354 



BELL SYSTEM TECHNICAL JOURNAL 



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Fig. 9 — Response of the receiver to square topped waves. 

 A — Measuring circuit — no receiver. 

 B — Early type receiver. 

 C — New type receiver. 



new receiver is due primarily to the falling off of its response above 

 3000 cycles per second. 



The substantially uniform response of the new receiver also renders 

 clicks and other surges much less objectionable. This is due to the 

 fact that the ear does not respond to the peak value of an oscillatory 

 transient alone but integrates the oscillation over an interval at the 

 beginning of the surge, hence the higher the damping the less ob- 

 jectionable the click. 



The non-linear distortion produced by a receiver of the new type is 

 negligible in its reaction on transmission, the harmonics in the output 

 being 35 db or more below the fundamental. 



Magnetic Circuit. — Inasmuch as the magnetic properties of the 

 diaphragm, as well as its mechanical properties, must be considered in 

 arriving at the preferred dimensions, it was necessary in designing the 

 new receiver to develop criteria which could be applied in determining 

 the optimum relationships between these factors. This study led to 

 the use of the ratio of the force factor to the effective mass of the 

 diaphragm for this purpose. For given magnetic materials in the pole- 

 pieces and the diaphragm and a given air-gap length, there is a pole 

 face area and diaphragm thickness for which this ratio is a maximum. 

 Typical data illustrating this relationship are shown on Fig. 10. The 

 available magnetic materials were studied using this technique and a 

 decision was reached to use permendur in the diaphragm and 45 per 

 cent permalloy in the pole-pieces. 



