Reduction of Nonlinear Distortion in Condenser Microphones Using a Simple Post-Processing Technique

TL;DR

This work tackles nonlinear distortion in single-backplate condenser microphones, including MEMS devices, by introducing a simple post-processing technique that relies on a single parameter $K_0$. The method inverts the quadratic distortion term $u(t) \approx K_0\,[y(t) - y^2(t)]$ to recover a distortion-free output, with $K_0$ either derived from microphone parameters or estimated from a straightforward measurement. Experimental results show substantial distortion reductions: the second harmonic is suppressed by about $40\text{ dB}$ at 1 kHz, while THD drops by factors of up to ~50 depending on SPL; intermodulation products in two-tone and multitone signals are reduced by roughly $20$–$25\text{ dB}$. The approach is hardware-friendly, robust to moderate errors in $K_0$ estimation, and applicable to ASICs or external analog/DSP hardware, offering a practical path to improve accuracy for MEMS and condenser microphones across wide frequency and dynamic ranges, with minimal complexity.

Abstract

In this paper, we introduce a novel approach for effectively reducing nonlinear distortion in single back-plate condenser microphones, i.e., most MEMS microphones, studio recording condenser microphones, and laboratory measurement microphones. This simple post-processing technique can be easily integrated on an external hardware such as an analog circuit, microcontroller, audio codec, DSP unit, or within the ASIC chip in a case of MEMS microphones. It significantly reduces microphone distortion across its frequency and dynamic range. It relies on a single parameter, which can be derived from either the microphone's physical parameters or a straightforward measurement presented in this paper. An optimal estimate of this parameter achieves the best distortion reduction, whereas overestimating it never increases distortion beyond the original level. The technique was tested on a MEMS microphone. Our findings indicate that for harmonic excitation the proposed technique reduces the second harmonic by approximately 40 dB, leading to a significant reduction in the Total Harmonic Distortion (THD). The efficiency of the distortion reduction technique for more complex signals is demonstrated through two-tone and multitone experiments, where second-order intermodulation products are reduced by at least 20 dB.

Figures (9)

• Figure 1: Schematic view of the measurement setup.
• Figure 2: Example of spectra of the measured sound pressure for a 1 kHz excitation (a) with the reference microphone, (b) with the MEMS microphone under test. Note that thanks to the harmonic correction, there are no higher harmonics measured by the reference microphone.
• Figure 3: Sound pressure level of first three harmonics for the microphone under test as a function of reference sound pressure level, measured at 1 kHz. Measurement results are denoted by dots, model results are depicted by dashed lines.
• Figure 4: Block schema of the proposed distortion reduction technique.
• Figure 5: Estimated value of the coefficient $K_0$ as a function of reference sound pressure level. The measurement results are denoted by dots, and the selected optimal value $K_0$ is represented by the gray dashed line.