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 plates stay parallel as their spacing oscillates. The team analytically modeled the microphone response in terms of mechanical and electrical parameters such as the separation between the plates, the Young’s modulus of the spacer ring, and the surface potential of the electret (red). The researchers’ initial devices demonstrated characteristics that would be adequate for many potential applications: sensitivities on the order of 5 mV/Pa, a fairly flat frequency response, and reasonable noise levels (down to 23 dBA). And compared with conventional electret microphones, the solid-plate design offers improved mechanical robustness, features low harmonic distortion, and is waterproof. (J. Hillenbrand, S. Haberzettl, G. M. Sessler, J. Acoust. Soc. Am. 134, EL499, 2013, doi:10.1121/1.4826915 .)