SPEECH POWER AND ENERGY (^ 



.iminetcr, or by the volume indicator. A graph of the mean power 

 may be obtained by drawing the average jiower in each vocal cycle 

 and then drawing a smootli curve tliroiigh the resulting broken hne. 

 This wouUl be an impracticable way of olitaining curves of mean 

 [)ower; actually they have been obtained independently of the P, 

 curves in this work, in a manner described later. 



X'owel sounds carry by far the most of the power and energy of 

 speech, and it was to them that the above considerations were tacitly 

 applied; but the definition of the mean power is similarly applicable 

 to the semi-vowels, voiced consonants, and fricative consonants. 



The peak factor is the square root of the ratio of a peak value of P, 

 to the corresponding value of Pm- 



Still another commonly used interpretation of power is made 

 in terms of its average over an entire svllable, word or speech. Such 

 an average, although the same for instantaneous and mean power, 

 is most easily determined by means of the latter: it is the total energy 

 divided by the time involved. Graphically it is the area of the P, 

 or Pm curve divided by the base. If the base includes the silent 

 intervals between syllables the result will be called the long average; 

 if the silent intervals are excluded from the base, the result will be 

 called the short average. 



Thus it is seen that the word "power" when applied to speech has a 

 variety of meanings and always needs to be qualified. For example, 

 the speech of a certain person may have shown a long average power 

 of 10 microwatts while the instantaneous power frequently rose to 

 2,000 microwatts. 



In obtaining the power, w-e obtain indirectly the pressure on the 

 condenser transmitter, which is located 9 cm. from the speaker's 

 lips. In the treatises on acoustics, the power of a simple-harmonic 

 wave is derived in terms of the pressure,- the numerical result being 

 at 20° C, 



where P is the power in microwatts across 1 sq. cm. of wave front, 

 and where either mean or peak value is taken for both power and 

 pressure. Here we are not concerned with simple harmonic waves, 

 but the same result holds for instantaneous, mean, or average values 

 in any kind of wave, since 



\0^ dl 

 • Scf, for example, Rayleigh: Theory of Sound, Vol. 2, page 16. 



