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BELL SYSTEM TECHNICAL JOURNAL 



the same order of magnitude as those observed looking into the 

 auditory canal of a human ear. In considering Fig. 11, it should be 

 noted that, due to the irregular contour of the auricle, one sectional 

 drawing cannot adequately portray the shape of the opening in the 

 molded soft rubber insert. 



The box shown in Fig. 12, in which the artificial ear is mounted, 

 houses a two-stage amplifier for use with the small condenser trans- 

 mitter. The mechanical structure around the receiver is used for 

 centering it on the coupler and to hold it in position at a definite 

 pressure, the force, of course, always exceeding the weight of the 

 receiver. 



The magnitude of the impedance (Z) indicated in Fig. 11 was 

 measured looking through the aperture of a conventional type of 

 receiver cap held in a normal manner to the ear by each of 14 men. 

 As might be expected, wide variations were observed between indi- 

 vidual ears, particularly for the lower frequencies. Considerable 

 variation was also observed on repeated tests on an individual. Table 

 I shows the magnitude and range for both the resistance and reactance 



TABLE I 

 Acoustic Impedance of Ears Looking through Aperture of Receiver Cap 



(1) With no leak. 



(2) With typical leak between receiver cap and ear. 



components of the acoustic impedance observed in the measurements 

 made at several frequencies. Supplementing these data is Fig. 13 

 which shows the acoustic resistance and reactance for typical human 

 ears with and without a leak between the receiver cap and the ear, 

 together with similar data on the artificial ear. It will be noted that 

 the various impedance curves for comparable conditions of the human 

 and artificial ears are quite similar in shape. There is, however, 

 some discrepancy in the magnitudes of the impedance for comparable 



