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

94 SOUND MOTION PICTURES factors upon the ears of the audience it is necessary to know something about the propagation of action and sound in a room. Sound spreads in spherical waves from its source, and the loudness or intensity decreases as the square of the distance. Thus, in the open air sound loses its "carrying qualities" at no great distance from the source, and there are complaints of lack of distinction. In different types of auditoriums the nature of the surface of the walls, the ceiling, the floor, or of other extensive areas, is the principal factor. Surfaces that are soft and thick, such as stage drapes, heavy curtains, carpets, upholstered theatre chairs, or the clothing of the audience, absorb or disperse sound waves. Elaborate ornamental plaster acts in the same respect. However, the quality of building materials reflects sound to the highest degree. Most building interior finishes, for instance, absorb only from i per cent, to 4 per cent, of sound (96 per cent, to 99 per cent, of the sound striking them being reflected). The result is that, as a prescribed area is surrounded with walls and then a ceiling, sound which would otherwise be rapidly dissipated in the open air is reflected from the walls to the ceiling, ceiling to floor, and floor to walls again, many times, with the outcome that much of this reflected sound reinforces and amplifies the original. What is the effect? A more even and a greater distribution of loudness than would be possible otherwise. To this extent the evolution of the modern theatre has been the means of providing increased and more equably distributed intensity in even extremely large auditoriums. On the other hand, the increasing demands of sanitation and fireproofing have brought about the production of harder and more dense interior finishes, so that the quality of sound reflection in materials has sometimes been overdone. Sound distribution in a theatre is quite similar to air