PHYSICAL CHARACTERISTICS OF SPEECH AND MUSIC 361 



the circuit is shown in Fig. 12. Two kinds of meters are used. The 

 first is a flux meter as shown in Fig. 12 for integrating the speech 

 energy over any desired interval. When the rectifier is designed to 

 give a value which is proportional to the average voltage, then the 

 deflection of the needle of the flux meter will be proportional to the 

 average pressure times the time. In other words, this device will 

 read the average pressure during any desired time interval. In this 



AMPLIFIER 



MESSAGE 



REGISTERS 



SYNCHRONOUS 

 MOTOR 



Fig. 13— Schematic of electrical circuit for measuring the peak power-frequency 



distribution of sounds 



way it is possible to find the average pressure in any one of the 14 

 bands. If the rectifier is adjusted so that the reading is proportional 

 to the square of the impressed voltage then the reading will correspond 

 to the average power. Knowing the calibration "* of the transmitter 



* "Speech and Hearing," page 305, and also paper entitled "Absolute Calibration 

 of Condenser Transmitters" by L. J. Sivian, Bell System Tech. Jour., Jan., 1931. 



