Acoustic Granulometry of Granular Avalanches: Laboratory Validation of the Size–Frequency Relationship

Abstract

Granular flows generate intense acoustic emissions, yet the quantitative relationship between particle size and sound frequency remains poorly characterized for natural, irregular materials. This study investigates the inverse relationship between mean particle size and the spectral centroid of acoustic emissions under controlled laboratory conditions. We conducted vertical-drop and inclined flume experiments using steel spheres as a control, alongside dacite and pumice sieved into narrow size fractions. Acoustic signals were acquired at a 96 kHz sampling rate and analyzed using the short-time Fourier transform (STFT) to compute spectral centroids. Vertical-drop experiments confirmed a robust inverse size–frequency trend across all materials, with spectral centroids decreasing systematically as particle diameter increased. Flume experiments preserved the general inverse relationship but exhibited greater variability and a systematic shift toward lower frequencies, indicating that boundary interactions and flow kinematics modulate the acoustic signature beyond pure particle size effects. The present work establishes a controlled physical basis for acoustics-based granulometric characterization of granular avalanches, as the first stage of a factor-isolation research program in which the particle size contribution is quantified under dry, single-phase conditions. Operational field deployment lies beyond the scope of this study; the results provide the controlled calibration baselines required before extensions to multiphase, field-scale flows can be meaningfully pursued and highlight the necessity of regime-specific calibration for confined flows.