of the lower frequencies depends largely on the overall acoustic characteristics of the auditorium. At this point the sound-service engineer may wish to put the finishing touches on the tonal balance of the system, adjusting the frequency response by means of the degenerativefeedback network of the amplifier and listening to a series of pure tones reproduced from test films. But if the acoustics of the auditorium are unusually bad, final adjustment of the frequency response may be deferred until every possible effort has been made to kill troublesome echoes and reverberations. The auditorium, unless specifically designed and treated for the reproduction of sound pictures, may present almost insuperable difficulties to the installation engineer. It is not always possible to obtain uniform distribution of sound in many of the older theatres having balconies without at the same time allowing the poor acoustics of the theatre to ruin the naturalness and clarity of the reproduction. Old-time theatres designed for movies exclusively tended to be long, (Continued on page 32) Musical Tones and Acoustics THE FUNDAMENTAL frequency of a musical or voice tone usually determines the pitch, the harmonic frequencies provide the characteristic timbre, or "tone." A sound system capable of level response from about 30 to about 10,000 cycles per second will reproduce the higher harmonics and provide natural sound with lifelike brilliance. Attenuation or exaggeration of certain frequencies makes the sound unnatural by altering the relative intensities of the fundamental and harmonics. Meaning of Harmonics The keyboard shown in Fig. 1 reveals the musical pitch of the various harmonics when "C" is the fundamental. (The 7th harmonic is omitted because no keyboard note corresponds to it.) A sound having few harmonics is "dull;" one rich in harmonics is "brilliant." The 2nd, 4th, and 8th harmonics are called "consonant" because they are 1, 2, and 3 octaves, respectively, above the fundamental, and thus are consonant with "unison pitch." These harmonics result in a fluty quality of tone. The sub-3rd, sub-5th, 3rd, 5th, and 6th harmonics (various E's and G's on the keyboard) are called "dissonant," and predominate in reedy and stringlike tones. The sub-fundamental (the C below fundamental C) imparts a deep, resonant quality to tones. When the sub-fundamental is as strong as the fundamental, it assumes the character of a fundamental and lowers the pitch of the tone one octave. The theatre pipe-organ, mightiest of instruments, is noted for the range and variety of its tones. The harmonic con(Continued on page 34) FIGURE 1 EjjpffiuSlCAL TONES consist of a FUNDAMENTAL plus one or more HARMONICS : Fundamental + ^/\/\+ WW = /Vv/K/vWAxA Harmonics Musical tone For the Key of C the fundamental artd harmonics are-. mi i i i i i m c £ «i c 3 <+* I JO 3 -I < Q Z ij international PROJECTIONIST • FEBRUARY 1956 11