512 



beLl system technical journal 



values of the elements of the complete network for a free transfer 

 of energy throughout an assigned frequency band, the analogous 

 mechanical elements may be determined in the same manner. 



Fig. 12 — Equivalent electric circuit of the electromagnetic recorder 



General Design of Mechanical Systems 



In designing mechanical systems of the band pass type, the problem 

 is three fold — first, that of arranging the masses and compliances such 

 that they form repeated filter sections; second, determining the magni- 

 tude of these quantities so that with or without transformers the separ- 

 ate sections all have the same cut-off frequenceis^" and characteristic 

 impedances; third, to provide the proper resistance termination. 

 Where the transmitted mechanical power has not been radiated as 

 sound this third part has been one of the most difficult to fulfill. 



In designing these systems, practical difficulties arose — first, the 

 difficulty of insuring that the parts vibrated in the desired degrees of 

 freedom only, and second, the difficulty of determining the magnitudes 

 of the various effective masses, compliances and resistances. Before 

 the work to be described could be carried out practically it became 

 necessary to develop a method of measuring mechanical impedances". 



1" It is of course permissible to have a section having a higher cut-off than the others 

 Ijrovidcd its characteristic impedance is the same as that of the others over the trans- 

 mission band of those having the lower cut-off. 



^' Kenneily, A. E. and Affel, H. A., "The Mechanics of Telephone Receiver Dia- 

 phragms, as L)erived from their Motional Impedance Circles," Proc. A. A. A. S., Vol. 

 51, No. 8, November, 1915. 



Kenneily, A. E. and Pierce, G. VV., "The Impedance of Telephone Receivers as 

 Affected by the Motion of their Diaphragms," Proc. A. A. A. S., Vol. 48, No. 6, Sep- 

 tember, 1912, 



