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

VARIOUS APPARATUS 71 they will reflect most of the sound striking them. Different frequencies, also, may be reflected in varying proportion. When surfaces are made of soft, thick materials such as heavy drapes, carpets, upholstery, or where the audience covers a relatively large space, a good portion of the sound is absorbed. Pilasters, mouldings, cornices, and such broken surfaces disperse sound waves and tend to damp them out quickly. The hard or reverberant type of theatre is especially annoying for speech selections, since the duration of audibility is so great that one sound remains to overlap the sound succeeding it. This gives an effect of poor articulation. For music reproductions, however, a reasonable amount of reverberation is acceptable, for it contributes to the impression of fullness and roundness. Another uncertain factor, which further complicates the situation, is the varying size of the audience. Theatres which may be entirely too reverberant when empty will sometimes give pleasing results when filled. Unfortunately, it is not possible to have a completely filled house at every performance. It will therefore be seen that, at best, a compromise must be tolerated. Reverberation depends upon the rate of decay of sound and is measured by its inverse, the duration of audibility of a sound of known initial intensity. The introduction of highly absorbent material into a room will have greater absorptive effect on the higher frequencies. If the thickness of the material is increased considerably, the absorption of the lower tones will be increased, while that of the higher frequencies will remain practically intact. Very thin fabrics, such as cheesecloth or bunting, absorb only extremely high frequencies. A considerably damped auditorium will cause sound to appear to lack brilliance and definition and will produce an effect of deadness, which in extreme cases may be almost