Response of an edge‐supported circular membrane electret earphone

Abstract

In the simplest model of an earphone, sound pressures are generated at the eardrum by adiabatic changes of the volume of air in the ear cavity. In capacitance earphones these volumetric changes result from the dynamic motion of the earphone diaphragm. In the treatment presented here the dynamic behavior of an edge‐supported, circular membrane electret earphone is studied analytically in a two‐step procedure. First, a general series solution for the static deformation of a foil electret suspended over a backplate is determined. It is possible to determine a stability criterion for the system, which if not met, results in collapse of the foil onto the backplate. Second, a series solution for the dynamic deflection is determined. The dynamic deflection may be divided into a linear response and a distortion response. Both depend on the static deflection. With this analysis it is possible to predict and optimize the response of an edge‐supported circular electret earphone.