The increasing popularity of sandwich and composite double panel structures stems from their better sound insulation properties over a wide frequency range with respect to their single panel counterparts. However, in the design of such multi-layer panels particular attention must be paid to the resonance dominated region of the Sound Transmission Loss (STL), where the Mass-Spring-Mass resonance causes a frequency region of poor acoustic performance. In the present contribution, a metamaterial unit cell endowed with an ultra-wide band gap is integrated with faceplates to create a novel acoustic insulationsandwich panel. The panel is realized with a single material and with a properly designed mechanical stiffness that does not affect results in terms of acoustic wave attenuation over a wide frequency range, making at the same time the internal core self-sustained. The STL of the panel is numerically evaluated through a plane wave tube model. The model represents a simple tool to predict and optimize the behaviour of the panel in the resonance dominated region of the STL. The acoustic performances of the panel are validated through an experimental campaign on a Nylon prototype, which is 3D printed through Selective Laser Sintering technique.