An acoustic resonator includes a bottom electrode disposed over a substrate, a piezoelectric layer disposed over the bottom electrode, a top electrode disposed over the piezoelectric layer, and a cavity disposed beneath the bottom electrode. An overlap of the bottom electrode, the piezoelectric layer and the top electrode defines a main membrane region of the acoustic resonator structure. The acoustic resonator further includes an acoustic reflector disposed over the substrate adjacent to the cavity, the acoustic reflector including a layer of low acoustic impedance material stacked on a layer of high acoustic impedance material.