, nThank you very much for your answer. nTaking the difference of the two absolute displacements is what I am currently trying to do, but that doesn't seem to work well for the case where the harmonic displacement is in the same direction as the deformation I am trying to measure. If I shake the support in the longitudinal direction with an amplitude of, for instance, 1mm, I get displacements of 1mm at both the points of interest. Basically, the deformation of the cavity is completely hidden by its rigid motion, which is many orders of magnitude bigger.n