No abstract

Over the past few years, acoustic gradient index metasurfaces (GIMs) have been actively studied for the numerous wave control capabilities that they facilitate. Previous research, however, has primarily focused on GIMs that operate in the audible frequency range, due to the difficulties in fabricating such intricate structures at the millimeter and submillimeter scales, for ultrasonic applications. In this work, we design, fabricate, and experimentally demonstrate the working of a hybrid resonant acoustic gradient index metasurface for airborne ultrasound at 40 kHz. The fabrication of such a GIM is made possible by projection microstereolithography, an emerging additive manufacturing technique. Numerical simulations were conducted to verify the metasurface design, and experiments were performed to corroborate these simulations. The stronger dissipation associated with airborne ultrasound is highlighted in this paper. The experimental demonstration of such a metasurface for airborne ultrasound could further its prospects as a candidate for miniaturized acoustic devices.

No abstract

Acoustic holograms have promising applications in sound‐field reconstruction, particle manipulation, ultrasonic haptics, and therapy. This study reports on the theoretical, numerical, and experimental investigation of multiplexed acoustic holograms at both audio and ultrasonic frequencies via a rationally designed transmission‐type acoustic metamaterial. The proposed metahologram is composed of two Fabry–Pérot resonant channels per unit cell, which enables the simultaneous modulation of the transmitted amplitude and phase at two desired frequencies. In contrast to conventional acoustic metamaterial‐based holograms, the design strategy proposed here provides a new degree of freedom (frequency) that can actively tailor holograms that are otherwise completely passive and significantly enhances the information encoded in acoustic metamaterials. To demonstrate the multiplexed acoustic metamaterial, the projection of two different high‐quality metaholograms is first shown at 14 and 17 kHz, with the patterns of the letters N and S. Then, two‐channel ultrasound focusing and annular beams generation for the incident ultrasonic frequencies of 35 and 42.5 kHz are demonstrated. These multiplexed acoustic metaholograms offer a technical advance to tackle the rising challenges in the fields of acoustic metamaterials, architectural acoustics, and medical ultrasound.