232 



MISCELLANEOUS TRANSDUCERS 



D. Inductor Telephone Receiver '^'^•^^. — The effect of the leak between the 

 ear and the ear cap upon the response of a telephone receiver has been out- 

 lined in the preceding sections. Obviously, from a practical standpoint 

 the performance of a telephone receiver should be independent of the leak 

 between the ear and the ear cap. In order to design the vibrating system 

 of the telephone receiver so that constant sound pressure will be delivered 



CONDUCTOR 

 BOLT 



CROSS -SECTIONAL VIEW 



Fig. 10.5. Cross-sectional view and equivalent circuit of the acoustical system of an inductor 

 telephone receiver. In the equivalent circuit Mi the inertance of the diaphragm and con- 

 ductor. Cai and tai the acoustic capacitance and acoustic resistance of the diaphragm 

 suspension. M2 and rAi the inertance and acoustic resistance of the bolt of silk. Ca2 and 

 TAi the acoustic capacitance and acoustic resistance of the cavity behind the diaphragm. 

 Mi the inertance of the tube. Cas the acoustic capacitance of the case volume. M5 and 

 TAi the inertance and acoustic resistance of the hole in the case. Me and tae and Cae 

 the inertance, acoustic resistance and acoustic capacitance of the ear. p the driving pres- 

 sure, p is/m divided by the area of the diaphragm. The graph shows the pressure response 

 frequency characteristic. B. Receiver feeding a closed cavity, /f. Receiver feeding an 

 artificial ear. 



to the ear, the nature of the acoustic impedance looking through the aper- 

 ture of the ear cap must be considered as a part of the vibrating system. 

 The impedance characteristic, looking through the aperture of the ear 

 cap of a telephone receiver, is shown in Fig. 11.15, Sec. 11.45. These 

 characteristics show that the impedance is positive and increases with 

 frequency up to 400 cycles; between 300 and 500 cycles it is practically 

 resistive and above 400 cycles it is negative and decreases with frequency. 

 A generalization of the requirements for maintaining constant sound pres- 



^ Olson and Massa, Jour. Acous. Soc. Amer., Vol. 6, No. 4, p. 240, 1935. 

 ^ Olson, H. F., Jour. Soc. Mot. Pic. Eng., Vol. 27, No. 5, p. 537, 1936. 

 ^^ Olson and Massa, " Applied Acoustics," P. Blakiston's Son and Co., Phila- 

 delphia. 



