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The quality factor of the traditional Solidly Mounted Resonator (SMR) is limited by substrate losses, as the traditionally employed acoustic mirror reflects longitudinal waves but not shear waves. Modern mirrors do reflect both waves, but design rules for such mirrors have not been published so far. We propose a systematic design procedure derived from optics leading to a novel embodiment for the acoustic mirror which effectively reflects both longitudinal and shear waves. This method can be applied for the acoustic mirror design for any given material combination. An analytical model is presented; its agreement with FEM simulations is good. With the optimized design, we can obtain a minimum transmission for longitudinal and shear waves of -25 dB and -20 dB at resonance frequencies for longitudinal and shear waves, respectively, for various reflector material combinations.