International projectionist (Jan-Dec 1956)

and dialogue is frequently "fuzzy." In many cases these defects are not sufficiently pronounced to be diagnosed with certainty by anyone except an experienced sound engineer, but patrons sense that something is wrong. Reproducing Speech Now, when patrons say that they can't hear everything spoken on the screen, the projectionist, ultimate recipient of such complaints, quite naturally raises the volume level. But this expedient usually makes matters worse: speech becomes even less intelligible, and people begin to complain that the sound is too loud. We have already said something about audience noises and the effect these have on the required sound volume. The presence of an audience also affects sound reproduction in other ways. A nearly empty theatre is "boomier" and more reverberant than a full one, as well as quieter. As the size of the audience increases, the sound becomes "thinner" and more directional. These annoying transient effects are minimized in up-to-date theatres by the use of heavily upholstered seats. Hard-back seats reflect sound waves rather strongly; and the size and distribution of the audience is thus permitted to modify the quality of the sound in theatres having such outmoded, uncomfortable seating facilities. The walls and ceiling are more constant factors in the acoustics of a theatre auditorium than is the seating area. These are the areas that receive the lion's share of attention from acoustic engineers. Not only do plain walls of wood, plaster, or concrete reflect a large proportion of the sound energy they receive, causing the sound waves to bounce back and forth for a long time after the original sound has ceased, but they may even vibrate under the impact of sound waves, reinforcing certain frequencies and suppressing others. This alters the relative proportions of harmonic frequencies in tones, making the sound unnatural. Reverberation Troubles The acoustical effect of hard walls and ceilings may be appreciated by comparing the sound of a phonograph or radio played out of doors with one played in a large, plain enclosure like an empty garage or railroad-station waiting room. Out of doors there is no reverberation, no resonance, and, in the absence of nearby hills and buildings, no "slap," or echo. Only the source of the sound is heard. But in the large, barren chamber, the reflected waves nearly drown out the source with a barrage of confused noise. Speech loses intelligibility under these conditions because each word continues to be heard while succeeding words are being spoken. The acoustic conditions that prevail out of doors are said to be "dead," while the reverberant conditions of an echoing hall are termed "bright," or "live." Concert halls are usually made sufficiently live to impart to instrumental music the rich fullness preferred by most people. (Electronic organs, for instance, sound "flat" and colorless when the reverberation circuit is switched off.) Theatres intended for the showing of sound pictures, on the other hand, should be comparatively dead. A trace of liveness in theatre auditoriums is nevertheless by no means fatal to good sound reproduction. In fact, a certain amount of reverberation without selective frequency characteristics, and without a time-lag no FIG. 1. The design of en "exponential" highfrequency horn is intended to provide the most efficient reproduction of sound. The lowest frequency reproduced by such a speaker depends on the length of the horn, which acts both as a baffle and as a "projector" of sound waves. The term exponential refers to the mathematical equation determining the form of the horn. The width of the horn at any point (s, s') is a function of its length (d, d') operating as the exponent of a constant small number (n). ticeable to the average observer, serves to reinforce the sound in a very desirable way, masking directional characteristics and decreasing the amount of sound energy (amplifier output in watts) needed for adequate volume. To be unobjectionable, reverberation must decay at a uniform rate, the reflected sound dying away quickly and without echo. The back wall of an auditorium is usually responsible for any "slap," or distinct echo, that may be heard. In Older Theatres Older theatres require a great deal of attention soundwise because very few of them, no matter how painstakingly refurbished with sound-absorbing panels and draperies, are entirely free from acoustic defects capable of ruining even the best sound. Improper positioning of the speakers invariably brings out the worst in such theatres — back-wall slap, ceiling echo, side-wall resonance, etc. The installation of stereophonic sound systems in these theatres, especially if long and narrow in shape, has often resulted in dislocation of the speaker placement formerly used with acceptable results. Reproduction from the stereophonic system may not live up to expectations in acoustically unsatisfactory theatres. Acoustic treatment of the backstage area is extremely necessary to proper functioning of a sound system. We examine this area first when installing or replacing speaker units. No amount of padding and upholstering in the auditorium can overcome the bad effects of acoustic deficiencies backstage. Even though modern speaker units are designed to minimize the amount of sound radiated from the back of the speaker, a small amount of "leakage radiation" usually occurs and must be absorbed to prevent the reflection of sound waves from the rear wall of the stage. Sound reflected from this wall and thrown forward through the screen produces interference effects detected as distortion. Rock Wool Blankets Blankets of loosely packed rock wool are effective absorbers of medium and high-frequency sound even when applied directly to the back wall of the stage. For better absorption of the low frequencies, the rock wool is applied to wire mesh set out from the wall by furring strips. Where this can INTERNATIONAL PROJECTIONIST • FEBRUARY 1956