  We presented an acoustic 'meta-atom' model of hollow steel tube (HST). The
simulated and experimental results demonstrated that the resonant frequency is
closely related to the length of the HST. Based on the HST model, we fabricated
a two-dimensional (2D) acoustic metamaterial (AM) with negative effective mass
density, which put up the transmission dip and accompanied inverse phase in
experiment. By coupling the HST with split hollow sphere (SHS), another kind of
'meta-atom' with negative effective modulus in the layered sponge matrix, a
three-dimensional (3D) AM was fabricated with simultaneously negative modulus
and negative mass density. From the experiment, it is shown that the
transmission peak similar to the electromagnetic metamaterials exhibited in the
double-negative region of the AM. We also demonstrated that this kind of
doble-negative AM can faithfully distinguish the acoustic sub-wavelength
details ({\lambda}/7) at the resonance frequency of 1630Hz.
