Comprehensive Acoustic Characterization of a Broadband Lateral MEMS Ultrasonic Transducer by Near-Field Acoustic Holography and Coupled Multiphysics Simulation
Broadband airborne ultrasonic transducers are attractive for applications such as ranging, echo-location, and acoustic material characterization, where wide bandwidths enable advanced excitation and signal-processing techniques. Recently, a lateral capacitive micromachined ultrasonic transducer (L-CMUT) employing electrostatically actuated in-plane microbeams was introduced as a broadband MEMS transducer concept. However, a comprehensive investigation of its acoustic radiation behavior and quantitative validation of corresponding multiphysics models have not yet been reported. In this work, we present a combined experimental and numerical characterization of a previously reported L-CMUT device operating between 10 kHz and 60 kHz. A coupled electrostatic--structural--thermoviscous finite-element model is combined with an acoustic boundary-element formulation to investigate the complete transduction chain from electrostatic actuation to acoustic radiation. The numerical predictions are validated experimentally using spherical near-field acoustic holography, enabling reconstruction of three-dimensional directivity patterns, radiated acoustic power, and spherical harmonic representations of the radiated sound field. The results show that the broadband behavior originates from strong thermoviscous damping within the narrow beam--stator cavities, resulting in strongly overdamped operation despite a mechanical eigenfrequency near 50 kHz. Good agreement between simulations and measurements is obtained for radiation patterns, radiated sound power, and spherical harmonic content. Furthermore, horn loading primarily redistributes acoustic energy and modifies directivity while only weakly affecting the total radiated acoustic power.