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Abstract: This work enunciates the principles of SAW design from a physical perspective, examining the standard engineering modelling assumptions in some detail. An integral form analytical expression based on 2D elastic isotropic theory is validated against a COMSOL Multiphysics® simulation, corresponding to the traditional COM approach. Next, a COMSOL model of the full 3D problem, including anisotropy, piezoelastic couplings, and surface electrical and mass loading effects is developed and solved. The results show quite sensitive phase variation in the output not captured by the standard approach, highlighting the need for careful design in phase-sensitive applications and the power of the FE multiphysics perspective in elucidating complex engineering design problems.