STANDARD VOLUME INDICATOR AND REFERENCE LEVEL 103 



be a phonograph pickup, a direct microphone pickup, or a program 

 circuit, is connected through control circuits to the ampHfier which is 

 to be overloaded, and thence through additional circuits to a loud 

 speaker. The loud speaker employed in the tests reported here was 

 a special high quality two unit loud speaker having a response which is 

 substantially flat from 40 to 15,000 cycles per second.^ Including the 

 power amplifier used with it, the overall response of the system was 

 substantially uniform from 40 to 11,000 cycles. 



The arrangement of the circuit is such that the volume level at the 

 output of the test amplifier may be raised or lowered while keeping the 

 overall gain of the system constant. Two controls are provided for 

 this purpose. One, operated by a key, transfers a 15 db loss from 

 ahead to behind the test amplifier. This permits comparing a test 



POWER 

 AMPLIFIER 



coupled attenuators 

 (sum of losses constant) 



test a 

 S>AAAr9 AMPLIFIER 9 9 



I 15 DB I 



loud 



SPEAKER 



VOLUME 



indicators 



2-position key 

 conditions a and B 



Fig. 3 — Arrangements for determining volume level at which overload 

 of amplifiers is audible. 



condition with a reference condition in which the load on the amplifier 

 is 15 db lower, while the loudness with which the program is heard 

 remains the same for either condition. The other control, represented 

 in Fig. 3 by the coupled attenuators, permits the load on the amplifier 

 for the test condition to be varied, also without changing the loudness. 

 The volume indicators to be compared are connected for convenience, 

 to a point where the volume level is unaffected by the controls. Their 

 readings are corrected for each test by the measured loss or gain be- 

 tween the point where they are situated and the output of the test 

 amplifier, so as to express the levels which would be read at the 

 amplifier output. 



Two techniques were employed for conducting tests with this equip- 

 ment. In one, the individual method, a single observer at a time 



2 "Auditory Perspective — Loud Speakers and Microphones," E. C. Wente and 

 A. L, Thuras, Electrical Engineering, January 1934. 



