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

108 SOUND MOTION PICTURES chord. The indicated limit for this room is perhaps twelve or fifteen pieces, with the organ. As to interior finish, this should be planned with both echo and reverberation in mind. A liberal use of coffering on ceiling and on sloping upper walls should effectually prevent echo from these sources. The interior materials should be calculated to give a reverberation time as indicated by the average range in Table I. Use should be made of panels of absorbing material in such quantity as may be necessary to reduce the reverberation time to a suitable value. Such materials, of several kinds, are now available commercially. In certain large buildings, as in the dome of St. Paul's Cathedral, London, sound is so focussed that a whisper at one side can be distinctly heard at the other. A similar "whispering gallery" in the Hall of Statues at the Capitol at Washington has recently been eliminated by remodelling the ceiling. Expert advice must always be sought in the handling of acoustical problems, and in this connection the best advice and the best materials are not only the most effective but the cheapest in the long run. The competent acoustical engineer, who has supplemented the theory of his science with years of practical experience, can determine or adjust the acoustics of any theatre to great exactness. It is important that theatre operators give this problem the attention that it deserves, for it is extremely difficult to live down a reputation for poor acoustics once it has been incurred. What it all comes to is this: Sound means not only new equipment in the motion picture theatre, but, in a partial sense, a new theatre. For reasons I have stated throughout this chapter, no producer can possibly anticipate, no manufacturer of machinery can possibly anticipate, the special conditions of the individual building. If these, by