Periodic Reporting for period 1 - MDS-APP-UAF (The Mosaic Design Strategy: Application to Underwater Acoustic Functionalities)
Période du rapport: 2024-02-01 au 2026-01-31
Acoustic and elastic wave manipulation plays a central role in a wide range of scientific and engineering applications, including underwater sensing, noise mitigation, and vibration control. However, effective control of low-frequency waves remains particularly challenging due to their long wavelengths and strong fluid–structure interactions, which often result in bulky structures, limited bandwidth, and reduced practicality. Conventional metamaterial approaches typically rely on strongly resonant or geometrically complex architectures, whose performance can be sensitive to fabrication tolerances and difficult to scale.
In this project, a mosaic design strategy is adopted in which the global wave response is primarily governed by the intrinsic dynamic properties of individual unit cells rather than by complex spatial patterning. By carefully engineering the mechanical and acoustic characteristics of each unit, desired macroscopic behaviors emerge naturally when these units are assembled into periodic structures. This approach allows wave control to be achieved through unit-level design, while maintaining structural simplicity and robustness at the system level.
Building on this concept, the project aims to establish a unified framework for low-frequency acoustic and elastic wave control that is applicable across multiple physical platforms. Rather than treating different applications independently, the project identifies common underlying mechanisms by which unit-cell properties shape global wave behavior. This framework is then applied to four interconnected research directions: mosaic-type underwater acoustic impedance tubes for low-frequency material characterization, lightweight airborne sound insulation enabled by tailored unit responses, ultrathin underwater absorbers based on soft-boundary unit designs, and tunable elastic metamaterials that allow flexible manipulation of wave propagation.
Through this unified perspective, the project seeks to provide scalable and physically intuitive solutions for low-frequency wave control, contributing to advances in underwater acoustics, noise mitigation, and adaptive wave-based systems.
In this project, a mosaic design strategy is adopted in which the global wave response is primarily governed by the intrinsic dynamic properties of individual unit cells rather than by complex spatial patterning. By carefully engineering the mechanical and acoustic characteristics of each unit, desired macroscopic behaviors emerge naturally when these units are assembled into periodic structures. This approach allows wave control to be achieved through unit-level design, while maintaining structural simplicity and robustness at the system level.
Building on this concept, the project aims to establish a unified framework for low-frequency acoustic and elastic wave control that is applicable across multiple physical platforms. Rather than treating different applications independently, the project identifies common underlying mechanisms by which unit-cell properties shape global wave behavior. This framework is then applied to four interconnected research directions: mosaic-type underwater acoustic impedance tubes for low-frequency material characterization, lightweight airborne sound insulation enabled by tailored unit responses, ultrathin underwater absorbers based on soft-boundary unit designs, and tunable elastic metamaterials that allow flexible manipulation of wave propagation.
Through this unified perspective, the project seeks to provide scalable and physically intuitive solutions for low-frequency wave control, contributing to advances in underwater acoustics, noise mitigation, and adaptive wave-based systems.
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.
(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.
This project advances the state of the art in acoustic and elastic metamaterials by establishing a unified mosaic-based design strategy for low-frequency wave control. Rather than relying on highly localized resonances or complex spatial patterning, the proposed approach emphasizes the intrinsic dynamic properties of individual unit cells, whose tailored responses collectively determine the macroscopic wave behavior of the system. In this way, global functionality emerges from unit-level design while maintaining structural simplicity and robustness.
Building on this framework, the project delivers a set of complementary results spanning underwater acoustics and structural wave control. These include the development of low-frequency underwater impedance tubes for accurate material characterization, lightweight acoustic metamaterials for airborne sound insulation, ultrathin underwater absorbers with enhanced low-frequency performance, and tunable elastic metamaterials enabling adaptive wave manipulation. Across these applications, the same mosaic-based principle enables effective performance without relying on heavy structures or highly complex geometries.
Beyond individual demonstrations, the main contribution of the project lies in establishing a transferable and physically intuitive methodology for designing wave-based systems. By linking the intrinsic behavior of unit cells with their collective response in periodic assemblies, the proposed approach provides a general pathway toward scalable, compact, and application-oriented metamaterials. The results open new opportunities for advanced acoustic measurement, noise mitigation, and adaptive wave-control technologies in underwater engineering, structural acoustics, and related fields.
Building on this framework, the project delivers a set of complementary results spanning underwater acoustics and structural wave control. These include the development of low-frequency underwater impedance tubes for accurate material characterization, lightweight acoustic metamaterials for airborne sound insulation, ultrathin underwater absorbers with enhanced low-frequency performance, and tunable elastic metamaterials enabling adaptive wave manipulation. Across these applications, the same mosaic-based principle enables effective performance without relying on heavy structures or highly complex geometries.
Beyond individual demonstrations, the main contribution of the project lies in establishing a transferable and physically intuitive methodology for designing wave-based systems. By linking the intrinsic behavior of unit cells with their collective response in periodic assemblies, the proposed approach provides a general pathway toward scalable, compact, and application-oriented metamaterials. The results open new opportunities for advanced acoustic measurement, noise mitigation, and adaptive wave-control technologies in underwater engineering, structural acoustics, and related fields.