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BELL SYSTEM TECHNICAL JOURNAL 



speaker's platform. This position is most desirable, as the illusion 

 produced is such that the voice appears to come from the speaker 

 rather than from the projectors, a factor, the importance of which 

 has already been mentioned. Moreover in this position the acoustic 

 coupling between the transmitter and the projectors is a minimum, 

 permitting the operation of the system at a satisfactory degree of 

 amplification with an ample margin below the point where singing 

 troubles are encountered. 



A public address equipment, similar to that just described, but 

 with a somewhat lower power output, has been developed for use at 

 the smaller open air meetings, and in all but the largest indoor audi- 

 toriums. Fig. No. 5 shows one of these equipments, mounted on an 

 automobile truck, which has been employed at a number of points 

 in the eastern part of the United States. This smaller system has 

 characteristics as good as the larger system in regard to faithful re- 

 production of speech and music, with a power output in the order of 

 one-tenth as great. An audience of 50,000 can be adequately covered 

 at an outdoor meeting with this system. 



Fig. No. 6 is a schematic arrangement of the equipment at an 

 installation of the type shown in Fig. 4. At the extreme left are the 



Transmitter Control 



Panel 



Transmitters 



Speech 



Input Power „ . 

 Amplifier Amplifier ™ 



Panel 



^H« 



Projectors 



Level Per 

 Projector Dep- 

 endent on 

 Number Connected 



1X10 Watts 



Fig. 6. 



transmitters where the sound waves are picked up. The output from 

 these transmitters is taken to a switching panel where means are 

 provided for cutting in the various transmitters. From this panel 

 the transmitter currents are taken to the transmitter amplifier, which 

 is capable of amplifying them to a power level suitable for input to 



