Wavefront shaping is a novel technique that has revolutionized the manipulation of light propagation in diffusive media. We have recently extended this concept for acoustic waves by designing and building the first "spatial sound modulator (SSM)," which is an actively reconfigurable acoustic metasurface. The SSM can bestow a set of phase factors to a complex wavefield, thereby controlling their interference outcome. The SSM has already shown exciting functionalities in re-shaping reverberating sound field on demand. We expect a bright future for the SSM as it will not only be used in our everyday life but also open new avenues to study wave propagation in complex and chaotic media.

Phononic crystals are the acoustic analogy to electron’s wave characteristics in crystalline semiconductors. Bragg scattering of an acoustic wave propagating in a periodic structure leads to the formation of acoustic passbands and bandgaps. Since their advent in the early 1990s, phononic crystals have become a versatile platform and an important test ground for generic wave phenomena, as well as advanced physical concepts. New findings in other areas of physics (in particular, optics and condensed matter physics), such as geometric phases, topological insulators, Weyl points, etc. are also keeping this field exciting.a

Physical observables are conventionally described by Hermitian operators, giving them real eigenvalues that represent steady states. Recently, a certain class of non-Hermitian Hamiltonians are found useful in describing wave systems that are associated with loss such as leakage or dissipation, and also with gain such as in a laser. It was further shown that two or more eigenstates can coalesce at a specific parameter point called an exceptional point, at the eigenvalues are degenerate and the eigenvectors become parallel. Thanks to their versatility, acoustic systems are an excellent platform for the study of non-Hermitian physics.