Electrical and Mechanical Analogies 

 By W. P. MASON 



Introduction 



During the past few years, apparatus which transfers electrical into 

 acoustical or mechanical energy has received wide application. This came 

 about through the popular use of radios, phonographs, public address 

 systems, and sound motion pictures. While the fundamental principles 

 of such electro-mechanical or electro-acoustic transducers have been known 

 for decades, it is safe to say that the rapid progress and excellent design 

 obtained have been due in a large part to the knowledge derived from the 

 related subject of electrical network theory. 



Two examples may be cited to show the nature and extent of the im- 

 provement. Barton in his "Theory of Sound" (1914) cites measurements 

 on the efficiency of acoustic foghorns operated from an electrical source of 

 power and finds that the efficiency of conversion from electrical into acoustic 

 energy is less than one per cent. Today large loud speakers have been 

 developed which can be used for similar purposes and these have efficiencies 

 of conversion greater than 50 per cent. Another and more striking example 

 is the mechanical phonograph. From the days of its invention by Edison, 

 mechanical phonograph reproducers had been constructed from such me- 

 ch'anical units as needles, diaphragms, horns, and their connecting mechani- 

 cal elements. As late as 1925 the best of such units was capable of 

 reproducing only three octaves. About this time, another mechanical 

 phonograph^ was constructed from the same sort of elements, but with their 

 dimensions and relationships designed according to relations developed 

 in electrical network theory, and the resulting structure was able to repro- 

 duce a frequency band corresponding to five octaves with greater uniformity 

 and an increase in the efficiency of conversion. 



The type of electrical network which has received the greatest application 

 in the design of mechanical and acoustical systems is the electrical wave 

 filter invented by Dr. G. A. Campbell. This may seen surprising at first 

 sight, since the filter is usually regarded as a device for attenuating unwanted 

 frequency bands while passing other frequency bands which it is desired to 

 receive. The filter has two properties which make it of interest in electro- 

 acoustic transducer systems. These are: first, the filter is able to coordinate 



' Maxtk-kl and Harrison, Bell Sys. Tech. Jour.. \'ol. 5, No. .S, p. 49,?, 1926. 



405 



