THE RING ARMATURE TELEPHONE RECEIVER 



137 



this results in a higher force factor. The high yield point and high modulus of 

 elasticity of permendur give better elastic properties so that the diaphragm 

 will restore over a wider range of deflections to which it may be subjected. 

 The 45% permalloy pole-piece has a high resistivity, resists corrosion with- 

 out the need of a finish to protect it, and is easily formed to the desired 

 shape. Since it has a high permeability, and is not too sensitive to strains, 

 it is well suited for pole-piece material. 



Application of Ring Armature Receiver to Other Sound Devices 



Several other applications of the ring armature transducer have been 

 made on an experimental basis in addition to that of the handset receiver, 



1000 2000 4000 



FREQUENCY IN CYCLES PER SECOND 



6000 



10,000 



F.g. 19 — Available power response-frequency characteristic of an experimental wide- 

 range ring armature receiver, compared to a W.E.Co. 711-A moving coil receiver, measured 

 on a 6 cc. closed coupler. 



such as its use as a wide-range receiver, as a miniature horn-type loudspeaker, 

 and as a microphone. 



By narrowing the ring armature and suitably proportioning the acoustic 

 networks, the receiver may be made to operate over a much wider frequency 

 range at some sacrifice of efficiency, and may thus be used for high quality 

 monitoring and audiometric work. A response characteristic of such a unit 

 is shown in Fig. 19. The response compares favorably with the Western 

 Electric 711-A moving coil type receiver used for the same purpose. Being 

 comparable in efficiency, it has also a similar frequency range of about 

 7000 cps. More important, however, are the greater ruggedness and sim- 

 plicity of the ring armature type receiver. Moreover, the impedance may be 

 made to suit the application over a considerable range of impedance values, 

 whereas the moving coil type is limited to a coil of about 25 ohms. While 

 somewhat less pure in tone than the moving coil type the harmonics are 

 50 db below the fundamental at a sound pressure of 20 dynes per cm^ in the 



