Abstract
This study investigates acoustic memory and post-loading acoustic relaxation in mechanically compacted granular materials. The objective was to determine whether acoustic measurements can characterize changes in the mechanical state of a granular packing and retain information about its previous loading history. Experiments were performed on granular assemblies subjected to controlled mechanical compaction at different strain levels. Acoustic signals transmitted through the packing were recorded after loading and analyzed in terms of signal amplitude, dominant frequency, spectral entropy, and relaxation behavior. Repeated loading cycles were additionally used to evaluate the development of acoustic memory during mechanical conditioning. Mechanical compaction produced a progressive increase in the stress response, accompanied by a rise in acoustic RMS amplitude from 0.797 ± 0.020 to 1.049 ± 0.055 mV. The dominant frequency increased from 19.83 ± 0.11 to 21.00 ± 0.12 kHz, while spectral entropy decreased from 2.85 ± 0.04 to 2.37 ± 0.06, indicating increasing concentration of acoustic energy around the dominant spectral components. The characteristic relaxation time increased from 36.6 ± 3.7 to 70.7 ± 6.4 s, whereas the stretching parameter decreased from 0.934 ± 0.022 to 0.742 ± 0.029, demonstrating progressively slower and more heterogeneous relaxation. The acoustic memory index increased from approximately 0.84 to 0.92 during repeated conditioning cycles. The results indicate that the measured acoustic parameters can provide a non-destructive means of tracking both the instantaneous mechanical state and loading history of granular materials. The combined analysis of amplitude, frequency, spectral entropy, and relaxation provides a practical framework for monitoring mechanical conditioning and memory in disordered granular media. A Friedman test for the repeated-cycle measurements gave χ²(4) = 18.40, p = 0.0010, with Kendall's W = 0.92.