Projection engineering (Sept 1929-Nov 1930)

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Projection Engineering, May, 1930 Page 17 • t Architectural Acoustics Each Syllable or Note Should Die Away Before the Next Arrives — About One-Tenth of a Second Allowed. Historical Origin THE scientific study of architectural acoustics is a thing of comparatively recent years. In 1895 Harvard University had just completed the Fogg Art Museum, containing an auditorium which proved almost unusable. The corporation of the university appealed to the scientific staff of the faculty for advice and assistance in the matter, and Prof. W. C. Sabine undertook the study of the case. Two years were spent in the investigation of the questions involved, in the course of which experiments were made in a number of existing and satisfactory auditoriums. As a result, certain fundamental but previously unrecognized principles became clear, which later enabled Professor Sabine to predetermine the acoustic design of the new Boston Symphony Hall (1)\ These investigations of Sabine were the pioneer scientific work in the subject. So completely and carefully were they carried out that subsequent workers have done but little in the way of extending the theoretical foundations of the subject, and have for the most part merely enlarged our knowledge of the acoustic properties of the various materials commonly used in building construction. Usual Defects of Auditoriums The usual defects of auditoriums are three — echo, dead spots, and reverberation. In the usual sense of the term, echo means a definite or articulate repetition of a sound after an interval at least equal to the total duration of the sound that is being repeated, while reverberation means a confused or inarticulate prolongation of the sound. Echo is always a bad feature in a hall ; reverberation, on the other hand, is desirable up to a certain point ; only in excess is it an evil. Of the two, echo is the more difficult to remove ; prevention by foresight in construction, aided by expert advice, if necessary, is the best plan. Echo Echo arises by regular reflection of sound from smooth walls, ceilings, or proscenium arches just as a mirror The fundamental principles governing the construction of an acoustically successful auditorium are no longer new, but are not yet generally understood by those engaged in such work. In this article these principles are stated and an example is worked out showing their practical application to the planning of, a new auditorium or to the curative treatment of one that has proved to be unsatisfactory. t Prepared hy Paul R. Heyl, senior physicist, in charge of sound laboratory, Bureau of Standards Washington, D. C. 1 The figures given in parentheses here and throughout the text relate to the reference numbers in the bibliography given at the end of this paper. may reflect a beam of light without scattering it. If, however, the surface of the mirror be roughened the reflected light will be diffused in all directions ; and if the walls and ceilings of a room be similarly irregular (on a sufficiently large scale) the reflected sound will be scattered, broken up, and its definite or articulate character destroyed. In this case we have what is called reverberation. The lapse of time before an echo is heard is due to the fact that the reflected sound has traveled a longer path than the sound which comes directly from the source. This difference of path may be such as to cause much mischief. The reflected sound of a spoken syllable or of a note of music may arrive at the ear at the same moment as the succeeding syllable or note which has traveled by the direct path, and so cause hopeless confusion. Generally speaking, auditoriums are less likely to exhibit troublesome echo when their outlines are rectangular. An instructive case of the trouble that may be caused by curved walls is cited by Watson (2) in the case of the auditorium at the University of Illinois, with an approximately circular floor plan and a hemispherical dome. The best that could be done in the way of after correction of the acoustics of the room was only partly satisfactory. Watson regards the complete cure of such a room as hopeless without "surgical treatment ;" that is, straightening the walls. Smooth, hard-finished walls, such as the usual plastered type, are excellent regular reflectors of sound and are consequently likely to produce echo. It becomes of importance, therefore, to break up such surfaces so as to produce irregular distribution of the reflected sound. This is usually done by coffering in the case of ceilings. Examples of this may be seen in many theatres of modern construction. The ceiling and, perhaps, the proscenium arch are broken up into depressions about 4 feet square, containing a succession of steps totaling a depth of, perhaps, 8 or 10 inches. An irregular surface of this character breaks up the reflected sound and distributes it in such a way as to minimize echo, and, in fact, to convert it into reverberation. The dimensions which should be assigned to such coffering are not a matter of taste or accident. If the wavelength of the incident sound is very large compared to the size of the irregularities it encounters there will be little dispersive effect produced; and if very small, the smooth spaces inside the coffering may act as regular reflectors. The size mentioned, 4 feet in diameter, is a compromise between the average wavelength of the male and the female voice. Dead Spots and Sound Foci Dead spots and sound foci occur as a consequence of echo-producing conditions. Sound travels through the air as a wave of alternate compression and rarefaction, and if a reflected sound wave is retarded by the proper amount it may happen that the compression of the directly transmitted sound and the rarefaction of the reflected sound arrive at the ear at the same time, neutralizing each other's effect and producing a diminution in intensity. If the reflected sound is retarded a little more it may happen that two compressions coincide, producing an unusually loud sound. The most usual cause of such sound foci, however, is a curved wall or ceiling which concentrates the sound to a focus. Since dead spots and sound foci arise from the same cause as echo their removal may be brought about by the same treatment. Some care and experience is necessary in order to locate the particular portion of the room which is responsible for the production of a dead spot. Often this can be found only by a cut-and-try experiment, as it is not possible to predict the path of reflected sound with the same accuracy as in the case of light. The reflecting portion once found must be treated in such a way as to decrease its power of regular reflection. W. C. Sabine mentions a case of a theatre, the ceiling of which contained a flat oval panel, to which such trouble was traced. In this case an irregular canopy, oval in plan and slightly larger than the panel, was hung just below it with good effect.