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BELL SYSTEM TECHNICAL JOURNAL 



assuming total reflection by the transmitter. It is further assumed 

 that the power flow is uniform over a hemisphere whose radius is the 

 distance from the speaker's Hps to the diaphragm. For this average 

 power flow with "normally modulated" voices, Crandall and Mac- 

 kenzie ^ found the value of 12.5 microwatts. More recently Sacia ^ 

 gave 7.4 microwatts, including pauses between speech sounds, which 



Fig, 



AVERAGE PRESSURE 

 2 — Time distribution of speech pressures. 



occupied about one-third of the total time. Some measurements 

 made since then have led to substantially similar values, and so the 

 value of 10 microwatts may be taken as fairly representative of a 

 normal level. The corresponding average pressure on the diaphragm, 

 at 5 cm. distance from the lips, was found to be about 5 bars. The 

 averaging, of course, is for the absolute values of the pressures. Some 

 notion of the level involved may be gained by noting th it is at 

 ^Phys. Rev., March 1922. 



