We report a design and experimental realization of a three-dimensional (3D)
acoustic double-zero-index medium (DZIM), whose effective mass density and
compressibility are nearly zero simultaneously. The DZIM is constructed from a
cubic lattice of three orthogonally-aligned metal rods in air. The combination
of lattice symmetry and accidental degeneracy yields a four-fold degenerate
point with conical dispersion at the Brillouin zone center, where the material
becomes a 3D DZIM. Though occupying a finite volume, the 3D DZIM maintains the
wave properties of a "void space," and enables rich applications. For
demonstration, we fabricate an acoustic "periscope" by placing the designed 3D
DZIM inside a 3D bending waveguide, and observe the unusual wave tunneling
effect through this waveguide with undisturbed planar wavefront. Our findings
establish a practical route to realize 3D DZIM as an effective acoustic "void
space," which offers unprecedented opportunities for advanced sound
manipulation
