WAVE FILTERS 69 



system, Fig. 4.7F, the attenuation will increase as the shunt acoustic re- 

 sistance is made smaller. 



L. " T " Type Resistance Network and the Mechanical and Acoustical 

 Equivalents. — The use of " T " resistive networks in electrical circuits for 

 introducing attenuation without reflections or for matching two dissimilar 

 impedances is well known. Figure 4.7G shows a " T " type electrical net- 

 work and the mechanical and acoustical equivalents are shown in Figs. 4.7/f 

 and 4.77. 



M. " TT " Type Resistance Networks and the Mechanical and Acoustical 

 Equivalents. — The " -k " type of network may be used for the same pur- 

 pose as the " T " network of the preceding section. Figure 4.7/ shows a 

 " TV " type electrical network and the mechanical and acoustical equiva- 

 lents are shown in Figs. 4.7X and 4.7L. 



4.10. Wave Filters. — A. Introduction. — The essential function of a 

 wave filter is to let pass desired frequency bands and to highly attenuate 

 neighboring undesired frequency bands. Wave filters may be either elec- 

 trical, mechanical or acoustical. Electric wave filters were invented by 

 Campbell ^ and have innumerable uses in electrical circuits. Acoustic 

 wave filters were invented by Stewart ^ and have been used extensively. 

 Mechanical filters have been employed in all types of mechanisms for many 

 centuries. Acoustical and mechanical wave filters are becoming very im- 

 portant for use imnoise reduction and control of vibration in all types of 

 machinery and appliances. It is the purpose of this section to illustrate 

 and describe the different types of electrical, mechanical and acoustical 

 wave filters. 



B. Types of Wave Filters. — The response frequency characteristics of 

 wave filters are widely different. The more frequently used types are 

 designated as follows: 



Low Pass Wave Filters 

 High Pass Wave Filters 

 Band Pass Wave Filters 

 Band Elimination Wave Filters 



A low pass wave filter is a system which passes currents, velocities or 

 volume currents of all frequencies from zero up to a certain frequency 

 termed the cutoff frequency/c and which bars currents, velocities or volume 

 currents of all higher frequencies. 



A high pass wave filter is a system which passes currents, velocities or 



1 Campbell, G. A., Bell System Tech. Jour., Vol. 1, No. 2, 1922. 



2 Stewart, G. W., Phys. Rev., Vol. 20, No. 6, p. 528, 1922. 



