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BELL SYSTEM TECHNICAL JOURNAL 



same as for normal levels. The decrease at low frequencies is most 

 pronounced in the band from 62 to 125 p.p.s., i.e. in the region of the 

 voice fundamental frequency at normal levels. This leaves two 

 possibilities. Either at the high speech level the fundamental fre- 

 quency is the same as for normal, but its amplitude is relatively small; 

 and the spacing of the overtones is the same as at normal levels. 

 Or else — and more probably as indicated by the auditory pitch sense 

 — passage to the high level is attended by an actual upward shifting 

 of the fundamental frequency, and a correspondingly larger spacing 



-130 



-140 



100 



1000 

 FREQUENCY -CYCLES PER SECOND 



10000 



Fig. 7 — Average pressures per frequency interval of 1 cycle per second — piano 

 selection — -Liszt's "Hungarian Rhapsody No. 2" — -average total pressure = 3.5 

 bars. 



of the overtones. In addition the amplitude of this new fundamental 

 would be relatively lower than for the normal level. The average 

 spectrum method is incapable of deciding between the two alternatives. 

 Some oscillograms were made at normal and high speech levels, 

 which clearly indicated that the second alternative is the correct one, 

 or at least the prevalent one. For most vowel sounds, the fundamental 

 frequency was found shifted from about 100 p.p.s. at normal to 

 about 200 p.p.s. at the higher level. This result has an intimate 

 bearing on the loss of naturalness encountered when speech originally 

 picked up at normal voice levels is subsequently reproduced at much 

 higher levels. A corresponding change, though in the opposite di- 

 rection, probably takes place in going from normal to subnormal 

 levels. Some evidence of this will be seen in the section on peak 

 amplitudes. 



The method of average pressure-frequency spectra is equally 



