SPEECH POWER AND ITS MEASUREMENT 



653 



determined not only by the amplitudes in the individual speech sounds 

 but equally by frequency of occurrence. Thus the fact that nearly 

 all vowels for male voices at normal levels have a fundamental fre- 

 quency between 80 and 150 p.p.s. tends to accentuate that region 

 even though the fundamental amplitudes in individual speech sounds 

 not be outstandingly large. 



The data in Fig. 4 are for close talking conditions, 5 cm. from the 

 lips to the diaphragm. Fig. 5 shows the effect of distance on the 

 spectral distribution. The two distances are 5 cm. and 90 cm. 

 respectively, in both cases the transmitter being set into a large 

 fiber-board wall. The shapes of the two spectra are almost identical 



uj Q 

 5^ 



< 



Pa: 



-40 



-50 

 -60 

 -70 

 -80 

 -90 



100 



1000 

 FREQUENCY -CYCLES PER SECOND 



10000 



Fig. 6^Effect of level on voice spectrum — average pressures per frequency interval 

 of 1 cycle per second — -composite, three male voices, distance 3 ft. 



indicating that, on the average, even at 5 cm. the condenser micro- 

 phone does not greatly affect the voice as a sound generator. The 

 largest difference between the two is in the lowest band, from 62 to 

 125 p.p.s., perhaps owing to the relatively low radiation efficiency of 

 the voice at those frequencies. 



The ratio of the distances is 18 : 1, that of the average pressures 

 14 : 1. The average pressure is nearly inversely proportional to the 

 distance, part of the difference probably being chargeable to the 

 more nearly total reflection for the distant condition. It is implied, 

 of course, that even for the latter condition the direct sound is large 

 compared with that reaching the microphone by reflections. 



So far the spectra discussed were those of normally modulated 

 voices. Fig. 6 shows what happens to the average spectrum when a 

 high, rather declamatory level is adopted. The higher level is rela- 

 tively poorer in frequencies below 500 p.p.s., relatively richer between 

 500 and 4000 p.p.s., and above 4000 p.p.s. its spectrum is nearly the 



