A VOICE AND EAR 



305 



The output of a receiver coupled to the ear is ii;ovcrne(l by several 

 factors, among which are: enclosed volume, leakage around the cap, 

 constriction of the ear canal and the yielding of walls and tymi)anum. 

 Resonances in the enclosed volume are also generally present, associ- 

 ated with the dimensions of the chamber. The effects of the acoustic 

 load of an ear coupled to a receiver are, in general, increased damping, 

 higher resonant frequency and a dropping off of the receiver response 

 at low frequencies. The receiver response also shows peaks due to 

 resonances in the enclosed chamber. 



Largely as a matter of convenience, couplers of simple construction 

 imposing a stiffness load have been frequently used for receiver 

 calibrations. It has been recognized that calibrations made in this 

 way do not agree with the performance realized when the receiver is 

 held to the ear and this has led to the development of an artificial ear 

 which will permit the measurement of receivers of any type under 

 conditions closely simulating those under which they are used. 



— AMPLIFIER 



ELECTRICAL 

 NETWORK 



MEASURING 

 APPARATUS 



Fig. 10 — Schematic arrangement of artificial ear. 



The schematic arrangement of this artificial ear is shown in Fig. 10. 

 It comprises: a special coupling device designed to present the same 

 acoustic load to a receiver as does a typical human ear, a small con- 

 denser transmitter serving as the measuring element in the coupler, 

 and means for amplifying and measuring the voltages generated by 

 the condenser transmitter. For certain purposes arrangements are 

 provided for introducing an electrical network of the constant-re- 

 sistance type ^ having a response-frequency characteristic correspond- 

 ing to that of the equal loudness curve ^ of human hearing at the 

 desired sensation level. 



The coupler is shown in Figs. 11 and 12. The cap of the receiver 



under test rests upon a molded soft rubber insert which has the internal 



contour of the auricle. Soft rubber was selected because of its yielding 



qualities which are of importance in two respects: it permits the 



receiver under test to be sealed to the coupler without the aid of such 



substances as petroleum jelly, and it can be readily molded to have 



the desired shape. In this rubber insert is an acoustic leak consisting 



^ Loc. cit. 



' "A Direct Comparison of the Loudness of Pure Tones," B. A. Kingsbury, 

 Phys. Rev., April, 1927. 



