  Topologically protected surface modes of classical waves hold the promise to
enable a variety of applications ranging from robust transport of energy to
reliable information processing networks. The integer quantum Hall effect has
delivered on that promise in the electronic realm through high-precision
metrology devices. However, both the route of implementing an analogue of the
quantum Hall effect as well as the quantum spin Hall effect are obstructed for
acoustics by the requirement of a magnetic field, or the presence of fermionic
quantum statistics, respectively. Here, we use a two-dimensional acoustic
crystal with two layers to mimic spin-orbit coupling, a crucial ingredient of
topological insulators. In particular, our setup allows us to free ourselves of
symmetry constraints as we rely on the concept of a non-vanishing "spin" Chern
number. We experimentally characterize the emerging boundary states which we
show to be gapless and helical. Moreover, in an H-shaped device we demonstrate
how the transport path can be selected by tuning the geometry, enabling the
construction of complex networks.
