Within this overarching framework, the project has pursued four interconnected research directions, each addressing a key challenge in low-frequency wave control while sharing a common mosaic-based design philosophy.
(1) Mosaic-type underwater acoustic impedance tube for low-frequency material characterization
The project developed a new generation of underwater acoustic impedance tubes based on a mosaic-type metamaterial architecture composed of hexagonal aluminum units connected by soft silicone elements. This configuration enables the formation of elastic bandgaps in the solid structure over the frequency range of approximately 800 Hz to 10 kHz, effectively suppressing structural resonances that typically limit the performance of conventional water-filled impedance tubes. As a result, accurate inversion of acoustic material properties in water becomes possible within this frequency range, overcoming the constraints imposed by structural resonances in traditional designs.
The lightweight and compact nature of the proposed metamaterial impedance tube eliminates the need for large water tanks, enabling low-frequency underwater acoustic measurements to be conducted in confined laboratory environments. In addition to the experimental realization of the metamaterial tube, the project also established a systematic methodology for material parameter inversion in fluid-filled waveguides, providing a practical and transferable framework for underwater acoustic characterization.
(2) Lightweight airborne sound insulation using mosaic metamaterials.
To overcome the classical mass–law limitation in airborne acoustics, the project developed lightweight metamaterial configurations that combine rigid angular constraints, compliant interlayers, and mosaic-arranged subwavelength plate elements. Through the interplay between structural constraint and elastic decoupling, the proposed designs enable the formation of a zero-frequency bandgap extending up to approximately 6.8 kHz. The effective sound insulation is achieved over a wide frequency range without relying on heavy or bulky materials, offering a lightweight and compact solution for broadband noise control in architectural and transportation applications.
(3) Ultrathin underwater sound absorption.
The project further addressed underwater sound absorption by developing mosaic-type absorbers based on acoustically soft boundaries. Through the collective response of periodically arranged unit cells, ultrathin structures with a thickness of approximately 5 cm achieve effective absorption below 1 kHz. Unlike conventional designs relying on rigid backings, this soft-boundary approach enables lightweight and flexible configurations, offering a compact and practical solution for broadband underwater acoustic absorption.
(4) Tunable elastic metamaterials for elastic wave propagation manipulation.
Finally, the mosaic design strategy was extended to elastic metamaterials with tunable functionalities, enabling the realization of a variety of unconventional wave phenomena. By tailoring the configuration of unit cells and boundary conditions, the project demonstrates programmable control over elastic wave dispersion, including the realization of tailored dispersion such as maxon-like and roton-like dispersion. Beyond dispersion engineering, the proposed approach enables advanced wave manipulation effects such as elastic-wave rainbow trapping and the emergence of higher-order topological states. These results highlight the versatility of the mosaic design framework in generating rich and controllable elastic wave behaviors, paving the way toward reconfigurable and multifunctional structural systems for tunable vibration control and wave-based signal processing.
