THE RING ARMATURE TELEPHONE RECEIVER 



139 



compares favorably with similar devices of the moving coil type. The maxi- 

 mum acoustic output, however, is lower, because the amplitude of the 

 diaphragm is limited by the air-gap. 



As a microphone, the ring armature structure may be modified to have 

 characteristics which are quite favorable for certain types of applications. 

 Figure 21 shows the field response of a ring armature unit modified for use 

 as a microphone and measured on an open circuit voltage basis. A special 

 housing of 30 cc. rear volume was used in this case, with a | inch diameter 

 orifice in the rear of the housing to act as a resonant circuit to produce the 

 low frequency resonance shown with a desirable cut-off at 250 cps. By 

 lowering the acoustic damping resistance of the unit, a second resonance was 

 produced in the middle of the frequency range as shown. The peak at the 

 upper end of the range is the normal characteristic of the instrument, but it 

 may be enhanced somewhat by the use of a cavity in front of the diaphragm. 



2 

 en u 



CD Z 



-55 



-60 



QCC -65 

 Z Q. 



o > 



^2 



■75 



-80 



500 



1000 2000 4000 



FREQUENCY IN CYCLES PER SECOND 



6000 



10,000 



Fig. 21 — Free field response-frequency characteristic of an experimental ring armature 

 microphone, at normal sound incidence. 



The output level of this microphone is about 9 dh above the Western Electric 

 633A moving coil type at the same impedance level, but over a more limited 

 frequency range. 



Conclusions 



It has been shown in the preceding sections that, by the use of the ring 

 armature structure, it has been possible to realize a substantially higher 

 grade of performance with regard to efficiency, frequency range, and leakage 

 noise level, as compared to other types of telephone receivers in current use. 

 To summarize, the ring armature receiver has been found to have the fol- 

 lowing advantages from a performance standpoint: 



1 . A gain in conversion efficiency of the order of 5 db as compared to the 

 HAl receiver and a corresponding increase in output capacity. 



2. A wider frequency range, with an upper frequency of 3500 cps as 

 compared to 3000 cps for the HAl receiver. 



