636 BELL SYSTEM TECHXICAL JOURNAL 



or "vibrant"; they have the greater carrying power, especially over 

 the telephone; they are rich in the musical sense and are therefore well 

 suited to singing, although main- such \-oiccs, unfortunately, are ne\er 

 applied to the art. 



To illustrate an application of the peak factor to engineering, we 

 shall again take into consideration the speech amplifier whose mean 

 efTective output voltage is indicated by a suitable device such as a 

 volume indicator. F'rom this, the peak value of the instantaneous 

 voltage is wanted; to find it necessitates a knowledge of the peak 

 factor. Now since the latter differs somewhat for different sounds and 

 speakers, it is necessary to use one factor which makes allowance for 

 the worst cases (highest voltage peaks) which can occur often. For 

 most purposes, the factor 5 will suffice, hence the rule is: the mean 

 effective voltage should not exceed one-fifth the overload voltage of 

 the system. 



Apparatus 



In order that the apparatus (see Fig. 5) be a faithful recorder, it 

 was made with the following characteristics: 



(1) A nearly distortionless reproduction of wave form by the con- 



denser transmitter and amplifier. 



(2) A full-wave parabolic rectification of the amplifier output. 



(3) Load capacity sufificient to transmit the high sharp peaks of 



speech waves without cutoff. 



(4) Uniform response, from to 6000 cycles in the oscillograph 



vibrator recording instantaneous power. 



The calibration of the amplifier and condenser transmitter is shown 

 in Fig. 6. To make the overall characteristics so nearly uniform it 

 was found necessary to use the resonant circuit in the output of the 

 second N tube, this compensating for an irregularity due mostly to the 

 45 feet of cable which leads from the transmitter and first stage of 

 amplification in the sound-proof room to the main part of the amplifier. 



The oscillograph (see Fig. 5) was provided with two series connected 

 vibrators one of which was sensitive to low frequencies only, and re- 

 corded the mean power. Although it did not completely suppress the 

 fluctuations of vocal frequency, it reduced them to the order of small 

 superimposed ripples through which the Pm curve could be drawn. 

 The instantaneous power was recorded by the other \ibrator whose 

 characteristics are noted in item (4) above. 



