Bandwidth Enhancement of Piezoelectric MEMS Speaker via Frequency Separation of Resonance and Zero Modes Using Curved Actuator and Joint Spring

This study presents a piezoelectric microspeaker composed of a central diaphragm, curved actuator, and joint spring, designed for wide bandwidths in sound pressure level (SPL) and total harmonic distortion (THD). In this design, the curved actuator generates a large out-ofplane displacement, while the joint spring increases the frequency gap between the in-phase and out-of-phase motions of the diaphragm and actuator, effectively broadening the SPL bandwidth. Moreover, by positioning a single resonance frequency within the human audible range ($20 ~\text{Hz}-20 \text{kHz}$) and shifting the zero mode beyond 20 kHz, the design further extends the THD bandwidth. Measurements show that the proposed microspeaker achieves more than a threefold increase in SPL bandwidth $(>70 ~\text{dB}, 1.4-13 \text{kHz})$ and over a 1.8 -fold improvement in THD bandwidth ($<5 \%, 4-15 \text{kHz}$) compared with the reference design, under a 0.7 Vrms driving signal in a pressure-field ear simulator.