Taking the flexural wave propagating in elastic thin plate as an example, we investigate the mechanism for gap opening in the resonator-based acoustic metamaterials. Results show that the band gap in such a kind of structure depends not only on the abrupt phase change of the wave when it is scattered by the resonators, but also on the retarded phase of wave when it is propagating in host. This means that the dispersion of wave in the structure can be adjusted either by the scattering or by the propagating phase. Based on this understanding, we show that the defect state at subwavelength scale (obtained either by changing locally the resonating property of the resonator or by changing locally the distance between the resonators) can be understood simply by the band gap condition. We show further in this paper that, because the dispersion of the metamaterial can be adjusted by the propagating phase, the structures with negative band at a subwavelength scale can also be achieved by arranging the resonators into a compound lattice.