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.

Acoustic/elastic metasurfaces as a kind of two-dimensional metamaterials are of subwavelength thickness and show remarkable ability of acoustic/elastic wave manipulation. They have potential applications in various fields such as acoustic imaging, communications, cloaking, camouflage, vibration/noise control, energy harvesting, and nondestructive testing. In this review, we mainly summarize recent developments in acoustic/elastic phase gradient metasurfaces, including design principles, design of functional elements, wave field manipulation with applications, design of tunable metasurfaces, as well as the emerging digital coding metasurfaces. At last, we outline the future research directions in this field.

Previously reported acoustic metasurfaces consisting of fixed channels are untunable to meet broadband requirements and alterable functionalities. To overcome this limitation, we propose a screw-and-nut mechanism of tunability and design a type of continuously tunable acoustic metasurface with unit components of helical cylinders which are screwed into a plate. The spiral channel length can be tuned continuously by the screwed depth, and thus a metasurface with a continuously tunable acoustic phase of independent pixels is attained. Different distributions of the unit components can form an arbitrary metasurface profile. We also develop an approximate equivalent medium model to predict the tunability of the unit component. As an example, we present the design details of a circular tunable metasurface for three-dimensional (3D) adjustable broadband acoustic focusing of different airborne sound sources in a wide frequency range. A sample of the metasurface is manufactured by polylactic acid plastic with 3D-printed helical cylinders. Experiments of the adjustable acoustic focusing are performed to validate the equivalent medium model and the finite element simulation. It is shown that the results of the equivalent medium model, the finite element simulation, and the experiments are in good agreement.