Speech Power and Energy 



By C. F. SACIA 



Introduction 



IN the (i.ist, iiukIi rescarcli lias been devoted to tlie determination 

 of tlte relative magnitudes of the frequency components of speech, 

 and the results of these explorations are useful and well known. 

 Thus the communication engineer is apprised of the frequency range 

 over which his apparatus should respontl uniformly in order that the 

 transmitted speech suffer no frequency distortion. But to provide 

 against load distortion, he requires the knowledge of a different kind 

 of data : numerical values of the magnitude of power involved in 

 sp)€ech waves as a whole. This investigation deals with the magni- 

 tudes and forms of speech wa\'es primarily in terms of power, and 

 is not concerned with frequency as the argument. 



Although the subject matter is not fundamentally new, this treat- 

 ment of it is somewhat of a venture. The broad classification of power 

 is a convenience here, but its future value will be dependent upon 

 engineering usage. I have also introduced the use of the peak factor, 

 which, being a simple index of the wave form, may perhaps find 

 application in vowel study and phonetics as well as in the technical 

 field. A condensed table of peak factors was incorporated in Mr. 

 Fletcher's compilation in the preceding issue of this Journal. 



Derivation 



The nature of power in a syllable of speech may be most easily 

 comprehended by reference to an illustration such as that shown in 

 Fig. 1. The representation of the instantaneous power (P,) is an 

 enlarged copy of a power oscillogram of the word "quite." Because 

 of its extreme jaggedness, the curve had to be represented by a profile 

 rather than by an outline. Although this is a quickly spoken syllable 

 it plainly displays a cyclic repetition; the cyclic interval (for example, 

 from a to 6 in the figure) is ordinarih- called the vocal f)eriod and its 

 reciprocal, the v'ocal frequency'). 



One feature of interest may be noted here: the irregularity in the 

 growth and decay of the peaks. This is evidence of a slight vocal 



' The power due to any pcrioflic force, containing only odd harmonics, fluctuates 

 with double the frequency of the fundamental; but in the case of any periodic force 

 containing even harmonics also, the power fluctuations have the same fundamental 

 frequency as the force. .Although speech sounds are not periodic an analogous 

 relation exists for them. 



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