  Acoustic metamaterials (AMMs) are engineered materials, made from
subwavelength structures, that exhibit useful or unusual constitutive
properties. There has been intense research interest in AMMs since its first
realization in 2000 by Liu et al. A number of functionalities and applications
have been proposed and achieved using AMMs. Hyperbolic metamaterials are one of
the most important types of metamaterials due to their extreme anisotropy and
numerous possible applications, including negative refraction, backward waves,
spatial filtering, and subwavelength imaging. Although the importance of
acoustic hyperbolic metamaterials (AHMMs) as a tool for achieving full control
of acoustic waves is substantial, the realization of a broad-band and truly
hyperbolic AMM has not been reported so far. Here, we demonstrate the design
and experimental characterization of a broadband AHMM that operates between 1.0
kHz and 2.5 kHz.
