Electro-Mechanical Frequency Response of a Piezo-MEMS Speaker Measured by Laser Diode Feedback Interferometry

Piezo-MEMS speakers based on the inverse piezoelectric effect are becoming increasingly attractive thanks to their low power consumption and are good candidates for miniaturized devices for in-ear audio systems. In this work, we report the use of laser diode feedback interferometry, or self-mixing interferometry, for analyzing the out-of-plane vibration performance of a commercial piezo-MEMS speaker. The photodetected signal provided by the monitor photodiode contained in the laser package was acquired in the frequency domain in the band 0 – 25.6 kHz to analyze the electro-mechanical spectral response of the piezo-MEMS, under conditions of small displacements, by driving it with electrical white noise. Two resonances were detected, one at $\approx 2.8$ kHz, with low quality factor Q and consistent with device specifications, and one at $\approx 21.4$ kHz, slightly above the audible frequency range for humans, with higher Q. To obtain the displacement sensitivity, the MEMS was actuated with sinusoidal signals at different frequencies and amplitudes. The photodetected signal, acquired in the time domain, was used to recover the diaphragm displacement amplitude and phase delay. In particular, we were able to highlight the non-linear spectral response of the high-Q resonance, under conditions of high stress and large displacement, and to reconstruct the hysteretic cycle for upward and downward frequency sweeps. The electro-mechanical frequency response detected with our extremely compact and low-cost interferometric configuration provides a complete overview of the vibration performance of the speaker, useful as a significantly easier preliminary investigation before the electro-acoustic characterization. [2024-0204]