Projection engineering (Sept 1929-Nov 1930)

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Page U Projection Engineering, September, 1929 Acoustics of Motion Picture Theatres Practical Information Pertaining to the Testing of Sound Conditions and Remedies for the Most Common Faults By John S. Parkinson* SO much unintelligible or misleading information on the subject of acoustics, has been directed at the theatre owners and managers since the inception of the talking movie that it seems desirable at this time to try to put the subject on some sort of a practical basis. The average theatre owner is not interested in abstract discussions of the effects of frequency and phase distortion, nor does he care to go into the refinements of the reverberation theory. Absolutely all he wants to know is whether or not his customers can hear comfortably. If they cannot, he wants to know how to place the blame, and what steps to take. In view of the fact that his needs are so simple, it seems almost strange that the various firms interested in the subject have not succeeded better in satisfying them. The answer lies in the fact that the talking movie arrived so suddenly that no one was prepared for it. Hundreds of theatres were confronted with problems outside of their experience, and a frantic cry arose for anyone who could offer assistance. For a time the situation amounted almost to hysteria ; many owners were stampeded into believing all sorts of impossible things about their houses. Now, however, the first rush of excitement has passed and there is time to give the matter a little coherent thought. As a matter SPECIAL ARTICLES BY CAMERON J-4.2kT.EiSf R. CAMERON, a member of our editorial staff, has begun the preparation of a series of special articles, to run exclusively in Projection Engineering. The first of the series of articles will appear in the forthcoming issue. Mr. Cameron is well known in the motion picture field, both personally and by the acknowledgment received through the numerous technical text books he has written, the most notable of which are, "Motion Picture Projection" and "Motion Pictures With Sound." Mr. Cameron's articles xoill deal with the technical and semitechnical considerations of both light and sound projection as they are specifically applied to the art of motion pictures. — Editor. * Staff Acoustical Engineer, Johns-Manville Corp. of fact a comparatively small percentage of theatres need acoustical treatment. In a much larger percentage flaws are apparent, but very often these flaws may be remedied by a few simple changes in apparatus or the furnishings. The fact remains, of course, that many theatres really do require expert advice and treatment, but the theatre owner or manager A view of the Fox Theatre in Detroit showing the areas on the ceiling where acoustical material was installed. should be able to determine himself whether this is the case. It is hoped that the data contained in the following paragraphs will facilitate this decision by describing a few simple experiments which any person can perform. Reverberation In order to understand the significance of these tests it will be necessary to review briefly the acoustical theory as it applies in each case. The most common acoustical defect is excessive reverberation. This produces an overlapping of successive syllables and musical sounds with an attendant loss ini intelligibility. The time of reverberation depends on the sound absorptive qualities of the interior surfaces of the room. It is influenced by carpets, upholstered seats, draperies, etc. These materials all increase the rate at which the sound decays, and consequently help to reduce the length of time during which reverberation lasts. Nearly every one has experienced the effect of excessive reverberation. A familiar case is the sound of one's steps reverberating in a long bare corridor. Another is that hollow lingering quality which one's words assume when talking into a well or cistern. The opposite effect is obtained in a library or in the rug department of a large store. The difference is obvious, and will convey a fairly accurate idea of how much good hearing conditions depend on the control of reverberation. In a theatre this effect is most noticeable at the end of a sentence, or after an abrupt pause in the dialogue. The sound continues to be heard for two or three seconds, sometimes apparently in far corners of the house. This phenomenon must not be confused with the same defect in the apparatus. Sometimes too much reverberation is allowed in the recording, but this can always be detected because the residual sound comes from the horn rather than from the body of the house. Reverberation in the recorded sound causes the sensation that the horn is "singing." Reverberation almost always makes itself felt equally in all parts of the house. Immediately in front of the stage it may be drowned out by the blare of the speakers, but elsewhere it causes an almost uniform reduction in intelligibility. By virtue of this fact it can often be detected, for it is the only acoustical defect except certain types of extraneous sound which annoys all the patrons equally. Here again we must except the defects in reproduction ; these of course will be equally annoying to everyone.