  Valley pseudospin, labeling quantum states of energy extrema in momentum
space, is attracting tremendous attention1-13 because of its potential in
constructing new carrier of information. Compared with the non-topological bulk
valley transport realized soon after predictions1-5, the topological valley
transport in domain walls6-13 is extremely challenging owing to the
inter-valley scattering inevitably induced by atomic scale imperfectness, until
the recent electronic signature observed in bilayer graphene12,13. Here we
report the first experimental observation of topological valley transport of
sound in sonic crystals. The macroscopic nature of sonic crystals permits the
flexible and accurate design of domain walls. In addition to a direct
visualization of the valley-selective edge modes through spatial scanning of
sound field, reflection immunity is observed in sharply curved interfaces. The
topologically protected interface transport of sound, strikingly different from
that in traditional sound waveguides14,15, may serve as the basis of designing
devices with unconventional functions.
