Sound motion pictures : from the laboratory to their presentation (1929)

ACOUSTICS 103 The limits given in the table are not to be regarded as rigid. Some auditoriums are known to exceed these limits in either direction by several tenths of a second, and yet are of fairly satisfactory quality. However, in planning a new auditorium it should be the aim to strike the average of the range which the table provides. The varying absorptive powers of different materials make an interesting study. The most complete absorber known is an open window! It is theoretically possible that a small amount of sound may be sent back by diffraction from the edges of the window, but this quantity is so small that it is permissible to say that an open window is a perfect absorber. The next most perfect absorber of sound is probably hair felt, which consumes perhaps half as much sound as an equal area of open window. In other words, it may be said that since an open window absorbs (or transmits) all the sound that falls upon it, its coefficient of absorption is unity. A sample of building material will reflect 99 per cent, of the sound striking it. The same surface covered with one inch of hair felt (the best sound absorber) will reflect only 47 per cent, of the sound. Increasing the thickness of the felt will further reduce the percentage. The harmful effects produced by a dome, a curved ceiling, or pendentives cannot be entirely overcome by acoustical treatment, because such surfaces cannot be made totally absorbent. When they are used the centre of curvature should be located a considerable distance from the location of seats. Surfaces covered with felt one inch thick will absorb 53 per cent, of the sound energy striking it. There will still be reflected 47 per cent., sometimes sufficient to be a source of complaint. Because every substance may be said to have its own absorption coefficient, Sabine was able to determine constants for a number of common materials, and later work