The Ring Armature Telephone Receiver 



By E. E. MOTT and R. C. MINER 



{Manuscript Received Aug. 15, 1950) 



A new type of telephone receiver is described, in which the permanent magnet, the 

 pole piece and the armature, which drives a light weight dome, are all ring-shaped 

 parts. This structure exhibits a substantially higher grade of performance than 

 present receivers of the bipolar type, with regard to efficiency, frequency range, 

 leakage noise level, and response when held off the ear. In addition to showing 

 the characteristics of this new receiver, an analysis of the various losses is given, 

 and ideal performance limits are estabhshed. The advantage of providing an aux- 

 iliary path for the air gap flux is indicated, and other applications of the device 

 as a transducer are described. 



Introduction 



tHE ring armature receiver is a new type of telephone receiver de- 

 veloped for use in the subscriber's telephone set. It differs from 

 other types in that the diaphragm consists of a thin, lightweight, dome- 

 shaped central portion made of low density, non-magnetic material whose 

 function is to radiate sound energy, surrounded by a narrow ring-shaped 

 armature to which it is attached. The ring armature is not clamped at the 

 outer periphery, but is held in place solely by magnetic attraction. It is 

 driven at its inner periphery by the magnetic force. A ring-shaped pole and 

 magnet structure serves as the motor element to drive the diaphragm. The 

 new receiver is shown in sectional view in Fig. 1. 



The advantage of the composite diaphragm construction in the new 

 receiver is that the central portion moves almost wholly Uke a piston and is 

 therefore nearly 100% effective and that its contribution to the total moving 

 mass is small, being of the order of \. For these reasons it has been found 

 possible to reduce the mass per unit area to approximately \ of that of the 

 diaphragm of the bipolar receiver. Because of the large effective diaphragm 

 area and the low mass, the acoustic impedance of the new receiver is low, 

 being about \ of that of the earlier receiver. Although the motor efficiency 

 is approximately equal to that of the bipolar receiver, the improved dia- 

 phragm construction yields a receiver of higher available power response,^ 

 wider frequency range, improved characteristic when the receiver is held ofT 

 the ear, and having greater discrimination against room noise. 



The ring armature construction is also applicable to devices other than 

 earphones, such as microphones and loudspeakers. 



^A.S.A. Standard Z24.9-1949 "Coupler Calibration of Earphones." 



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