  Time reversal (T) invariant topological insulator is widely recognized as one
of the fundamental discoveries in condensed matter physics, for which the most
fascinating hallmark is perhaps a spin based topological protection, the total
cancellation of scattering of conduction electrons with certain spins on matter
surface. Recently, it has created a paradigm shift for topological insulators,
from electronics to photonics, phononics as well as mechanics, bringing about
not only involved new physics but also potential applications in robust wave
transport. Despite the growing interests in realizing topologically protected
acoustic wave transport, T-invariant acoustic topological insulator has not yet
been achieved. Here, we report the first demonstration of acoustic topological
insulator: a strongly coupled metamaterial ring lattice that supports one-way
propagation of helical edge states under T-symmetry, backscattering immune to
boundary abrupt variations. The very unique thing is the formation of
spin-filtered interface states due to lattice dislocations. The mechanism
underlying the formation of topologically protected edge states and interface
states is applicable in various other wave systems or higher dimensions.
