316 



BELL SYSTEM TECHNICAL JOURNAL 



(a) The presence of the search transmitter does not appreciably affect 



the deflection of the disc. 

 {b) The output of the transmitter is determined solely by the position 



of the mouth of the tube in the sound field; i.e., it is independent 



of the angle at which the tube may be pointing, and of any 



rotation about its axis, 

 (c) The output of the transmitter with the mouth of the tube closed 



must be small compared to that with the mouth open. 

 {(l) The form of the sound wave must be such that the relation between 



the velocity and pressure at the mouth of the tube is known. 



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F"ig. 18 — Search transmitter calibration by means of Rayleigh disc. 



Measurement oj Pressure in Ear Canal Without Receiver 

 Pressure measurements are made as far inward in the canal as 

 consistent with perfect safety to and comparative comfort of the 

 subject. Most ears show a distinct bend in the canal accompanied 

 by a flange which is mainly responsible for obstructing the view of the 

 tympanum. It has been found possible to make measurements at a 

 distance of from 0.5 cm. to 1 cm. past that flange, toward the 

 tympanum. It is doubtful if measurements can be made at a point 



