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ARCHITECTURAL ACOUSTICS 311
size, or what is the average power of speakers' voices in rooms of different sizes.
(b) The average Acoustic Power of Speakers' Voices in Auditoriums. The data on the effect of loudness upon speech articulation indicate that it would be desirable to maintain the loudness of speech in auditoriums at a level of not lower than 50 db. The question now naturally arises whether the average speaker in an auditorium maintains a loudness level as high as 50 db. It will be seen presently that he does in small rooms, but in large, non-reverberant auditoriums it requires considerable effort on the part of the speaker, and in very large auditoriums it will be impossible to maintain this level without the aid of amplifiers.
The approximate loudness of speech in an auditorium can determined from simple calculations based upon some numerical constants of speech and hearing obtained by Bell Telephone engineers. The data of Sacia and Sivian at Bell Laboratories indicate that the average speech power generated by an average speaker in normal conversation is about ten microwatts. The actual power output of different speakers, and even of the same speaker, varies widely from this average value. For example, they found that the peak power may sometimes rise to two thousand microwatts.
Every public speaker is fully aware that he must raise the intensity of his voice above the ordinary conversational level in order to be heard in a large auditorium. It is evident therefore that his energy output, particularly in very large auditoriums, will be considerably above the average conversational level of ten microwatts. In order to determine the approximate value of the average power of the average speaker's voice in an auditorium, the writer has obtained some measurements on the loudness of speakers' voices in a small and also in a moderately large auditorium. The measurements were made with the help of a microphone (suspended near the middle of the auditorium), an amplifier with an attenuation circuit and a head-set in its output, and a high-quality electric phonograph. The electric phonograph, with a calibrated volume control, was first used for a source of speech in the auditorium. The loudness of the reproduced speech was maintained at different measured levels, and at each level, the attenuation circuit associated with the amplifier (which was located in a remote room) was ad