120 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



quency. The Ul receiver has considerably lower acoustic impedance than 

 the HAl receiver and, since the acoustic impedance of the receiver shunts 

 the ear coupling chamber impedance, the same degree of noise leakage under 

 the Ul receiver generates a lower noise pressure in the ear cavity with a 

 resultant decrease in the masking effect on a given signal. Figure 8 shows 

 data on this characteristic. Tests were made by measuring the pressure in a 

 6 cc. coupling chamber closed by the receiver except for a leakage path 

 having acoustic resistance and mass values approximating those of the worst 

 leakage condition shown by the data presented in Fig. 6. The measurements 

 were made in a highly absorbent room, with a loud-speaker as the sound 

 source. The curves show that below 1500 cycles per second the noise leakage 

 sound pressure generated in the coupler when the Ul receiver is used is less 

 than that of the HAl receiver by as much as 5 db over a considerable portion 



a -20 



400 600 800 

 FREQUENCY IN CYCLES PER SECOND 



2000 



Fig. 8 — Relative noise leakage sound pressure in a 6 cc. coupler having a simulated 

 ear leak. Reference level is the pressure at the coupler with the receiver removed. 



of the frequency range. The effect of this difference has been observed in 

 listening tests using real voices* and in subjective tests] while measuring 

 the shift in the threshold of intelligibility between the two receivers under 

 quiet and noisy conditions. Since under certain conditions a reduction in the 

 noise may be equivalent to a corresponding gain in signal strength, this 

 feature of the ring armature receiver represents a distinct improvement. 



A characteristic of considerable importance in the development and de- 

 sign of a telephone receiver is the manner in which the receiver output level 

 varies with direct current superimposed on the alternating current flowing 

 in the receiver coils. The direct current may be applied in such a way that it 

 develops flux in the magnetic circuit which either aids or opposes the polar- 



*Unpublished work by W. D. Goodale, Bell Telephone Laboratories, 

 t Unpublished work by R. H. Nichols, Bell Telephone Laboratories. 



