Figure of merit for piezoelectric MEMS speakers

Piezoelectric MEMS speakers, an emerging technology with great promise, face significant challenges in performance evaluation and rational design. Their broadband nature means that responses at every frequency point across the whole operating bandwidth contribute to performance, yet there is no widely recognized weighting approach for fair evaluation. This absence of quantitative criteria makes objective comparisons of different designs difficult, slowing the adoption of new design concepts; and it leads to ambiguous design goals without response balance across frequency bands. Additionally, the current design methods rely on labor-intensive simulations, further prolonging the development process. To address these challenges, two figures of merit (FOMs) obtained via theoretical deduction are proposed in this study. These FOMs facilitate the evaluation of key metrics, such as sound pressure level and energy efficiency over a wide frequency range, enabling quantitative comparisons among various speaker designs. On the basis of FOMs, the design process can be simplified into a single-objective optimization problem, significantly streamlining the speaker design. Using this method, piezoelectric MEMS speakers with ultra-high FOMs and superior performance are demonstrated. The normalized SPLs at 1 and 10 kHz reach an impressive 76.6 and 86.6 dB/mm²/V_rms, respectively, with normalized sensitivities of 91.2 and 91.5 dB/mm²/mW. This achievement validates our FOM theory, representing a notable advancement in the field.