  We report on the theoretical and experimental realization of a
double-zero-index elastic waveguide and on the corresponding acoustic cloacking
and supercoupling effects. The proposed waveguide uses geometric tapers in
order to induce Dirac-like cones at $\vec{k} = 0$ due to accidental degeneracy.
The nature of the degeneracy is explored by a $k\cdot p$ perturbation method
adapted to thin structural waveguides. Results confirm the linear nature of the
dispersion around the degeneracy and the possibility to map the material to
effective medium properties. Effective parameters, numerically extracted using
boundary medium theory, confirm that the phononic waveguide maps into a double
zero index material. Numerical and experimental results confirm the expected
cloacking and supecoupling effects typical of zero index materials.
