  Metasurfaces formed of arrays of subwavelength resonators are often tuned to
'critically couple' with incident radiation, so that at resonance dissipative
and radiative damping are balanced and absorption is maximised. Such design
criteria are typically derived assuming an infinite metasurface, whereas the
absorption characteristics of finite metasurfaces, even very large ones, can be
markedly different in certain frequency intervals. This is due to the
excitation of surface waves, intrinsic to resonant metasurfaces, and especially
meta-resonances, namely collective resonances where the surface waves form
standing-wave patterns over the planar metasurface domain. We illustrate this
issue using a detailed model of a Helmholtz-type acoustic metasurface formed of
cavity-neck pairs embedded into a rigid substrate, with geometric and
dissipation effects included from first principles (R. Brand\~ao and O.
Schnitzer, Wave Motion, 97 102583, 2020).

Applied Physics;Fluid Dynamics