The advent of acoustic metamaterials in the beginning of 2000s and very recently of acoustic metasurfaces has created tremendous excitement and efforts in the field of materials science and physics by introducing and building real transformative research and dealing with unprecedented physics and applications. The acoustic/elastic metamaterials and metasurfaces, which can simply be described as designed artificial materials with unusual physical properties, form the core of the present Special Topic published by the Journal of Applied Physics.

Acoustic metamaterials and metasurfaces derive their novel characteristics from the interaction between acoustic waves and designed materials. The field is pushed forward by the desire of controlling acoustic wave propagation using architectured materials and is increasingly driven by fundamental geometric and physical principles which provide both the design strategy and the basis of wave functionalities. Wave manipulation using artificial materials has become a central topic in materials physics due to the fact that material design plays an important role in determining the characteristics of the wave fields either within or outside the material. As such, there has been an exquisite connection between wave physics and materials science and engineering. Acoustic metamaterials and metasurfaces have then emerged as a central field combining the sciences of materials and waves.

The science of acoustic metamaterials and metasurfaces is in a burgeoning stage. They have drawn significant attention from both physics and engineering communities as they are at the crossroads of materials and acoustics engineering. This field, although relatively young, has branched out in many directions, and it has been evidenced that acoustic metamaterials and metasurfaces can manipulate waves in unprecedented ways.

In the present Special Topic (ST) section, we provide the most recent and advanced research on acoustic/elastic metamaterials and metasurfaces and their applications. The theoretical, numerical, and experimental research and analyses on these artificial materials, as well as the demonstration of their performances and potential applications, are discussed.

Acoustic Metamaterials in Transition starts this Special Topic with an interesting invited Perspective article by Ying Wu, Min Yang, and Ping Sheng on the future of acoustic metamaterials and emergent directions of the topic. The authors timely delineate the recent and important developments of metamaterials, metasurfaces, pt-symmetric acoustics (Parity-Time), and the future challenges for the real-world applications that will need breaking the constraints of metamaterials and pushing the boundaries of possibilities.

Acoustic and elastic metamaterials, metasurfaces, topological metamaterials, and phononic crystals form the main topics that have been reported in the invited contributed papers of this Special Topic Section. As an introduction, very basic definitions of the two reported designed artificial materials, metamaterials, and metasurfaces are provided in this Preface to ease the reading for a broader audience even of non-specialists in this field.

1. Acoustic Metamaterials

Acoustic metamaterials can be presented as architectured artificial materials composed of periodic or random distributions of sub-wavelength resonating meta-atoms hosted in a matrix (1D, 2D, or 3D). Under some specific conditions of the composition, design, geometry, and sometimes symmetry, these designed materials can be seen by the impinging waves as homogenous materials (continuum) with unusual physical parameters. The latter can indeed be negative, thus enabling the generation of new physical properties, functionalities, and applications.

2. Acoustic Metasurfaces

Acoustic metasurfaces are planar materials of subwavelength thickness capable of providing local phase shifts over a 2π span or extraordinary sound absorption. The main originality of metasurfaces lies in their unique ability to freely tailor the wave fields such that the phase or/and amplitude can be fully controlled. The acoustic metasurface concept is based on arrays of subwavelength resonating units, including but not limited to Helmholtz resonators, membranes, and coiling-up space structures.

As Guest Editor, I hope that this Special Topic Section will help the readers to gain a better understanding of the acoustic metamaterials and metasurfaces and their unique properties, functionalities, and applications. I also hope that it will inspire the readers to generate new research ideas to broaden the understanding of this exciting and promising relevant topic. This ST includes a Prospective paper and about 30 invited contributions. I would like to thank all the authors who accepted my invitation to contribute to this ST section and who provided their most recent and original research to make it very successful. Many thanks as well to the reviewers for providing useful and constructive feedback on the submitted papers.

Finally, I would like to thank Dr. Andre Anders, Editor-in-Chief of Journal of Applied Physics, for discussion and continuous advice and support, Dr. Benedetta Camarota, Journal Manager at AIP Publishing, for all the assistance on this ST, and many other staff members of AIP Publishing for their great communication and support to disseminate our research results. I acknowledge support from “La Région Grand Est” in France and “Institut CARNOT ICEEL.”