SPEECH POWER AND ITS MEASUREMENT 



661 



given length of speech. The ordinates give the ratios of the peak 

 pressures to the average total speech pressure. In Fig. 13 the peaks 

 are those of speech as a whole; in Fig. 14 the ordinates give the peak 

 pressures in the several frequency bands. This is done for three 

 widely different average levels spread over a range of 30 db. From 



UJ O 



§5 



Qz -30 



1000 

 FREQUENCY -CYCLES PER SECOND 



Fig. 15 — Peak pressures in piano music — Liszt's "Hungarian Rhapsody No. 2" 

 — each peak is the maximum instantaneous pressure during 1/8 sec. — average total 

 pressure 4.0 bars. 



the Standpoint of providing apparatus overload capacity it is some- 

 times important to know not only the maximum values of all peaks 

 (which might be uneconomical to provide for) but the upper limit of 

 a certain percentage of all the peaks. The lower half of Fig. 14 

 illustrates this method of analyzing the peak data. It is interesting 

 to note how much larger the transmission capacity must be to take 

 care of the highest 25 per cent of the peaks. 



Fig. 15 gives a similar picture of the peak amplitude distribution 

 in a piano composition, for which the average amplitudes are given in 

 Fig. 7. The microphone was about 7 ft. laterally from the center 

 of the keyboard, and the measurements were made in a room having 

 an average reverberation of about 1 second. 



My thanks are due to Dr. H. K. Dunn and Mr. S. D. White of 

 Bell Telephone Laboratories, who have obtained most of the data 

 discussed in this paper. 



43 



