138 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



coupler, and therefore are not noticeable under ordinary listening conditions. 

 The attainment of wide frequency range in magnetic type units is unique, 

 and is due in part to the use of a peripherally driven diaphragm. Centrally 

 driven diaphragms used in magnetic type receivers usually have parasitic 

 modes of vibration at these frequencies, which places a limit on the frequency 

 range for which they can be designed. 



As a loudspeaker, the ring armature structure has been found to have 

 some experimental application when used with a horn, both for speech and 

 as a sound source for measuring purposes. Response characteristics of such 



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FREQUENCY IN CYCLES PER SECOND 



Fig. 20 — Available power response-frequency characteristic of a ring armature receiver, 

 measured as a loudspeaker with and without a horn. 



a unit are shown in Fig. 20 for the instrument with and without a horn. In 

 the case of the unit without a horn the unit had a receiver cap as used in the 

 handset receiver, and the acoustic circuits were similar to those of the normal 

 receiver. For the case of the horn attached to the unit a spherical plug was 

 used to couple the horn closely with the diaphragm, and no acoustic damping 

 circuits beneath the diaphragm were necessary. The horn pictured in the 

 sketch was spaced 15 inches away from the measuring microphone, and the 

 same distance was used for the curve without a horn. It is apparent that, 

 with the horn, 15 db is added to the sound level on the axis, and the fre- 

 quency is widened by a factor of 2 or more. The efficiency shown in Fig. 20 



