TESTING OF LOUD SPEAKERS 



263 



pass filter, admitting only the fundamental, should be employed. The 

 microphone and amplifiers may be the same as those used for response 

 measurements. The harmonic analyzer may be any of the various types 

 employed in distortion measurements on amplifiers. 



In making the test, the output of the power amplifier is connected to the 

 loud speaker. The sound is picked up by the microphone and then ampli- 

 fied and the measurement of harmonics is carried out in the conventional 

 manner. The output switch is now thrown to the dummy load, the 

 resistance of which should be the same as the impedance of the loud 

 speaker at the measurement frequency. The variable attenuator is 

 adjusted until the output of the microphone amplifier is the same as that 

 obtained with the sound. The harmonic content under these conditions 

 should be negligible. The purpose of this operation is to insure that no 

 distortion is introduced by the associated measuring equipment. In the 



HETERODYNE 

 OSCILLATOR 



MODULATOR 



> TUNED 

 jAMPLIFIER 



3 PHASE 



AND 



'AMPLITUDE' 



BALANCE 



oAPPARATUSofJ 



UNDER 

 » TEST 



A B 



Fig. 11.12. Harmonic analyzers. A. Heterodyne analyzer. B. Balance bridge analyzer. 



above discussion the possible distortion in the microphone has been neg- 

 lected. The distortion generated by the microphone may be measured 

 as outlined in Sec. 11. 2C 



Harmonic distortion measurements should be made outdoors to elimi- 

 nate errors due to standing waves. If it is necessary to make these mea- 

 surements in a room, they should be made under a sufficient variety of 

 conditions with respect to frequency and microphone placements to give 

 average values which are not appreciably affected by the errors associated 

 with room reflections. 



Two systems which are in common use as harmonic analyzers are shown 

 in Fig. 11.12. 



The heterodyne analyzer ^^ is shown schematically in Fig. \\.\2A. The 

 incoming signal, mixed with a carrier supplied by the heterodyne oscillator, 

 is fed to the modulator. A balanced modulator is usually used so that the 

 carrier will be suppressed. The heterodyne oscillator is adjusted so that 

 the sum of its frequency and that of one of the components of the signal 



Arguimbau, L. B., General Radio Experimenter^ Vol. 8, p. 1, June, July, 1933. 



