418 



BELL SYSTEM TECHNICAL JOURNAL 



500 1000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 13 — Calibration of receivers for tests on the No. 3A audiometer tone 



can be readily calculated with the formula developed here, a com- 

 parison of observed and calculated results will be shown. In Fig. 14A, 

 the ordinate is the reduction in loudness level resulting when a No. 

 3A Audiometer tone having a loudness level of 42 db was changed by 

 the insertion of a filter which eliminated all of the components above 

 or below the frequency indicated on the abscissa. The observed data 

 are the plotted points and the smooth curves are calculated results. 

 A similar comparison is shown in Figs. 14B, C and D for other levels. 



TABLE XV 

 A. Recent Tests on No. 3 A Audiometer Tone 



