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318 CINEMATOGRAPHIC ANNUAL ,
general it may be stated that the reverberation should be nearly uniform for tones of all pitch, and that the greater part of the absorptive material should be located in the neighborhood of the listeners rather than in the neighborhood of the speaker.
Both of these features just mentioned — the variation of reverberation with pitch and the distribution of absorptive material in the auditorium — will have an effect upon the reduction factor kr. However, it is not probable that either of these features has a very significant effect upon the "percentage speech articulation" in an auditorium. It is probable therefore that, for a first approximation, the value of the time of reverberation for a tone of 512 d. v., as is commonly employed in current practice, can be used for determining, by means of Fig. U, the appropriate value of kr for any auditorium. The two features of reverberation under discussion are of unquestioned value in auditorium design, but their most important significance is in relation to the preservation of naturalness of speech and music rather than the improvement of speech articulation.
(d) Effect of Noise upon the Reception of Speech in Auditoriums. The interfering effect of noise upon the hearing of speech was considered in the preceding lecture. The principal results of the effect of noise upon hearing is indicated by the curve in Fig. 6. It will be noted that the articulation decreases almost uniformly as the loudest of the noise increases. Further, it will be seen that even a slight noise produces an appreciable interference. The complete absence of noise is thus seen to be an important factor for ideal hearing.
The curve in Fig. 7 is derived from the curve in Fig. 6. It gives the value of kn for different loudness levels of noise. The loudness of the noise is here represented by the ratio of the noise, in db, to that of the speech, also in db. Thus, when the noise is at the same loudness level as the speech, the abscissa in Fig. 8 is 1.0. The value of kn for no noise is taken as unity, and all other values of kn are obtained by taking the ratio of the ordinate (in Fig. 6) for the loudness level in question to the ordinate for zero noise. This method of determining kn is not strictly rigorous, but it gives a close approximation which is sufficiently accurate for practical problems in auditorium acoustics.