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•INEMATOGRAPHIC ANNUAL
justed until the speech, as heard in the head-set, was reduced to the minimal threshold of audibility. A similar adjustment of the attenuation circuit when a speaker was speaking in the auditorium, gave a measure of the loudness of his voice. The method is essentially a substitution method in which the average loudness of the speaker's voice is compared with a measurable loudness from the electric phonograph. The loudness of the speaker's voice is expressed in db.
By means of known data for the sensitivity of the ear, and the relation between the intensity of sound in a room and the total amount of absorption in the room, it is possible to calculate the acoustic power of a speaker's voice in an auditorium. Measurements and calculations based upon this method and the experiments described in the preceding paragraph, indicate that the average acoustic power of the average speaker in a room having a volume of 27,000 cubic feet is 27.4 microwatts, and the average loudness of the average speaker is 50.7 db. In a large auditorium, one having a volume of 240,000 cubic feet, the average acoustic power of the average speaker is 48.9 microwatts, and the average loudness is 45.7 db. Thus, as the size of the room increases, the speaker generates more speech energy but not enough to maintain as high a loudness level as he maintains in a smaller room. The average acoustic power of the average speaker in rooms of different sizes is indicated by the curve in Fig. 1. The
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2Q30 5 6 60 I/3QO 22600 65 200 Cum lOOpOO 200,000 600,000 &00,000 1,600,000 Cu ft.
12,500 25,000 50,000
Volume FIGURE 1.
Curve showing the probable speech power of the average speaker in auditoriums of different sizes,