Audible Focal Spot Generation via a Metasurface-Enabled Parametric Array Loudspeaker

Parametric array loudspeakers (PALs) produce highly directional sound beams but are constrained by poor low-frequency performance and unwanted low-path attenuation. Existing beam focusing engineering alleviates these limitations but relies on active phased arrays, which are costly and requires complex hardware for practical applications. Here, we present a passive acoustic metasurface (AMS) lens that enables broadband focusing of the audible beam generated by an ordinary PAL. The AMS is constructed from Fabry-Pérot-based annular unit cells, whose width is smaller than the carrier ultrasound wavelength, allowing fine spatial phase control for achieving tight focusing. Both simulations and experiments confirm the generation of strongly localized audible focal spots, with measured full-width at half-maximums (FWHMs) below 30mm axially and 6mm laterally across a wide frequency range from 500Hz to 4kHz. Our passive AMS design delivers stronger low-frequency output and steeper path attenuation than conventional PALs, supporting private delivery in public installations, vehicles, healthcare, and immersive environments. Unlike phased-array PALs that rely on computationally intensive beamforming and complex control hardware, the passive AMS offers a compact and low-complexity alternative for direct applications.