652 



BELL SYSTEM TECHNICAL JOURNAL 



used to obtain the data will be described in detail elsewhere. Briefly, 

 a series of band-pass filters are used to separate the frequency compo- 

 nents. The output of any one filter goes into a rectilinear vacuum 



52 



Qi 



-10 

 -20 

 -30 

 •40 

 -50 

 -60 

 -70 

 -80 



-no 



-Tt 



L.._ 



COMPOSITE- FIVE MALE VOICES 



AV. TOTAL PRESSURE - 7.7 BARS 



COMPOSITE -THREE FEMALE VOICES 



AV TOTAL PRESSURE = 6.3 BARS 



...X 



4 5 6 8 



==T. 



"^ 



100 



1000 

 FREQUENCY -CYCLES PER SECOND 



10000 



Fig. 4 — -Average speech pressures per frequency interval of 1 cycle per second — 

 normal conversational voice. Distance 2". 



tube rectifier, and a fluxmeter integrates the rectified current over the 

 duration of the speech. Simultaneously with this measurement the 

 total spectrum also is rectified and integrated. The integration 

 periods used were 15 seconds long. The pressure in any one band is 



-40r 



d2 



(OCT 



-50 

 -60 

 -70 

 -80 



toco 



FREQUENCY-CYCLES PER SECOND 



10000 



Fig. 5 — -Effect of distance on voice spectrum — -average pressure per frequency 

 interval of 1 cycle per second — normal speech-composite, three male voices. 



given by the product: (lO"-"^" X band width in cycles X total pres- 

 sure), where a. is the ordinate expressed in decibels (db). In using 

 average spectra of this sort it is well to remember that they are 



