230 



MISCELLANEOUS TRANSDUCERS 



that it is possible to predetermine the response and to evaluate the effect 

 of changes in the constants of the component parts. Comparing the re- 

 sponse of Figs. 10.1 and 10.2 it will be seen that large gains have been 

 effected in uniform response over the entire range and in sensitivity from 

 1500 to 3000 cycles. 



B. Crystal Telephone Receiver. — A crystal telephone receiver^ consists 

 of a light diaphragm connected to a Rochelle salt crystal, Fig. 10.3. The 

 crystal as a driving system was considered in Sec. 6.5. The three corners 

 of a " bender " crystal are fastened to the case. The fourth corner is 



GB"t 





CRYSTAL L 



ELECTRICAL 

 miCMifMi CIRCUIT 



ESISTANCe S 



-CASE 

 CROSS-SECTIONAL VIEW 



1000 

 FREQUENCY 



EQUIVALENT CIRCUIT 



Fig. 10.3. Cross-sectional view, electric circuit and equivalent circuit of the mechanical system 

 of a crystal telephone receiver. In the electrical circuit: e the voltage of the generator. 

 rE\ the resistance of the generator plus the external series resistance, rsc and Cec the resis- 

 tance and electrical capacitance of the crystal. In the equivalent circuit: ma the mass of 

 the diaphragm. Cmv, and r^/o the compliance and mechanical resistance of the suspension. 

 m\, Cm\ and vmx the mass, compliance and mechanical resistance of the crystal. Jm the force 

 generated in the crystal. The graph shows the pressure response frequency characteristic. 

 B. Receiver feeding a closed cavity. A. Receiver feeding an artificial ear. (After Williams.) 



fastened to the diaphragm. The impedance of a crystal telephone is pri- 

 marily a capacitive reactance. Because of this fact the low frequency re- 

 sponse may be raised relative to the high frequency response by connecting 

 a high resistance in series with the receivers. A high resistance must be 

 used because the impedance of the crystal is very high, being 80,000 ohms 

 at 10,000 cycles. 



A response frequency characteristic feeding a plain cavity (acoustic ca- 

 pacitance) is indicated by 5, Fig. 10.3. The response frequency charac- 

 teristic taken on an artificial ear is indicated by A, Fig. 10.3. 



C. Dynamic Telephone Receiver. — A dynamic telephone receiver^ con- 

 sists of a light diaphragm coupled to a voice coil and a suitable acoustical 



2 Williams, A. L., Jour. Soc. Mot. Tic. Eng., Vol. 32, No. 5, p. 552, 1939. 

 ' Wente and Thuras, Jour. Acous. Soc. Amer., Vol. Ill, No. 1, p. 44, 1932. 



