  We propose a simple two-dimensional acoustic crystal to realize topologically
protected edge states for acoustic waves. The acoustic crystal is composed of a
triangular array of core-shell cylinders embedded in a water host. By utilizing
the point group symmetry of two doubly degenerate eigenstates at the \Gamma
point, we can construct pseudo-time-reversal symmetry as well as pseudo-spin
states in this classical system. We develop an effective Hamiltonian model for
the associated dispersion bands around the Brillouin zone center, and find the
inherent link between the band inversion and the topological phase transition.
With numerical simulations, we unambiguously demonstrate the unidirectional
propagation of acoustic edge states along the interface between a topologically
nontrivial acoustic crystal and a trivial one, and the robustness of the edge
states against defects with sharp bends. Our work provides a new design
paradigm for manipulating and transporting acoustic waves in a topologically
protected manner. Technological applications and devices based on our design
are expected in various frequency ranges of interest, spanning from infrasound
to ultrasound.
