Mechanically-open and acoustically-closed MEMS loudspeakers for in-ear applications

MEMS loudspeakers for in-ear applications have been attracting a growing interest in recent years, promising to overcome the limits in terms of form factor, power efficiency and cost of non-MEMS loudspeakers. The piezoelectric actuation principle is one of the most investigated, thanks to the introduction of high precision piezoelectric thin films with high electro-mechanical coupling coefficients in the microfabrication processes. This work presents a new set of high-performance piezoelectric MEMS speakers, consisting of PZT-driven thin plates properly shaped through a set of narrow slits. The latter serve the twofold purpose of enhancing the mechanical compliance of the speaker diaphragm and limiting the acoustic short-circuit between the speaker front and rear side. The diaphragm geometry, evolution of a design previously proposed by the Authors, is properly modified to improve the acoustic output and power efficiency. The proposed devices feature a Sound Pressure Level (SPL) at 30Vpp greater than 108 dB SPL in the whole audible range, a Total Harmonic Distortion (THD) at 1 kHz at 94 dB SPL lower than 1% and a compact footprint of 4.5x4.5 mm2. Experimental results are compared with a FEM-assisted lumped element model demonstrating a good match between them.