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THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



to the auxiliary magnet to be held there by magnetic attraction and insure 

 that the armature will always return to its seat in case it is dislodged by 

 mechanical shock. Another function which is built into the dome serves to 

 prevent the outside edge of the armature from coming into contact with the 

 inside surface of the cylindrical main portion of the magnet. Contact of this 

 nature has been found to produce irregularities in the response-frequency 

 characteristic of the receiver and a variability of the output level of a few- 

 decibels. These undesirable variations are controlled by six spokes of the 

 dome material which extend outward beyond the edge of the armature a few 

 thousandths of an inch and thus tend to prevent contact between the arma- 

 ture and magnet. Finally, the dome contains a small hole which introduces 

 a low frequency cut-off in the response-frequency characteristic of the re- 



250 



500 



750 1000 1500 2000 



FREQUENCY IN CYCLES PER SECOND 



3000 



Fig. 4 — Available power response-frequency characteristics measured with a source 

 impedance of 128 ohms on a 6 cc. coupler, except as noted. 



ceiver, which has been found to be desirable to reduce interference picked up 

 in telephone circuits from electrical power circuits. 



Performance Characteristics 



A partial evaluation of the performance improvements of the ring arma- 

 ture receiver as compared with its predecessor is illustrated in Fig. 4, which 

 shows available power response-frequency characteristics** for the two re- 

 ceivers. The two solid curves in this figure show the relative sound pressure 

 output of the new Ul receiver and the present standard HAl receiver over 



^Response-frequency characteristics in this article are shown on a new frequency scale 

 which gives a well balanced visual emphasis to the various frequency bands. The scale is 

 linear from to 1000 cycles per second, and logarithmic from 1000 to 10,000 cycles per 

 second, the two sections having a dimensional ratio of 4 to 9. See "A New ?>equency 

 Scale for Acoustic Measurements", W. Koenig, Bell Laboratories Record, August 1949. 



