  We report the experimental demonstration of broadband negative refractive
index obtained in a labyrinthine acoustic metamaterial structure. Two different
approaches were employed to prove the metamaterial negative index nature:
one-dimensional extractions of effective parameters from reflection and
transmission measurements, and two-dimensional prism-based measurements that
convincingly show the transmission angle corresponding to negative refraction.
The transmission angles observed in the latter case also agree very well with
the refractive index obtained in the one-dimensional measurements and numerical
simulations. We expect this labyrinthine metamaterial to become the unit cell
of choice for practical acoustic metamaterial devices that require broadband
and significantly negative indexes of refraction.
