256 



MEASUREMENTS 



quency of the sound source is varied. Figure 11.7 shows how a pressure 

 characteristic can be made with the sound level recorder. A dark colored 

 tape coated with white wax is moved under a stylus by a motor which 

 changes the value of the beat frequency generated at the same time. The 

 loud speaker under test is connected to the output of the beat frequency 

 generator and the variations in response are recorded on the paper directly 

 on a decibel scale by a stylus which scratches through the wax coating on 

 the recording paper. 



The rectifier output incorporates a control circuit which causes direct 



OSCILLATOR, 



CM 



* 



LOUD SPEAKER 



POTENTIOMETER 



MICROPHONE 



zmi 



m) 



^ 



LINEAR 

 DETECTOR 



m) 



CLUTCH 

 DRIVER 



Fig. 11.7. Schematic arrangement of the apparatus used in a high speed level recorder for 

 automatically recording a sound pressure frequency characteristic. (After Wente, Bedell, 

 and Swartzel.) 



current to flow through one circuit when the rectifier current is less than a 

 certain critical value and through a second circuit when it is greater than 

 a second critical value. In the first case, the control circuit operates a 

 magnetic clutch which causes a potentiometer to operate and increases 

 the voltage. In the second case, the voltage is decreased. 



The output of the rectifier is kept balanced to within the voltage change 

 produced by a change in potentiometer corresponding to the smallest unit 

 of the attenuator calibration. The motion of the potentiometer is com- 

 municated to the stylus which gives a trace on the recording paper. The 

 same motor which drives the oscillator frequency control moves the po- 

 tentiometer by means of the magnetic clutches. 



The speed with which changes in sound level are recorded may be varied 

 from 10 to 560 db per second through alteration of the speed of rotation 

 of the clutches. 



