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100 EDWARD W. KELLOGG [j. s. M. p. E.
and detrimental overlapping. The auditors near the speaker could hear better without any reverberation, but in the remote parts of the room the voice is too faint without the assistance of echoes. If the sound from an echo follows the original sound very quickly it is helpful, whereas if it is delayed it produces only confusion. The effect of an echo on speech perception might be represented by a curve such as Fig. 3A. The curve would obviously be different if the initial loudness were changed. Thus if the initial sounds were loud enough, there would be no help from echoes. Since, however, the purpose of the curves of Fig. 3 is simply to help us form a mental picture of how increasing the reverberation period shifts the balance from no help where there is zero echo, through a condition of substantial help, to one of serious detriment, it is only necessary that curve A be a reasonable approximation to the over-all gain or loss from an echo in the rear part of an auditorium where an average voice would fail to carry without reinforcement. Since reverberation consists in a series of echoes of gradually decreasing amplitude, the first and strongest of these will help and the later weaker echoes (resulting from repeated reflections) will hinder. Let Fig. 3B represent the reverberation curve showing the manner in which the sound dies out in a room with considerable damping. The net help or hindrance to the auditor would be represented by Fig. 3C, whose ordinates are the product of those of A and B. The area above the zero line exceeds that below, which means that the echoes have been more help than hindrance. On the other hand, with the same initial loudness and a longer period of reverberation as illustrated in Fig. 3D, the product curve, E, has a larger area below the zero line, or there is more hindrance than help. There is an intermediate value of reverberation time at which the maximum help is obtained. This optimum reverberation is, as has been pointed out, shorter for speech than for music, and well-designed auditoriums have usually aimed at a compromise between the best values for each.
ALTERED REQUIREMENTS RESULTING FROM ELECTRICAL SOUND EQUIPMENT
I have so far simply attempted to review the fundamental principles of auditorium acoustics in so far as they can be expressed in terms of reverberation, and to suggest some of the reasons for the acceptable reverberation being what it is. During the past ten years much information has been accumulated, many new and better materials have become available for acoustic treatment, and