TESTING OF LOUD SPEAKERS 



257 



A cathode ray tube/^-^" with a long persistence screen, may be used as 

 a response indicator and recorder, Fig. 11.8. A motor drives the beat 

 frequency oscillator and a potentiometer. The potentiometer varies the 

 voltage on the horizontal deflection plate of the cathode ray tube and 

 thereby drives the cathode ray beam across the tube in synchronism with 

 the oscillator. A reversing switch changes the direction of the motor 

 travel at the upper and lower limits of the audio frequency range. The 

 output of the oscillator actuates the loud speaker. The sound is picked up 

 by the microphone and amplified. The output of the amplifier is detected 

 by a linear or logarithmic detector and fed to a low pass filter. The output 



LOUD 

 SPEAKER 



*n 



OSCILLATOR 



/I 



° LINEAR °; 



LOGARITHMIC 

 DETECTOR 



LOW 

 PASS 

 FILTER 



<3 <J 



HORIZONTAL 



DEFLECTION 



INPUT, 



m nf 



VERTICAL 



DEFLECTION 



INPUT 



D C 

 AMPLIFIER 



Fig. 11.8. Schematic arrangement of the apparatus employing a cathode ray tube with a 

 long persistence screen as a pressure response frequency indicator and recorder. (After 

 Hackley.) 



of the filter is amplified by a d-c amplifier, the output of which is con- 

 nected to the vertical plates of the cathode ray tube. The cathode ray 

 beam traces the response characteristic upon the persistence image screen. 

 The ordinates are in decibels when the logarithmic detector is used. The 

 ordinates are proportional to the sound pressure when the linear detector 

 is used. The time required to trace a response frequency characteristic 

 of a loud speaker is about 30 seconds. The apparatus is very useful for 

 development work because the motor sweeps through the range again and 

 again. The operator is free to make changes in the equipment under test 

 and note these changes upon the response. In case it is desirable to record 

 the characteristic, this may be done photographically or by tracing the 

 curve left upon the screen. 



19 Hackley, R. A., Broadcast News, No. 28, p. 20, July, 1938. 

 2° Sherman, J. B., Proc. I. R. E., Vol. 26, No. 16, p. 700, 1938. 



