The Swiss plate geophone (SPG) system is an indirect bedload surrogale monitoring lechnique used to quantify bedload transport and retrieve information about the grain-size distribution from the recorded seismic signal. However, the PG system still requires direct measurements of bedload transport for calibration. In addition to a field calibration campaign carried out in 2019 at the Navisence stream in Switzerland, a series of prototype flume experiments was conducted to further investigate the variability of the SPG system for different bedload transport conditions. By reproducing natural flow conditions in the flume and using grain size distributions of bedload samples collected at the Navisence stream, we identified the importance of vibrations caused by larger grains impacting a neighboring plate or the surrounding concrete. This improves our understanding how the SPG signal response change with increasing grain sizes.