RECORDING AND REPRODUCING OF MUSIC AND SPEECH 515 



bottom curve shows an early type of highly damped singly resonant 

 system. The middle curve is a calibration of a low pass mechanical 

 filter type using lumped loading in the resistance line. The upper 

 cur\e shows the calibration of the recorder shown in Fig. 10. 



The compliance — Co is a mechanical (}uantity for which there is no 

 simple electric analog. In a balanced armature t\pe of structure such 

 as that shown in Fig. 11, the action of the field on the armature, when 

 it is at its center point, is balanced. If. howe\er, the armature be de- 



100 500 1000 



CYCLES PER SECOND 



Fig. 14 — Calibration curve of three types of electromagnetic recorders 



fleeted, a small distance from this equilibrium, there is exerted by the 

 magnetic field a torque tending to pull the armature further away from 

 its center position. The value of this torque for small amplitudes is 

 proportional to the angular displacement. It is therefore seen that this 

 quantity is of the nature of a compliance but that the back force is in a 

 reverse direction to that required for a positive compliance. 



Design of the Reproducing Apparatus 



As the analogy between the mechanical and electrical filter is more 

 perfectly shown in the case of the reproducing equipment, its detailed 

 quantitative description will now be given. Figs. 15 and 16 show re- 

 spectively a diagram of the reproducing system and its equivalent 

 electric circuit. From these diagrams it is evident which units in the 

 mechanical system correspond to the various electrical parts. As 

 the series compliances c^, Ci and Co have been made so large that 



