  We report on a detailed study of the optical response and $T_c-\rho$ phase
diagram ($T_c$ being the superconducting critical temperature and $\rho$ the
normal state resistivity of the film) of granular aluminum, combining transport
measurements and a high resolution optical spectroscopy technique. The
$T_c-\rho$ phase diagram is discussed as resulting from an interplay between
the phase stiffness, the Coulomb repulsion and the superconducting gap
$\Delta$. We provide a direct evidence for two different types of well resolved
sub-gap absorptions, at $\omega_1\simeq\Delta$ and at
$\Delta\lesssim\omega_2\lesssim2\Delta$ (decreasing with increasing
resistivity).
