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BELL SYSTEM TECHNICAL JOURNAL 



in the sound field. Care was taken to suspend the transmitter and its 

 small associated amplifier between small poles in such a way that any 

 possible reflections from such objects in the sound field would not reach 

 the transmitter position. As for reflections from the ground, the 

 distance of the loud speaker from the ground with the consequent sound 

 divergence from the radiating surface, and also the absorption and 

 diffraction at the ground, caused by the magnitude of the sound 

 reflected to the transmitter position to be quite undetectable. 



One of the loud speakers was a 115 cycle cut-off exponential horn 

 with a moving coil type receiver.^ The mouth of the horn was located 

 at the platform edge with the axis making an angle upward from 



50 100 500 1000 5000 10000 



FREQUENCY 



Fig. 7 — Response-frequency characteristic of lYi' piston diaphragm loud speaker. 

 Measured outdoors 12 feet from and on a Hne perpendicular to the center of the 

 diaphragm. 



the horizontal of approximately 15". Fig. 2 shows the response 

 frequency characteristic with the condenser transmitter on the axis at 

 a distance of 12' from the mouth. Except for variations near the 

 horn cut-off frequency due to the horn itself, note the absence of any 

 large irregularities and also the rising trend of the curve with frequency. 

 The diameter of the horn mouth was 30" so that 12' is greater than the 



distance 



4500 



feet discussed above. 



Fig. 3 shows response-frequency characteristics of the same loud 



speaker measured under exactly the same conditions except that the 



condenser transmitter was located on the axis only 2" from the horn 



^ This type of receiver was described by Wente and Thuras in The Bell System 

 Technical Journal, for January, 1928. 



