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308 CINEMATOGRAPHIC ANNUAL
exacting requirements. There is no warrant for the superstition, or belief, still held by many that the acoustical qualities of an interior cannot be known until the structure is completed. It is true that difficulties may arise during the design or construction of a building, because of the limitations imposed by good acoustical design, but there are devices and materials the proper use of which will overcome the difficulties and secure entirely satisfactory results.
2. The hearing of Speech in Enclosed Spaces.
(a) General Considerations. We shall first consider the limited problem of the hearing of speech in auditoriums. This is not only one of the most important asuects of architectural acoustics, but it is a problem which I wish to discuss in some detail because it will illustrate the value of scientific research in solving a specific problem in acoustical design.
Perhaps the most important single factor which affects the acoustic qualities of an enclosed space is reverberation. When sound is generated in an enclosure it is reflected back and forth by the boundaries until the sound energy is all converted into heat. The persistence of sound in a room after the source of the sound has been stopped is called reverberation. The time of reverberation is a measure of the time required for a sound to die away to one-millionth of its initial intensity; that is, the time required for the sound to be reduced in loudness 60 db. Ordinarily the time of reverberation is referred to a tone of 512 d. v., although it is necessary to know the time of reverberation for tones of all pitch used in speech and music, namely from about 50 d. v. to 5000 d. v. If the time of reverberation in an enclosed space is long, say several seconds, the successive sounds of speech or music remain audible so long that they overlap and confuse. The method of calculating and controlling the reverberation in rooms has been largely worked out by W. C. Sabine. The actual method of carrying out the calculations will be given later in this lecture.
Ever since the monumental work of W. C. Sabine on reverberation there has been a growing tendency, especially in America, to rate the acoustic quality of an auditorium almost solely in terms of its time of reverberation. It is true that reverberation (which determines the rate of growth and decay of sound in a room) has been, and yet is, the most important factor