234 



MISCELLANEOUS TRANSDUCERS 



cross-sectional view and the equivalent circuit of a mechanical phonograph 

 is shown in Fig. 10.6. The system consists essentially of a diaphragm ruz 

 coupled to a needle Cm\ driven by a phonograph record. To improve the 

 radiation efficiency, the diaphragm is coupled to the horn Zmh- The record 

 mechanical Zmr impedance is usually large compared to the impedance of 

 the remainder of the system save at the high frequencies. The record 





CROSS-SECTIONAL VIEW f HtuuLNi-r 



Fig. 10.6. Cross-sectional view and equivalent circuit of a mechanical phonograph. In the 

 equivalent circuit: Zmr the mechanical impedance of the record. Cmi, Cmi, Cms, Cui, 

 Cui, Cui, and Cmi the compliances of the needle, the needle holder arm pivot, the needle 

 holder arm, the connector, the spider, the suspension and the air chamber. m\, m^ and rriz 

 the mass of the needle holder arm, the spider and thediaphragm. %mh the impedance at the 

 throat of the horn, ju the force generated by a constant velocity generator. The graph 

 shows the pressure response frequency characteristic of a console type mechanical phono- 

 graph. (After Maxfield and Harrison.) 



mechanical impedance is a function of the type of material. Obviously, 

 it is higher for the harder materials. The generator in the equivalent cir- 

 cuit of this system is of the constant current type. That is, Jm delivers 

 constant velocity to the equivalent circuit. That is, the velocity is indepen- 

 dent of the impedance of the load. 



The velocity response frequency characteristic of a typical phono- 

 graph record for constant voltage input into the microphone amplifier 

 (see Sec. YIAF) is shown in Fig. 10.7. To prevent overcutting the groove 

 the system is compensated so that the amplitude for constant input is es- 

 sentially independent of the frequency below 500 cycles. Therefore, the 

 velocity under these conditions falls off 6 db per octave below 500 cycles. 

 From this characteristic and the equivalent circuit, the performance of the 

 system may be determined. 



The response frequency characteristic of a mechanical phonograph of 

 the console type is shown in Fig. 10.6. 



B. Phonograph Pickups. — A phonograph pickup is an electromechanical 



