  Topological insulators are new states of matter in which the topological
phase originates from symmetry breaking. Recently, time-reversal invariant
topological insulators were demonstrated for classical wave systems, such as
acoustic systems, but limited by inter-pseudo-spin or inter-valley
backscattering. This challenge can be effectively overcome via breaking the
time-reversal symmetry. Here, we report the first experimental realization of
acoustic topological insulators with nonzero Chern numbers, viz., acoustic
Chern insulator (ACI), by introducing an angular-momentum-biased resonator
array with broken Lorentz reciprocity. High Q-factor resonance is leveraged to
reduce the required speed of rotation. Experimental results show that the ACI
featured with a stable and uniform metafluid flow bias supports one-way
nonreciprocal transport of sound at the boundaries, which is topologically
immune to the defect-induced scatterings. Our work opens up opportunities for
exploring unique observable topological phases and developing practical
nonreciprocal devices in acoustics.
