238 



MISCELLANEOUS TRANSDUCERS 



sponse frequency characteristic with record characteristic shown in Fig. 10.7 

 is shown in Fig. 10.9. The output below 1000 cycles is similar to that of 

 the record. Some compensation in the amplifier must be provided to com- 

 pensate for this drooping characteristic. The peak at the low frequencies 

 is due to the resonance of the total mass of the pickup and tone arm W2 

 with the compliances Cmi and Cm2- Below this resonance frequency the 

 mass reactance due to m2 becomes small compared to the impedance of the 

 compliance elements and the output falls off rapidly with decrease in 

 frequency. 



3. Dynafnic Pickup. — A dynamic pickup is a phonograph pickup in 

 which the output results from the motion of a conductor in a magnetic 



Fig. 10.10. Cross-sectional view and equivalent circuit of the mechanical system of adynamic 

 pickup. In the equivalent circuit: Zmr the mechanical impedance of the record. w?i 

 the mass of the needle and voice coil. Cmi and tmi the compliance and mechanical resis- 

 tance of the suspension system, mi the mass of the pickup and tone arm. /m the force 

 generated by a constant velocity generator. The graph shows the voltage response charac- 

 teristic with the record characteristic of Fig. 10.7. 



field. Fig. 10.10 shows a cross-sectional view of a dynamic pickup^'' for 

 the reproduction of hill and dale records. The principal mechanical im- 

 pedance is due to the mass of the needle and coil. The output of the coil 

 is proportional to the velocity. Therefore, the response characteristic is 

 similar to that of the magnetic pickup. 



C. Distortion in Record Reproduction^^'^'^. — The recording and repro- 

 ducing of a phonograph record is a complicated process and there are many 

 sources of nonlinear distortion. The record does not present an infinite 

 impedance to the needle. As a consequence, the vibrating system of the 

 pickup is shunted by the effective mechanical impedance of the record at 

 the needle. Nonlinear distortion will be introduced if the record is a vari- 

 able element. 



1" Frederick, H. A., Jour. Soc. Mot. Pic. Eng., Vol. 18, No. 2, p. 141, 1932. 

 " Di Tore, M. J., Jour. Soc. Mot. Pic. Eng., Vol. 29, No. 5, p. 493, 1938. 

 12 Pierce and Hunt, Jour. Acous. Soc. Amer., Vol. 10, No. 1, p. 14, 1938. 



