314 BELL SYSTEM TECHNICAL JOURNAL 



directly compared and tlie results used to great advantage in the 

 engineering and maintenance of the telephone plant. 



A further valuable application of the artificial voice and ear is as 

 an adjunct to a reference telephone system. At the present time un- 

 controllable differences in technique and testing personnel at various 

 points where it may be desirable to employ such reference systems 

 involve discrepancies difficult to eliminate or explain. The use of the 

 artificial voice and ear with suitable phonograph records makes it 

 possible to have identical testing means at any point desired. 



Further experience with the artificial voice and ear will undoubtedly 

 open up new possibilities and applications for these instrumentalities 

 and enable more accurate investigation of instruments with less 

 expenditure in effort and time. 



APPENDIX 



Response-Frequency Characteristics of Receivers 

 ON THE Human Ear 



In Figs. 15 and 16 are shown response-frequency characteristics of 

 receivers on the human ear. One method by which such charac- 

 teristics may be determined is shown schematically in Figs. 11 -A 

 and \1-B. With the auricle projecting through an aperture (provided 

 for purposes of definite location of the ear) tovv^ard the sound source 

 as shown, the pressure at each frequency produced at the tympanum 

 or as near to it as possible is measured by means of the calibrated 

 transmitter and search tube. 



With the pressure measured and the search tube removed from the 

 ear, the observer listens to the sound from the source, as shown in 

 Fig. 17 -Ba. Then he listens to the receiver to be calibrated as shown 

 in Fig. 17 -Bb. The electrical input to the receiver is adjusted until 

 the observer judges the sensation to be equal to that from the source. 

 This then gives the pressure produced by the receiver for a given 

 frequency and for a given input to the receiver. 



The Search Transmitter and Its Calibration 



The purpose of the search transmitter is to furnish an instrument of 

 small external dimensions to meet the following requirements: 



1. It must admit of calibration at single frequencies in terms of 



electrical output per unit pressure at the mouth of the tube. 



2. It must be small enough to admit of insertion into the ear canal 



without material distortion of the sound field in the latter. 



