A High-SPL Full-Range PZT MEMS Speaker for In-Ear Applications

To address the challenge of insufficient full-range sound pressure level (SPL), particularly in the low-frequency range (20–100 Hz), in piezoelectric micro-electromechanical system (MEMS) speakers with limited area, a novel quasi-closed diaphragm design is proposed. The 6.25 mm² diaphragm incorporates double-drive electrodes, a flexible coupling structure, and a central mass, subdivided by slits. Parylene-C acts as an acoustic isolation layer to prevent air leakage, a flexible coupling structure to reduce diaphragm stiffness, and significantly lowers the stress at the ends of the slits, enabling higher excitation voltages for improved SPL. Additionally, the flexible coupling structure reduces both electrode area and power consumption, compared to the rigid coupling structure. Tests conducted on an ear canal simulator show that compared to the rigid coupling design, the proposed speaker achieves a 13–15 dB SPL improvement from 20 Hz to 1 kHz under 2 Vpp excitation, with a 20% reduction in power consumption. Under 60 Vpp excitation, the proposed speaker produces SPL > 93 dB across the frequency range from 20 to 100 Hz, and SPL > 100 dB from 100 Hz to 20 kHz. Under 100 Vpp excitation, the design delivers SPL > 97 dB across the frequency range from 20 to 100 Hz. This work demonstrates the advantage of achieving high full-range SPL with a small diaphragm area through optimized design, offering a promising solution for the advancement of MEMS speakers.