Microphones step up to the plate

A majority of today’s microphones, including those in mobile phones, laptops, and some recording studios, rely on electrically polarized thin plastic films known as electrets. (See the article by Siegfried Bauer, Reimund Gerhard-Multhaupt, and Gerhard Sessler in Physics Today, February 2004, page 37.) The electret is incorporated in a parallel-plate capacitor, one of whose electrodes—sometimes the electret itself—is a thin membrane, or diaphragm. When sound waves induce mechanical vibrations in the diaphragm, an alternating voltage is generated across the capacitor. But the diaphragm, typically only a few microns thick, stretches as it oscillates, which can introduce harmonic distortion. Joachim Hillenbrand, Sebastian Haberzettl, and Gerhard Sessler of the Darmstadt University of Technology now report on an electret microphone design that features a stiff aluminum plate (yellow in the sketch) some 500 microns thick as the moving electrode. Separated by a thin elastic ring (light blue), the capacitor’s two...