RECORDING AND REPRODUCING OF MUSIC AND SPEECH 505 



in the filter have impediinces similar in character but different in abso- 

 lute magnitude they may be joined by a suitable transformer. 



Many early attempts were made to design mechanical transmission 

 systems having a wide frequency range in which highly damped single 

 or multi-resonant systems were employed. In these attempts both of 

 the obvious methods of increasing the damping were used, namely, 

 that of adding a resistance to the system and that of increasing the 

 value of the compliance and decreasing mass in such proportion as to 

 maintain the same natural frequency. The former of these methods 

 reduces the sensitivity of the system at the point where it is most efft- 

 cient (See Fig. 9), while the second method increases the response at 

 the points where the system is less sensitive, namely, away from its 

 resonance point. Fig. 9 shows four curves — first, a singly resonant sys- 



1000 



FREQUENCY 



Fig. 9 — -Velocity response for various values of mechanical constants 



tem, Curve A ; second, the same system with friction added. Curve B; 

 third, the same system without the added friction but with an increase 

 in compliance and a decrease in mass such that the natural period re- 

 mains the same. Curve C; and fourth, a band pass type of circuit 

 whose resistance impedance is the same as that of the system shown in 

 Curve A. (See Curve D.) 



The results of filter theory have shown how these resonances should 

 be coordinated so that when a proper resistance termination is used 

 high elftciency and equal sensitivity are obtained over a definite band 

 of frequencies by elimination of response to all frequencies outside the 

 band. With the electrical case of a repeated filter, each section con- 

 sidered by itself resonates at the same frequency but when combined 

 into a short-circuited filter of n sections, there will be n natural fre- 

 quencies. However, when such a system is terminated with a resist- 



