  This letter investigates a flow-free, pseudospin-based acoustic topological
insulator. Zone folding, a strategy originated from photonic crystal, is used
to form double Dirac cones in phononic crystal. The lattice symmetry of the
phononic crystal is broken by tuning the size of the center "atom" of the unit
cell in order to open the nontrivial topological gap. Robust sound one-way
propagation is demonstrated both numerically and experimentally. This study
provides a flexible approach for realizing acoustic topological insulators,
which are promising for applications such as noise control and waveguide
design.
