LAUSR

In ideal fluids, acoustic waves are known as longitudinal sound waves, the particle velocity of which is parallel to the wave traveling direction. The emergence of metasurfaces offers extensive flexibility in acoustic wave manipulations. However, achieving a sound wave with transversely polarized particle velocity by means of metasurfaces is not yet achievable. Here, we demonstrate that a sound wave with polarized particle velocity perpendicular to the traveling direction can exist in an acoustic metawaveguide. The metawaveguide is made of a pair of spaced acoustic metasurfaces, each comprising alternatingly arranged membranes that resonate phase reversibly. Bearing transversely polarized particle velocity, the sound wave shows lateral dependence, frequency selectivity, and insensitivity to longitudinal irregularity, all distinct from that in a traditional waveguide. More interestingly, the transverse particle velocity is axis symmetric, and thus, acoustophoresis traps are formed in the center of the metawaveguide. The advent of a sound wave with transverse particle velocity may open another avenue for more sophisticated acoustic applications, such as resistance-free insulation, acoustic ``circuits,'' and wave-matter interaction related bioacoustics.