  Using the concepts of slow sound and of critical coupling, an ultra-thin
acoustic metamaterial panel for perfect and omnidirectional absorption is
theoretically and experimentally conceived in this work. The system is made of
a rigid panel with a periodic distribution of thin closed slits, the upper wall
of which is loaded by Helmholtz Resonators (HRs). The presence of resonators
produces a slow sound propagation shifting the resonance frequency of the slit
to the deep sub-wavelength regime ($\lambda/88$). By controlling the geometry
of the slit and the HRs, the intrinsic visco-thermal losses can be tuned in
order to exactly compensate the energy leakage of the system and fulfill the
critical coupling condition to create the perfect absorption of sound in a
large range of incidence angles due to the deep subwavelength behavior.
