Acoustic metamaterials can manipulate and control sound waves in ways that are not possible in conventional materials. Metamaterials with zero, or even negative, refractive index for sound offer new possibilities for acoustic imaging and for the control of sound at subwavelength scales. The combination of transformation acoustics theory and highly anisotropic acoustic metamaterials enables precise control over the deformation of sound fields, which can be used, for example, to hide or cloak objects from incident acoustic energy. Active acoustic metamaterials use external control to create effective material properties that are not possible with passive structures and have led to the development of dynamically reconfigurable, loss-compensating and parity–time-symmetric materials for sound manipulation. Challenges remain, including the development of efficient techniques for fabricating large-scale metamaterial structures and converting laboratory experiments into useful devices. In this Review, we outline the designs and properties of materials with unusual acoustic parameters (for example, negative refractive index), discuss examples of extreme manipulation of sound and, finally, provide an overview of future directions in the field. Acoustic metamaterials can be used manipulate sound waves with a high degree of control. Their applications include acoustic imaging and cloaking. This Review outlines the designs and properties of these materials, discussing transformation acoustics theory, anisotropic materials and active acoustic metamaterials.

Controlling sound with acoustic metamaterials

Dynamics of Phononic Materials and Structures: Historical Origins, Recent Progress, and Future Outlook

We report a new type of phononic crystals with topologically nontrivial band gaps for both longitudinal and transverse polarizations, resulting in protected one-way elastic edge waves. In our design, gyroscopic inertial effects are used to break the time-reversal symmetry and realize the phononic analogue of the electronic quantum (anomalous) Hall effect. We investigate the response of both hexagonal and square gyroscopic lattices and observe bulk Chern numbers of 1 and 2, indicating that these structures support single and multimode edge elastic waves immune to backscattering. These robust one-way phononic waveguides could potentially lead to the design of a novel class of surface wave devices that are widely used in electronics, telecommunication, and acoustic imaging.

Topological Phononic Crystals with One-Way Elastic Edge Waves

Phononic crystals (PCs) and metamaterials (MMs) can exhibit abnormal properties, even far beyond those found in nature, through artificial design of the topology or ordered structure of unit cells. This emerging class of materials has diverse application potentials in many fields. Recently, the concept of tunable PCs or MMs has been proposed to manipulate a variety of wave functions on demand. In this review, we survey recent developments in tunable and active PCs and MMs, including bandgap and bandgap engineering, anomalous behaviors of wave propagation, as well as tunable manipulation of waves based on different regulation mechanisms: tunable mechanical reconfiguration and materials with multifield coupling. We conclude by outlining future directions in the emerging field.

Tunable and Active Phononic Crystals and Metamaterials

Coding acoustic metasurfaces can combine simple logical bits to acquire sophisticated functions in wave control. The acoustic logical bits can achieve a phase difference of exactly π and a perfect match of the amplitudes for the transmitted waves. By programming the coding sequences, acoustic metasurfaces with various functions, including creating peculiar antenna patterns and waves focusing, have been demonstrated.

Coding Acoustic Metasurfaces

We designed a two-dimensional acoustic metamaterial with periodical array of split hollow sphere (SHS) and investigated its transmission behaviors in the impedance tube system. The results showed that the acoustic metamaterial exhibited a transmission dip and inverse phase close to 5 kHz, which indicated the local resonance of SHS. Based on the homogeneous-media theory, the effective modulus of the acoustic metamaterial was calculated to be negative. The finite element method and resonant model simulation results also confirmed the local resonance and negative response in acoustic metamaterial, which agreed well with experiment results. This metamaterial is able to achieve the double negative acoustic metamaterial conveniently by arraying acoustic “atoms” with negative mass density in the matrix.

Two-dimensional acoustic metamaterial with negative modulus

Nowadays, the acoustic devices are developing toward miniaturization. However, conventional materials can hardly satisfy the requirements because of their large size and complex manufacturing process. The introduction of acoustic metasurfaces has broken these restrictions, as they are able to manipulate sound waves at will by utilizing ultrathin planar metamaterials. Here, a simple acoustic metasurface is designed and characterized, whose microstructure is constructed with a cavity filled with air and two elastic membranes on the ends of cavity. By appropriately optimizing the configurations of microstructures, the steering of transmitted wave trajectory is demonstrated, and some extraordinary phenomena are realized at 3.5 kHz, such as planar acoustic axicon, acoustic lens, the conversion from spherical waves to plane waves, and the transformation from propagating waves to surface waves.

Manipulation of transmitted wave front using ultrathin planar acoustic metasurfaces

This paper presents an acoustic metasurface (AMS) model consisting of split hollow sphere (SHS) resonator arrays with the property of negative modulus. It shows that the AMS can imprint phase discontinuities on an acoustic reflected wave as it traverses the interface between two media. By designing suitable phase gradients, the AMS enables the perpendicularly incident acoustic wave to be converted to a surface wave or reflected in any angle. Four kinds of AMSs, which can anomalously manipulate the reflected wave's direction, are simulated to fulfill the generalized Snell's law. The results provide an available and simple path to experimentally achieving the AMS.

https://iopscience.iop.org/article/10.1088/0022-3727/48/4/045303/pdf

The anomalous manipulation of acoustic waves based on planar metasurface with split hollow sphere

We presented an acoustic “meta-atom” model of hollow steel tube (HST). The simulated and experimental results demonstrated that the resonant frequency is closely related to the length of the HST. Based on the HST model, we fabricated a two-dimensional (2D) acoustic metamaterial (AM) with negative effective mass density, which put up the transmission dip and accompanied inverse phase in experiment. By coupling the HST with split hollow sphere (SHS), another kind of “meta-atom” with negative effective modulus in the layered sponge matrix, a three-dimensional (3D) AM was fabricated with simultaneously negative modulus and negative mass density. From the experiment, it is shown that the transmission peak similar to the electromagnetic metamaterials exhibited in the double-negative region of the AM. We also demonstrated that this kind of double-negative AM can faithfully distinguish the acoustic sub-wavelength details ( λ / 7 ) at the resonance frequency of 1630 Hz.

Double-negative acoustic metamaterial based on hollow steel tube meta-atom

This work presents a double-negative acoustic metamaterial (AM) constructed using unitary meta-molecule structure. Analogy to electromagnetic metamaterials, transmission and reflection parameters are obtained by experiments in free space. The refractive index, effective mass density, and effective bulk modulus are found to be negative in a specific frequency region by retrieving the effective parameter from experimental data. In contrast to the reference acoustic field distribution through sponge substrate, the slab focusing phenomenon of the fabricated metamaterial lens is clearly displayed within the range of double-negative frequency at 5.7 kHz. The negative refraction is demonstrated in a triangular sample with refraction index of −0.78, which agrees well with the derived results. The proposed meta-molecule model is an effective approach to preparing integrative AMs, and the method of studying AMs is extended to completely free space.

https://iopscience.iop.org/article/10.1088/0022-3727/46/47/475105/pdf

Changlin Ding

Papers

Two-dimensional acoustic metamaterial with negative modulus

Manipulation of transmitted wave front using ultrathin planar acoustic metasurfaces

The anomalous manipulation of acoustic waves based on planar metasurface with split hollow sphere

Double-negative acoustic metamaterial based on hollow steel tube meta-atom

Double-negative acoustic metamaterial based on meta-molecule