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THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



heavy magnetic material needed for armatures. These two factors permitted 

 a very substantial reduction in the effective mass to be made in the moving 

 system. In addition it has been observed that the peripherally driven 

 diaphragm moves as a piston over a wide frequency range, while a centrally 

 driven cone type diaphragm, as in Fig. 2(a), is more likely to have parasitic 

 modes of vibration in the frequency range of interest. However, the re- 

 ceivers of the type shown in Fig. 2(b) needed small air gaps in order to get 

 the force factor necessary to attain a high efficiency. Moreover, the thin 



(a) 



COIL 



-f — POLE PIECE 

 Tfj- MAGNET 



(b) 



Fig. 2 — (a) Early composite diaphragm receiver. 



(b) Simple ring armature receiver. 



(c) Ring armature receiver with auxiliary magnet. 



magnet mounted between the inner and outer pole pieces presented manu- 

 facturing problems because of curved surfaces that had to be ground to close 

 tolerances. The magnet in this case had to be of low reluctance and large 

 section area, and this resulted in a rather tall structure. 



By adding an auxiliary ring magnet overlying the front of the diaphragm, 

 as in Fig. 2(c), radially magnetized in aiding relation to the lower magnet, a 

 large increase in force factor was attained with larger air gaps than in 

 Fig. 2(b).'*' ^ In addition an upright main magnet, as shown, could then be 



^U. S. Patent No. 2,249,160, E. E. Mott, Filed May 19, 1939. 

 »U. S. Patent No. 2,249,158, L. A. Morrison, Filed July 15, 1941. 



