Nonlinear acoustic impedance of circular vent holes in miniaturized speakers.

Back volume vent holes are designed to boost the low-frequency response of miniaturized electro-acoustic transducers in hearing aids and earphones. The acoustic impedance of these small vent holes considerably alters the frequency response of the speaker. The large built-up pressure within the small back volume of these compact speakers produces high particle velocity through the vent hole, introducing acoustic nonlinearities. These nonlinearities significantly affect the hole impedance, rendering linear prediction models inaccurate. In this work, the impedance of circular holes is calculated for a range of applicable hole sizes and velocities using finite element simulations. Empirical expressions are proposed to find the resistive and reactive components of the impedance as functions of hole diameter, frequency, and particle velocity.