254 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



By combining this acoustic system with a diaphragm and associated car- 

 bon chamber that resonates at the anti-resonant frequency of the acoustic 

 network, it is possible to obtain an overall response free from resonances 

 with uniformly rising output with frequency to about 2500 cps, followed by 

 a broad maximum output extending to approximately 3500 cps, then 

 dropping off in output at higher frequencies as shown in Fig. 11. This shows 

 the complete equivalent circuit, the computed response and the response 

 measured with constant sound pressure at the diaphragm. 



The carbon chamber of the new transmitter unit, although similar in 

 general to previous designs, has been modified in many important details 

 in order to decrease the mechanical impedance of tl>e carbon in the interest 

 of higher modulating efficiency. Also, better positional performance has been 

 obtained by changes in the carbon chamber contour and effective head of 

 carbon. 



Receiver 



The receiver unit of the new handset, as shown in Fig. 9 differs radically in 

 design from any previous commercial receiver. It is referred to as a "ring ar- 

 mature" receiver and employs a completely new magnetic and vibratory sys- 

 tem.^ The diaphragm, which in previous receivers has been a simple disc 

 of magnetic alloy, is now a composite design consisting of a ring of magnetic 

 material (permendur) with a center of phenolic impregnated fabric material 

 formed in the shape of a dome. This required, of course, an entirely new type 

 of magnetic circuit. The magnetic ring or armature is supported at its outer 

 edge on a ring of non-magnetic material which provides the diaphragm seat. 

 The inner edge of the armature is associated in the design with a ring pole 

 piece which carries the flux from a ring-shaped permanent magnet. The use 

 of the composite diaphragm in the new receiver results in a lower mechanical 

 impedance and an appreciable increase in the ratio of effective area to 

 effective mass. This accounts for an improvement in receiving efficiency as 

 compared to the previous handset receiver of approximately 5 db along with 

 an extension of the frequency range. Also, because of the lower mechanical 

 impedance of the diaphragm system, the loss in intelligibihty when it is held 

 off the ear, as may occur in service, is greatly reduced. 



Because of the higher efficiency and greater power output capacity of 

 the new receiver as compared to its predecessor, a peak limiting device (click 

 reducer) is provided to prevent the user from receiving uncomfortably high 

 acoustic levels. A copper oxide varistor element is therefore incorporated in 

 the design as an integral part of the receiver. This varistor also protects the 

 receiver magnet from possible demagnetization caused by transient electri- 

 cal disturbances. Less magnet material is therefore needed. 



