Symmetry-Protected Acoustic “Ghost Tunnels”

By leveraging nonlocal airborne acoustic metamaterials, we propose acoustic "ghost tunnels" that enable perfect tunneling while simultaneously functioning as an "empty" space for external waves incident upon their lateral boundaries. This remarkable property originates from two groups of eigenstates shifted along different directions in the momentum space: one exhibiting doubly degeneracy with directional linear dispersions, which is strictly protected by glide symmetries, and the other showing broad-angle impedance matching, characterized by arc-shaped equal-frequency contour. Upon selective excitation of such eigenstates through different boundaries, acoustic waves perceive the same metastructure, either as a perfect tunnel with strict bounds or as a reflectionless empty space. This exotic acoustic tunnel, verified through both simulations and experiments, establishes a versatile paradigm for novel acoustic manipulation.