revised Supplemental Material

This supplemental material contains a description of the numerical calculation of $\gamma$-ray intensity passing through a multi-frequency vibrating absorber. The calculations are also simplified to optimize the computation time by avoiding negligible expressions. Furthermore, details of the experimental setup, mainly the piezo-transducer, are provided. Next, the procedure of measuring the frequency response function of the piezo-transducer is described. Finally, the normalization of experimentally measured $\gamma$-ray intensity is specified so it can be compared with simulations.