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ARCHITECTURAL. ACOUSTICS 315
value. On the other hand, the combined mask and megaphone was of unquestioned value for augmenting the loudness of the voice — at the same time, however, distorting the natural quality of the voice.
The use of these two devices — the megaphone and the resonator— most clearly indicates that in their open air theatres the Greeks were handicapped by the same difficulty that was revealed by the recent investigation of the loudness of speech in auditoriums; namely, that the natural, unaided voice does not provide an adequate supply of speech energy for good hearing in large auditoriums.
From the dotted line curve in Fig. 3 it is possible to calculate the value of kb which will be useful in connection with equation 1. Ki is taken as unity at a loudness level of 70 db and the value of ki at any other loudness level is obtained by taking the ratio of the percentage articulation at that loudness level to the percentage articulation at a loudness of 70 db. The solid line curve in Fig. 3 gives the values of k! at different loudness levels.
(c) Effect of Reverberation upon the Reception of Speech in Auditoriums. We shall next consider the interfering effect of reverberation upon the hearing of speech in auditoriums. It would be expected, and experience bears out this expectation, that hearing conditions would be very unsatisfactory in reverberant rooms, owing to the overlapping and confusing of the separate syllables and words of articulated speech. The curve in Fig. U shows how the speech articulation depends upon the time of reverberation, measured at a frequency of 512 d. v., in a group of auditoriums having about the same shape and volume (200,000 to 300,000 cubic feet) but different times of reverberation. The small circles in Fig. U show the observed value of percentage articulation for the corresponding measured times of reverberation in the auditoriums investigated. The lower curve is drawn to represent the most probable fit with the observed data. It will be noted that, approximately, the articulation decreases six per cent for each additional second of reverberation.
The lower curve in Fig. 4, which represents the mean result of the experimental determinations, was not obtained for a constant loudness level of speech, because the loudness is de