  We theoretically and experimentally investigate visco-thermal effects on the
acoustic propagation through metamaterials consisting of rigid slabs with
subwavelength slits embedded in air. We demonstrate that this unavoidable loss
mechanism is not merely a refinement, but it plays a dominant role in the
actual acoustic response of the structure. Specifically, in the case of very
narrow slits, the visco-thermal losses avoid completely the excitation of
Fabry-Perot resonances, leading to 100% reflection. This is exactly opposite to
the perfect transmission predicted in the idealised lossless case. Moreover,
for a wide range of geometrical parameters, there exists an optimum slit width
at which the energy dissipated in the structure can be as high as 50%. This
work provides a clear evidence that visco-thermal effects are necessary to
describe realistically the acoustic response of locally resonant metamaterials.
