speech Power and Its Measurement ^ 



By L. J. SIVIAN 



The paper is chiefly concerned with the important speech power quanti- 

 ties — frequency spectra, distributions of instantaneous, average, syllabic 

 and peak amplitudes, etc. — as they obtain in actual speech for a large range 

 of voices, talking levels, and subject matters. The analysis is not nearly 

 so complete nor so fine-grained as that which, in principle, can be derived 

 from oscillographic records of individual speech sounds. Its advantage is 

 in the speed with which data can be secured, under widely varying condi- 

 tions and on a scale which warrants statistical conclusions. Some of the 

 methods in use for measurements of this type are described. A "level 

 analyzer" has been developed, primarily for the measurement of average 

 and peak pressure amplitudes in speech and music, both as to magnitude 

 and as to position in the frequency spectrum. Illustrative results are 

 given for samples of speech, music and noise. 



SPEECH sounds are so variable from one to another, from one 

 individual to another, from one conversation to another, as 

 to make it necessary to deal with speech power in a statistical 

 manner. This is particularly true of engineering applications, as 

 distinguished from studies in phonetics and voice dynamics. It is 

 largely from the viewpoint of the former that the subject is here 

 treated. 



Speech power occurs in several states, e.g. acoustic, electric, mag- 

 netic, mechanical, optical, thermal, etc., but its measurement largely 

 refers to speech in the acoustic or in the electric form. Acoustically, 

 the simplest quantity to define is the instantaneous power transmitted 

 through unit area tangent to the wave front. That power is L = P-U 

 where P is the pressure and U the air particle velocity. With P 

 expressed in bars (dynes per cm.-), and U in cm. /sec, L is given in 

 ergs /cm.- X sec. We do not directly measure the product P- U. 

 No suitable means for doing that has been developed. In a progressive 

 plane wave, or with good approximation in any progressive wave at 

 sufficient distance from the source, P and U are in phase, and the 

 expression for L simplifies down to L = P~/pc = IP- pc, where pc is a 

 constant equal to 41.5 mechanical ohms — the sound radiation re- 

 sistance of air. Hence for the type of waves mentioned the wattmeter 

 type of measurement (pressure X velocity, corresponding to voltage X 

 current in the electrical case) may be replaced with the simpler 

 measurement of pressure (voltage) or velocity (current) alone. 



What are the acoustic voltmeters and ammeters that are capable 



1 Presented before Acoustical Society of America, May 11, 1929. 



646 



