Dual-domain metamaterials co-integrated with a compact ultrasonic transducer for highly directional audio generation.

Directional audio systems, enabling personal sound sweet spots and spatially selective audio delivery, typically rely on parametric acoustic arrays that leverage nonlinear interactions of ultrasound beams to overcome the directivity constraint dictated by the wavenumber-aperture product. Such systems, known as parametric array loudspeakers (PALs), traditionally require phased arrays comprising tens to hundreds of ultrasonic emitters-posing persistent challenges for widespread adoption due to high cost, system complexity, and limited design flexibility. Here, we show an alternative form factor: a compact, single-body ultrasonic transducer integrated with dual-domain metamaterials, termed metamaterials-integrated parametric array loudspeakers (MiPALs). Unlike conventional PALs, the MiPAL utilizes a single piezoelectric driver co-integrated with dual-domain metamaterials-consisting of a co-designed acoustic metasurface and elastic meta-units-to yield a simple, cost-effective, and highly integrated architecture. To realize this, we exploit a unified design strategy that integrates theoretical modeling, metamaterial-transducer co-design, and comprehensive quantitative analyses of linear and nonlinear acoustic fields. Experiments demonstrate that the MiPAL generates ultra-broadband directional sound over four octaves, spanning from 500 Hz to 10 kHz. These findings underscore the potential of metamaterial-transducer co-integrated platforms for advanced audio systems, laying the foundation for immersive audio, spatial sound delivery, and extended reality.