Geophysical granular flows exert basal forces that generate seismic signals, which can be used to better monitor and model these severe natural hazards. A number of empirical relations and existing models link these signals’ high-frequency components to a variety of flow properties, many of which are inaccessible by other analyses. However, the range of validity of the empirical relations remains unclear and the models lack validation, owing to the difficulty of adequately controlling and instrumenting field-scale flows. Here, we present laboratory experiments investigating the normal forces exerted on a basal plate by dense and partially dense flows of spherical glass particles. We measured the power spectra of these forces and inferred predictions for these power spectra from the models proposed by Kean et al. (2015), Lai et al. (2018), Farin et al. (2019), and Bachelet (2018), using Hertz theory to extend Farin et al. (2019)’s models to higher frequencies. The comparison of these predictions to our observations shows those of Farin et al. (2019)’s ‘thin-flow’; model to be the most accurate, so we examine explanations for this accuracy and discuss its implications for geophysical flows’ seismic signals. We also consider the normalisation, by the mean force exerted by each flow, of the force’s mean squared fluctuations, showing that this ratio varies by four orders of magnitude over our experiments, but is determined by a bulk inertial number of the flow.