SPL and THD improvement of a cantilever-diaphragm piezoelectric MEMS loudspeaker with Double-S actuators.

Piezoelectric MEMS loudspeakers based on cantilever diaphragms have demonstrated promising electroacoustic efficiency and low-frequency sound pressure level (SPL). However, their total harmonic distortion (THD) significantly increases near the first resonant frequency, and high-frequency SPL (above 10 kHz) rapidly decreases due to the resonance frequency and bandwidth limitations, severely affecting sound quality. This work presents a piezoelectric MEMS loudspeaker featuring a 2.7 µm-thick sputtered PZT film, comprising a cantilever diaphragm and four sets of Double-S actuators. The first resonance frequency of the cantilever diaphragm is 3.2 kHz, and the Double-S actuators introduce an additional resonance frequency at 21.3 kHz, addressing the issues of insufficient high-frequency SPL and poor THD performance. Testing on a 711-ear simulator reveals that, under 1-3 Vpp excitation, incorporating the Double-S actuators leads to an average SPL increase of 23 dB and an average THD reduction of 80% that remains below 0.6% across the 3.2-20 kHz range. Thus, both SPL and THD performance in the mid- to high-frequency range are improved. This work paves the way for the development of high-fidelity piezoelectric MEMS loudspeakers, offering new opportunities to improve sound quality and extend the frequency range for in-ear applications.