Electrical and Mechanical Analogies

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 t h a t the rapid progress and excellent design obtained have been due in a large p a r t to the knowledge derived from the related subject of electrical network theory. T w o examples m a y be cited to show the nature and extent of the improvement. Barton in his "Theory of Sound" (1914) cites measurements on the efficiency of acoustic foghorns operated from an electrical source of power and finds t h a t 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 mechanical units as needles, diaphragms, horns, and their connecting mechanical elements. As late as 1925 the best of such units was capable of reproducing only three octaves. About this time, another mechanical phonograph 1 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 reproduce a frequency band corresponding to five octaves with greater uniformity and an increase in the efficiency of conversion.