The effective operation of certain electronic, medical, industrial, and testing equipment relies on high-quality focusing and imaging capability, which also plays a vital role in the field of wave physics. Therefore, continuously improving the resolution capacity is essential. However, in a homogeneous medium dominated by the diffraction limit, the best resolution for wave focusing and imaging could only reach half the wavelength corresponding to the lowest operating frequency, significantly hindering the relevant application value. The development of phononic crystals (PCs) and acoustic metamaterials (AMMs) has realized sub-wavelength focusing and super-resolution imaging and attracted increasing research attention in physics, mechanics, engineering, and biomedical science. This Tutorial explained the basic principles and traditional methods of acoustic focusing and imaging. Then, the implementation principles and related research progress of sub-wavelength focusing and super-resolution imaging based on artificial acoustic devices, including but not limited to PCs and AMMs, were systematically discussed. Moreover, a method was introduced to realize sub-wavelength or sub-diffraction focusing by integrating these artificial devices into the time-reversal procedure. Finally, the potential development trends and practical application prospects were presented.

Many advanced physical properties can be realized by using well-designed acoustic metamaterial (AM) structures, which have significant application value in engineering. In particular, thin-walled membrane, plate, and shell-type structures with deep subwavelength thicknesses that can meet light weight requirements have attracted the attention of many researchers and engineers from various specialized fields. This Tutorial systematically introduced the structural design methods, acoustic/elastic wave attenuation and regulation principles, and engineering applications of thin-walled AMs for low-frequency sound insulation, sound absorption, and vibration reduction. In particular, the design methods and sound insulation/absorption properties of thin-walled AMs for realizing narrow-band and broadband sound attenuation were explored. Furthermore, the local resonance bandgap characteristics, quantitative extraction method for the bending wave bandgap, vibration suppression properties, and the design method for local resonance vibration dampers for elastic wave regulation by thin-walled elastic metamaterials were summarized successively. Moreover, other thin-walled AM applications, such as the wavefront steering performance of thin-walled acoustic/elastic metasurfaces, and the active thin-walled AMs, were introduced as well.

We present a theoretical and experimental realization of a thin multi-unit metasurface with multi-order sound absorption that exhibits a continuous near-perfect absorption spectrum in the broadband range of 450 Hz–1360 Hz. The metasurface unit is a perforated composite Helmholtz-resonator (PCHR) that is constructed by inserting one or more separating plates with a small hole into the interior of a Helmholtz resonator (HR). The multi-order sound absorption mechanism can be achieved so that with the original absorption peak and the structural size unchanged, multiple near-perfect peaks are obtained in higher frequencies by a PCHR unit. This extraordinary multi-peak performance is the result of the upgraded multi-degree-of-freedom system with the separating plates, which is explained well by the equivalent acoustic circuit. The specific absorption properties of the PCHR unit are investigated thoroughly with a theoretical approach similar to the plane wave expansion method, and verified via the finite element simulations. On this basis, by precisely balancing the parameters of each unit, the absorption bandwidth of the subwavelength 8-unit metasurface is dramatically broadened about 65% by the proposed mechanism. This work would offer a new guidance for the achievement of the wider absorption band and has great potential in engineering applications.

An artificial acoustic soft boundary on the inner wall of an absorber is constructed by an extremely soft PVC gel, and excellent sound absorption enhancement in the broadband frequency range is obtained.

In this paper, we propose a general design concept for acoustic metamaterials that introduces a ubiquitous synergetic behavior into the design procedure, in which the structure of the design is driven by its functional requirements. Since the physical properties of the widely used, resonant-type metamaterials are mainly determined by the eigenmodes of the structure, we first introduce the design concept through the modal displacement distributions on two typical plate-type structures. Next, by employing broadband sound attenuations that involve both the insulation and absorption as the typical targets, two synergetic coupling behaviors are systematically revealed among the dense resonant modes and multi-cell. Furthermore, through plate-type multiple-cell structures assembled from nine oscillators, the design is shown to realize strong broadband attenuations with either the average sound transmission loss (STL) below 2000 Hz higher than 40 dB or the absorption approximately 0.99 in the range of 400–700 Hz wherein the average absorption below 800 Hz remains higher than 0.8. Finally, two multi-cell plate-type samples are fabricated and then used experimentally to measure the STLs in support of the proposed synergetic coupling design method. Both the computational and experimental results demonstrate that the proposed synergetic design concept could effectively initiate a design for metamaterials that offer a new degree of freedom for broadband sound attenuations.