No abstract

Multifunctional structures such as ventilated sound barriers, have become the focus of recent research on the noise reduction and environmental comfort. However, its large size and complex inner structure hinder its potential applications. Novel structures with side-branch sectorial Helmholtz resonators (SHRs) and double-layered perforated slit plates enlightened by macro- perforated plates to enhance the soundproof performance and facilitate natural ventilation are proposed and experimentally validated. Compared with simple muffler ducts, the combinations with slit plates provide a smoother transmission loss (TL) curve with a broad and continuous TL band. We also study the influences of the independent parts and interactive effects of the assembly on the sound field, including the frequency migration and plate vibration. The proposed sub-wavelength structures with a thickness of 15mm can obtain TL values up to 25dB with a broad bandwidth from 930Hz to 1600Hz. Moreover, soundproof walls can be fabricated by using these structures with plenty of ventilated slits to freely exchange air and heat. This ventilation sound barrier is suitable for acoustic landscape buildings as it covers the main frequency spectrum of a human equal loudness contour.

Broadband sound absorption has consistently been a challenge in designing underwater sound absorption structure. Most research of underwater sound absorption structures achieve broadband sound absorption through structural optimization, which curbs the freedom of designing, and commonly alights it at the expense of increased thickness. In this paper, a method is reported to broaden the frequency band of the underwater sound absorption structure by embedding a membrane-type resonator into the cavity, which forming a membrane-type underwater acoustic absorption metamaterial. We demonstrate the mechanism of membrane-type metamaterial by theory, and verify it by simulation and experiment. The experimental results show that the sound absorption coefficient in the frequency range of 2000-10000 Hz is significantly improved after implanting the membrane-type resonator into the cavity. The average sound absorption coefficient is increased by nearly 17%, and the improvement effect of the sound absorption covers to each frequency point, which is consistent with our expectation. As the case of applying membrane-type metamaterials to the design process of underwater acoustic structures, this research possesses great application potential in acoustic wave communication and device compatibility design technologies.

Camouflage skills are important for understanding the relationship between predators and prey in ecological systems. Mimicry is a common strategy used by creatures to alter their appearance in order to blend in with their surroundings and avoid detection. This paper unveils a previously unknown camouflage strategy that creates false object information and positions of an object in the visual range of radar sensor vision. A dispersion‐modulated illusionary metasurface to realize focal spot coding in a 3‐dimensional space to fit the radar imaging process is proposed. As the radar image indicates, an aircraft image is created by a flat metasurface plane with good polarization adaptation ability. Design, simulation, and experimental results validate the study. This camouflage strategy indicates the complexity of the food chain and can be used by both predators and prey, providing accessible insights for maintaining biological diversity. The metasurface‐based system can be potentially utilized in future radar markers for marine traffic control systems.

No abstract