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BELL SYSTEM TECHNICAL JOURNAL 



sound pressure at 100 c.p.s. where maximum amplitudes of vibration 

 are encountered. Only odd harmonics are present, the greatest in 

 magnitude, the third, being about 15 db below the fundamental at 

 the highest pressure used. At other frequencies or lower intensities, 

 the harmonics are of even less importance. At the average intensity 

 employed in telephone conversations no appreciable harmonics what- 

 ever are present. The results of these measurements are shown in 

 Fig. 5. 



-6 -4 -2 2 4 6 8 10 12 14 



INPUT TO ARTIFICIAL MOUTH IN DECIBELS RELATIVE TO 1.0 VOLT 



Fig. 5 — Harmonics in output of artificial mouth. 



It has been mentioned that the sound field distribution of the 

 human and artificial voices should be alike. One method of deter- 

 mining agreement in this respect has been to measure the output of 

 several different types of transmitters when the instruments were 

 placed at various distances from both the artificial mouth and the 

 human mouth. Eight persons, four men and four women, were 

 employed in the actual voice tests, to call the testing phrases, "Joe 

 took father's shoe bench out" and "She was waiting at my lawn." 

 These phrases include all of the important speech sounds and are 



