Dual membrane condenser microphones: Post-processing strategies for nonlinear distortion reduction

This paper studies nonlinear distortion in a dual membrane condenser microphone and evaluates three post-processing methods for reducing residual second-harmonic distortion. A nonlinear capacitance model is developed using two asymmetry parameters that represent electrostatic sensitivity mismatch and geometric gap mismatch between branches. The model predicts partial intrinsic cancellation of quadratic distortion in the differential signal, with residual distortion governed by branch asymmetries. Three correction strategies are analyzed: branch-wise correction before recombination, direct correction of the differential signal, and weighted branch combination. Experiments are performed on a dual membrane micro-electro-mechanical systems microphone using a low distortion acoustic excitation setup with harmonic feedback control. All three methods show significant reduction of the second-harmonic distortion in the differential output of approximately 35-40 dB in the measured 110-125 dB SPL range. Practical aspects of distortion reduction in dual membrane micro-electro-mechanical systems microphones are discussed, including issues related to specific microphone geometry and parameter estimation.