118 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



trated by Fig. 7, which shows available power response-frequency character- 

 istics for the two receivers when they are raised one-eighth of an inch from 

 the normal sealed position on a standard coupler. The Ul receiver shows 

 better response than the HAl receiver at both high and low frequencies. 

 The low frequency end is cut off less in the Ul receiver, because it has 2.5 



Fig. 5 Mclhod of luta.^uiii 

 microphone. 



rt-Ciivcr res[)Orise on a liuiuan car, usin^ a sc 



irch tube 



times larger effective area and therefore is a better radiator of sound at low 

 frequencies. The high frequency end is better because of the inherent exten- 

 sion of frequency range in the Ul receiver. 



Another interesting characteristic of the new receiver is its performance 

 under conditions of high ambient noise levels. Noise leakage between the 

 receiver cap and the external ear generates an acoustic noise pressure in the 

 ear cavity which may mask the sound signal from the receiver. This leakage 



