Topological phase transition and edge states in two-dimensional acoustic systems

The discovery of topological acoustics has revolutionized the fundamental concepts of sound propagation, giving rise to strikingly unconventional acoustic edge modes immune to scattering. In this review we introduce the concept of topological acoustics and several topologically nontrivial states in two-dimensional acoustic systems. We mainly focus on the phononic quantum Hall effects based on circulating flow, Floquet topological insulators based on coupled resonator ring waveguides, phononic topological insulators mimicking quantum spin Hall effects and the phononic valley Hall effects.