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ARCHITECTURAL ACOUSTICS 327
Referring again to Fig. 10, and recalling that curve (a) was based upon distortionless amplification, it will be noted that if the amplifier introduces considerable distortion, the amplifier may be a hindrance rather than an aid to better hearing. Thus if the distortion factor kd be less than .90, the added distortion will more than offset the advantage gained from increased loudness for all speakers except those with weak voices. It is important therefore that public address systems for auditoriums or reproducing equipment in motion picture theatres be of the high-quality type.
The curves in Fig. 8 are of considerable value in placing a quantitative estimate on the acoustic merit of different auditoriums. They also indicate the limits of size and reverberation which can be tolerated if the percentage articulation is to be maintained at a satisfactory level. Experience has shown that if the average articulation in an auditorium be seventy-five per cent or more, the hearing conditions are regarded as satisfactory. It is possible to understand speech when the articulation is as low as sixty-five per cent, but it requires normal acuity of hearing and strained attention. If it be desired to keep the articulation above seventy-five per cent, and it seems to the writer that seventy-five per cent should be regarded as the admissible minimum, the size and time of reverberation of the auditorium are limited to values above the broken horizontal line in Fig. 8. Thus, it would seem advisable to regard about 800,000 cubic feet (22,600 cubic meters) as the upper limit to the size of an auditorium which is to be used for speaking, unless some amplifying equipment be installed for increasing the loudness. It should be borne in mind that this limitation is based upon the requirements for the average speaker. For speakers with moderately weak voices (see Fig. 10) , the size should not exceed 400,000 cubic feet (11,330 cubic meters) ; and for speakers with very weak voices, the size should not exceed 100,000 cubic feet (2,839 cubic meters).
The admissible limits of the time of reverberation, in order that the speech articulation be above seventy-five per cent, are also indicated plainly in Fig. 8. Thus, in an auditorium having a volume of 400,000 cubic feet, the time of reverberation should not exceed 2.4 seconds. The 2.4 seconds should be regarded as the upper admissible limit for the time of reverberation in such