Projection engineering (Sept 1929-Nov 1930)

Page 16 Projection Engineering, September, 1929 clothes or hangings. Such a condition is easily recognized. If the investigator will sit in various parts of the house and listen to the variations in sound energy rather than to the actual words or music he will very soon be able to detect any marked changes in the loudness of the sound. Then if the area in question shows attenuation so great as to strain the hearing, we have conclusive evidence that the energy distribution is faulty. For example, if one walks down the aisle from the stage and beneath the balcony of the average theatre a distinct reduction in intensity will be noted. If this phenomenon is coupled with complaints against the hearing conditions under the balcony the case is almost complete. To be sure, in almost any auditorium the energy shows a marked reduction toward the rear of the house. The point to observe is whether this reduction is so great that the ear begins to have a sensation of strain, such as is felt when listening to a person a long distance away. These are the major acoustical problems and the methods which can be used to analyze them. Once the analysis has been made more than half the battle is won, for then the responsibility for correction can be placed. The Remedies If the problem is one for the acoustical companies it will be desirable first to have the preliminary conclusions checked by a thorough analysis, and where necessary, by actual tests in the house. In general, however, a complete set of plans furnishes all the information required for the analysis. If the problem is found to be reverberation the solution is generally simple — requiring only the installation of a certain amount of absorbent material. This material may take the form of draperies, carpets, etc., or it may be supplied by various commercial acoustical materials. From a standpoint of A sound wave photograph of the reflections fron? the ceiling in a model section of a Chicago theatre. sound absorbing efficiency those materials which are specially designed for the purpose are far cheaper. Sound absorption should always be considered in terms of the cost per absorption unit unless the installation must also fulfill some special requirement of design or utility. Permanence and maintenance cost are also important, as also are such considerations as fire hazard and ease of decoration. Problems involving echo and socalled "dead spots" generally have a similar solution, but the material must be installed in the proper areas, else the correction is useless. To this end it is important to have a competent engineer check the analysis, since the determination of echoes is largely a matter of experience. Echoes should, of course, be detected and corrected during the process of design, but it is often possible to remedy the difficulty even after the theatre has been built. If the survey indicates the presence of extraneous sound, the discovery of A typical sound wave photograph showing the reflections from the walls In a model plan of a Chicago theatre. the cause generally indicates the remedy also. Almost always the remedy is suggested by common sense; there are really no general rules that can be set down. Wherever possible the sound should be destroyed at its source, or if this is not possible barriers in the form of doors, curtains, vestibules, etc., should be erected. If direct contact with the theatre is essential, as in the case of a ventilating system, the application of sound absorbent material around the source is an effective cure. Energy distribution and kindred problems must in general be turned over to the acoustical engineer for solution. No single formula applies, and sometimes radical changes are necessary. Sometimes changes in the position or power of the horns will ameliorate the condition. Sometimes changes in furnishings and draperies tend to improve matters. Occasionally the construction of suitable reinforcing or equalizing surfaces has provided the solution. But in general each new case calls for a different treatment. The point which it is wished to emphasize, however, is that in all cases a brief preliminary survey on the part of the owner or manager is desirable and possible. The simple tests described require no previous experience, and ask no more of the ear than it is equipped to give. The handicap under which the science of acoustics has always labored is the universal tendency to place it in the realm of the unknowable, if not the supernatural. But after all it has been developed solely to assist the human ear out of its various difficulties, and what more natural than to ask the ear to do its share of the work? Once the layman realizes that he carries with him always two of the most delicate acoustical instruments ever devised he will discard the compound of abstractions and superstitions" which has been bewildering him and will begin to offer the intelligent criticism which is so badly needed.