ELECrRJC.U. A.\n M/rilAX/CAI. ASM.OCIES 41,? 



early mufflers and introduce considerably less back pressure on the engine 

 or blower. 



Other uses to wiiicti mechanical filters have been put are in obtaining 

 shockproof mountings and vibration damping devices, in obtaining vibra- 

 tion and noiseproof walls and tloors, and in obtaining constant. speed motors 

 in which the effects of gear irregularities are removed b}' the use of a low- 

 pass mechanical tilter. 



Mfxhanical and Electro-Mkchanical Countp:rparts of Electrical 



Filters 



Although combinations of electrical elements were first studied and 

 applied in w?.ve filters and other structures, it does not follow that they 

 have any inherent advantages over analogous combinations of mechanical 

 or electro-mechanical elements which can be used as filters. In fact, ele- 

 ments which depend on mechanical motion have the great advantage that 

 they have very little energy dissipation associated with their motion and, 

 hence, the equivalent mechanical elements have a higher ratio of reactance 

 to resistance, or "Q," than do their electrical counterparts. The result is 

 that considerably more selective filters can be made from mechanical or 

 electrcM-mechanical elements than can be obtained by employing electrical 

 coils and condensers. 



The first attempts'- '■* along this line were made in substituting masses 

 for coils and springs for condensers in standard electrical filter configura- 

 tions. This work resulted in usable filters up to several thousand cycles 

 in frequency, which have been used for certain special purposes. 



More recently, electro-mechanical elements have been used to take the 

 place of some or all of the electrical elements of a filter and this work has 

 resulted in filters with markedly superior characteristics to those obtained 

 with filters using only electrical elements. The type of electro-mechanical 

 element which has been used most extensively in selective filters is the piezo- 

 electric crystal and particularly the quartz crystal. This element has the 

 advantage of an electro-mechanical converting system in the piezo-electric 

 effect and a very high mechanical (). Moreover, a quartz crystal is very 

 stable mechanically and can be cut so that its frequency changes very little 

 over a wide temperature range. For these reasons, quartz crystals have 

 been applied extensively when it is desirable to obtain a narrow band filter 

 or a very selective filter. 



'- This work was carried out principally ])>• Mcssers. Hartley, Lane and Wegel. 



'' The use of a mechanical tiller in visually studying the properties of a wave filter is 

 described in a paper ".\ Mechanical Demonstration of the Properties of Wave Filters," 

 C. E. Lane, SM.P.E. Jour. Vol. 24, pp. 206 220, .March l^vy 



