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In this paper we present the impact of acoustic resonances excited by electrostriction in BST thin-films on the Q-factor of a varactor in the GHz range. By changing the process-parameter substrate to target distance while sputtering the thin-films, we observe a change in the acoustic resonance frequencies. To characterize the material properties which determine the acoustic resonance frequencies a model based on a 1D acoustic transmission line is used. The extracted material properties results in a proportional dependency to the changed process parameter. For verification of the acoustic transmission line model 3D simulations have been performed which show good agreement to the line model for lower resonance frequencies, where the difference at higher frequencies becomes significant due to additional higher order modes.