Bandwidth enhancement of piezoelectric MEMS microspeaker via central diaphragm actuation and filter integration

This study presents the piezoelectric microspeaker design consisted of the central-diaphragm, connecting-spring, and cantilever-plate actuators to create two resonances in the desired frequency range. In addition to the cantilever-plate actuator, the electrical routing and piezoelectric film are designed to drive the central-diaphragm independently. According to the stress distributions on the microspeaker structure for both lower and higher modes, the all-pass filter circuit is designed and implemented to manage the phase of input signals to the central-diaphragm, thereby changing the motion of the proposed design. Thus, the sound pressure level (SPL) beyond 1 kHz is improved and the SPL zero at specific frequency range is avoided. As a result, the bandwidth enhancement is achieved by the proposed microspeaker. Measurements are conducted under 0.707 V_rms with 9 V_DC driving voltage in standard ear simulator to evaluate the performances of the proposed design. A reference design without a piezoelectric film on the central-diaphragm is also implemented for comparison. Measurements indicate, in the low frequency range (before 4 kHz), the proposed designs have over 3 dB SPL enhancement due to the excitation of central-diaphragm. Moreover, compared to the reference design, proposed designs prevent the occurrence of an SPL zero near 10 kHz (between lower and higher modes) and achieve over 15 dB SPL enhancement. When the driving frequency exceeds the higher mode (14 kHz), the proposed design with the all-pass filter eliminates the SPL zero (at 16.8 kHz) with nearly 8 dB enhancement in the 15–18 kHz frequency range. Thus, this study demonstrates the bandwidth enhancement by the proposed microspeaker design with central-diaphragm actuation and all-pass filter integration.