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[Display omitted] • An acoustic metafluid made of hollow regular polygons immersed in water is proposed, which can realize independent manipulation of mass density and bulk modulus. • A theory model is established to accurately calculate the effective acoustic parameters of the proposed metafluid. • The underwater impedance-matched gradient index lens and carpet cloak have been designed and experimentally demonstrated based on the proposed metafluid. Tuning the mass density and bulk modulus independently is the key to manipulate the propagation of sound wave. Acoustic metamaterials provide a feasible method to realize various acoustic parameters. However, the relevant studies are mainly concentrated in air, and the huge impedance difference makes it difficult to directly extend these airborne structures to underwater application. Here, we propose a metafluid to realize independent manipulation of the mass density and bulk modulus underwater. The metafluid is composed of hollow regular polygons immersed in the water. By adjusting the side number of the hollow regular polygons and choosing proper materials, the effective mass density and bulk modulus of the metafluid could be modulated independently. Based on the flexible adjustment method, metafluids with same impedance but different sound velocities are designed and used to realize an underwater impedance-matched gradient index lens. In addition, by combining the proposed metafluid with other artificial structures, acoustic parameters with great anisotropy can be achieved, which is exemplified by the design and demonstration of an impedance-matched underwater acoustic carpet cloak. This work can expand the practicability of underwater metamaterials and pave the way for future potential engineering applications in the practical underwater devices. [ABSTRACT FROM AUTHOR]

Efficient acoustic communication across the water–air interface has always been expected in the field of ocean exploration. However, the existing research works are mainly concentrated on the narrow-band transmission based on resonance, which greatly limits the transmission capacity and efficiency. Here, we combined air-based and water-based metafluids to realize an exponential gradient impedance matching layer for broadband water–air sound transmission. By cooperatively adjusting the sound velocity and thickness in the matching layers, we modulated the required acoustic parameters of each layer into a reasonable range, which can be conveniently achieved by the proposed metafluids. A matching layer sample was constructed and validated in a water tank. Experimental results show that the proposed matching layer can achieve an average sound energy transmission enhancement above 16.7 dB from 880 to 1760 Hz across the water–air interface. A water–air acoustic communication scene was further experimentally demonstrated through transmitting a multicolor picture from air to water, which shows extremely high communication capacity and accuracy. Our work is promising for more applications based on water-air transmission and opens an avenue to the design and implementation of the extreme impedance matching case. [ABSTRACT FROM AUTHOR]

We present the design, architecture, and detailed performance of a three-dimensional (3D) underwater acoustic carpet cloak (UACC). The proposed system of the 3D UACC is an octahedral pyramid, which is composed of periodical steel strips. This underwater acoustic device, placed over the target to hide, is able to manipulate the scattered wavefront to mimic a reflecting plane. The effectiveness of the prototype is experimentally demonstrated in an anechoic tank. The measured acoustic pressure distributions show that the 3D UACC can work in all directions in a wide frequency range. This experimental verification of 3D device paves the way for guidelines on future practical applications. [ABSTRACT FROM AUTHOR]

We present a practical design of an underwater acoustic carpet cloak with 2-dimensional version of the pentamode lattice. The quasi-conformal transformation, which is achieved by inverse Laplace's equations with Neumann and Dirichlet boundaries, is used to obtain the required parameters of the impedance matching carpet cloak. The theoretical carpet cloak is pre-divided into 300 cells and then filled with the corresponding pentamode material unit cells to achieve the latticed pentamode carpet cloak. The simulation results indicate that the proposed carpet cloak has a good and broadband cloaking effect. Moreover, the technique in this work can also be used to design arbitrary shaped devices. [ABSTRACT FROM AUTHOR]

Porous materials with corrugated surfaces are widely used in the field of noise control, as they can effectively convert sound energy into heat resulting in sound absorption. It is important to predict the absorption coefficients of sound-absorbing devices for the design of appropriate shape and size. In this study, a semi-analytic method of layered rigorous coupled-wave analysis (LRCWA) is proposed to predict the absorption of nonplanar periodic materials. Starting from the division of corrugated surfaces into multiple layers, we process the sound behavior in each layer as in a rectangular periodic modulation structure. By connecting the interlayer boundary continuity conditions, the acoustic coupling equation of the whole structure can be established. The effectiveness and practicability of the LRCWA method is validated based on the comparisons with the experimental data and the simulation of finite element method. Besides the absorption coefficient that is mainly discussed in this work, the proposed method is universal in analyzing the physical properties of nonplanar periodic structures, which can greatly accelerate the delicate design and optimization of such structures. [ABSTRACT FROM AUTHOR]

Among the different approaches proposed to design acoustic cloaks, the one consisting on the use of an optimum distribution of discrete scatters surrounding the concealing object has been successfully tested. The feasibility of acoustic cloaks mainly depends on the number and shape of the scatterers surrounding the object to be cloaked. This work presents a method allowing the reduction of the number of discrete scatterers by optimizing their external shape, which is here defined by a combination of cubic Bézier curves. Based on scattering cancellation, a two-dimensional directional cloak consisting of just 20 Bézier scatters has been designed, fabricated and experimentally characterized. The method of fundamental solutions has been implemented to calculate the interaction of an incident plane wave with scatterers of arbitrary shape. The acoustic cloak here proposed shows a performance, in terms of averaged visibility, similar to that consisting of 120 scatterers with equal circular cross sections. The operational frequency of the proposed cloak is 5940 Hz with a bandwidth of about 110 Hz. [ABSTRACT FROM AUTHOR]