660 



BELL SYSTEM TECHNICAL JOURNAL 



laboratory investigations of quantities affecting it merit more funda- 

 mental study than can be made with instruments of the volume indi- 

 cator type. The quantities of interest are the instantaneous amplitudes 

 of the wave peaks, the frequency of their occurrence and their distri- 

 bution in the frequency spectrum. The apparatus used to obtain the 

 data described below will be described in detail elsewhere. It is suffi- 

 cient to state here that it is capable of directly (and automatically) 

 registering the magnitudes and frequency of occurrence of the in- 



§ 20 

 ^ to 



■20 



S -30 

 O 10 



HIGH LEVEL 



■NORMAL LEVEL 



LOW LEVEL 



O -40 



-50 



-20^=^=^^ 



-30 



100 



1000 

 FREQUENCY-CYCLES PER SECOND 



10000 



Fig. 14 — -Peak pressures of speech — composite, three male voices — each peak is 

 the maximum instantaneous pressure during 1/8 sec. — average total pressure: 

 High (declamatory) level — -8.7 bars; 

 Normal (conversational) level — 1.6 bars; 

 Low (confidential) level — 0.5 bar. 



stantaneous peaks over a range of 60 db, corresponding to a power 

 ratio of 1,000,000 : 1. The peak amplitudes were measured for the 

 whole spectrum and also for restricted frequency bands selected by 

 filters. 



Fig. 13 and Fig. 14 show the results of some measurements with 

 undistorted speech. Each individual observation gives the magnitude 

 of the peak pressure in a 1/8 second interval. This is about as short 

 an interval as one can use and still retain a high degree of probability 

 that the individual measurements will give the maximum peak 

 amplitudes in syllables. Otherwise, from the standpoint of the 

 apparatus, the individual observations could be confined to much 

 shorter time intervals, resulting in many more measurements for a 



