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326 CINEMATOGRAPHIC ANNUAL
It will be noted further, by referring to the curves in Fig. 10, that the optimal time of reverberation is different for different speakers even in the same auditorium, varying from about .85 second for the loudest speaker to 1.40 seconds for the weakest speaker. However, a time of reverberation of about 1.00 to 1.25 seconds will quite satisfactorily approximate the optimal reverberation for all speakers.
Curve (a) in Fig. 10 has been calculated upon the assumption that the speech has been amplified, without distortion, to an energy level corresponding to a loudness of 60 db in this same auditorium when the time of reverberation is 1.0 second. The advantage of such distortionless amplification of speech is clearly indicated. Thus, with a time of reverberation of less than 1.0 second, the articulation is ninety per cent, which could be regarded as practically perfect for the hearing of speech. These calculations seem to indicate that suitable amplifiers (public address systems) are imperatively needed in large auditoriums. In addition, such amplifiers are also beneficial in smaller auditoriums, especially if the auditorium is beset with disturbing noise.
At a loudness level of 60 db there is no necessity of reverberation for the usual purpose of enhancing the loudness. In fact, the reverberation should be kept as low as is consistent with other considerations, such as maintaining sufficient brilliance and resonance in the room to meet the requirements for music. Curve (a) in Fig. 10 indicates that the time of reverberation should not exceed 1.0 second for speech. Even for music, there seem to be no physical factors which would warrant a time of reverberation much in excess of 1.0 second (which is about the optimal reverberation for speech and music in a small room) if the loudness be maintained at about 60 db.
The present public address systems or the reproducing equipment in motion picture theatres ordinarily introduce a certain amount of distortion because of the limitations of the electrical and acoustical equipment used, and therefore a distortion factor kd should be introduced in equation (1) when such amplifiers are used. In a properly designed public address system the value of kd probably will be no less than .95. This point, however, requires further investigation.