62 ANALOGIES 



equivalent to nii, m^, Cmi, Cmi and Vm in the mechanical system and Mi, 

 M2, Cai, Ca2 and r^ in the acoustical system. These equations also show 

 that the currents in the electrical system are equivalent to the velocities 

 in the mechanical system and to the volume currents in the acoustical 

 system. Therefore, the action of a mechanical or acoustical system may 

 be predicted by employing the equivalent electrical circuit of these systems 

 and solving the circuit by the conventional electrical theory. 



ELECTRICAL MECHANICAL ACOUSTICAL 



Fig. 4.4. Electrical, mechanical and acoustical systems of three degrees of freedom. 



4.9. Corrective Networks. — A. Introduction. — A corrective network 

 is a structure which has a transmission frequency characteristic that is 

 more or less gradual in slope. Such a characteristic is obtained when an 

 inductance, capacitance or the combination of both are shunted across a 

 line. Another type of corrective network is an inductance capacitance or 

 combination of both connected in series with a line. Various types of 

 resistance networks may be used as attenuators for matching dissimilar 

 impedances. It is the purpose of this section to illustrate further analogies 

 between electrical, mechanical and acoustical systems having similar trans- 

 mission characteristics. 



B. Inductance in Shunt with a Line and the Mechanical and Acoustical 

 Equivalents . — In Fig. A^.SA an inductance is shunted across a line. If 

 the impedance of the inductance is small compared to the input and out- 

 put impedances, the transmission will be small. If the impedance of the 

 inductance is large compared to the input and output impedances, the 

 attenuation introduced by the inductance will be negligible. Since the 

 impedance of an inductance is proportional to the frequency, the current 

 transmission will increase with frequency as shown by the characteristic 

 of Fig. 4.5D. 



When the mass reactance in the mechanical system of Fig. 4.55 is small 

 compared to the load impedance or driving impedance, the mass will move 

 and there will be very little velocity (motion) transmitted to the load. If 

 the mass reactance is comparatively large, the mass will remain station- 

 ary and the behavior will be practically the same as a directly coupled 

 system. Since the impedance of a mass is proportional to the frequency. 



