  Wave localization is a ubiquitous phenomenon. It refers to situations that
transmitted waves in scattering media are trapped in space and remain confined
in the vicinity of the initial site until dissipated. Here we report a phase
transition from acoustically extended to localized states in arrays of
identical air-filled bubbles in water. It is shown that the acoustic
localization in such media is coincident with the complete band gap of a
lattice arrangement of the air-bubbles. When the localization or the band gap
occurs, a peculiar collective behavior of the bubbles appears.
