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.