Optimization of dual-electrode ratio and driving phase of piezoelectric MEMS speakers for improving sound pressure level

The integration of dual-electrode driving to enhance diaphragm deformation, thereby increasing the diaphragm's average out-of-plane displacement, has proven to be effective in improving the sound pressure level (SPL) of MEMS speakers. In this work, a piezoelectric MEMS speaker with a quasi-enclosed diaphragm and a dual-top electrode (inner and outer) configuration is proposed. With the inner electrode area ideally configured at 70 % of the whole diaphragm, the MEMS speaker with a diaphragm of 2.5 × 2.5 mm² demonstrates an SPL higher than 73.5 dB in the frequency range from 1 kHz to 20 kHz at a driving voltage of only 2 V_pp. The acoustic performance of the dual-electrode MEMS speaker was also investigated by applying a series of driving signals with different phases and bias voltages to the inner and outer electrodes. It is found that the optimal driving phase varies across different frequency ranges and is dependent on the bias voltage level as well. Moreover, an adequate bias voltage was noted to incrementally enhance the SPL. With proper driving signals optimization, the proposed MEMS speaker achieves an SPL over 88 dB from 100 Hz to 20 kHz at 15 V_pp AC and −2.5 V_DC bias voltage. This study furnishes an effective electrode design methodology and driving scheme for the practical deployment of MEMS loudspeakers in future applications.