Hearing the heat: a proof-of-concept study of nonlinear sound measures for distinguishing phases of rapid organic decomposition

Acoustic recording can follow decomposition non-destructively, but which acoustic measures carry process information is largely untested. In a proof-of-concept experiment, four nonlinear complexity measures and six conventional ecoacoustic indices were compared in eight octave bands, using 150 recordings from five buckets of one vermicompost-hay batch during one mesophilic heating-cooling cycle. Internal temperature served as a proxy for the system state. Because smooth trajectories can agree by chance, a four-parameter temperature model with a heating-cooling term was evaluated on held-out buckets and against 100000 surrogate temperature courses with the same values and smoothness but randomised timing. For the nonlinear measures, the model predicted held-out buckets with a median skill of 0.48, better than a linear time trend, a time model with the same heating-cooling break and the surrogates (p = 0.00003). The conventional indices reached 0.12 and were not distinguishable from the surrogates (p = 0.21). The group difference exceeded that of the surrogates (p = 0.0002). At equal temperature, the nonlinear measures differed between heating and cooling. Their skill was highest at 128–256Hz (median held-out skill 0.70). The temperature model nearly matched a data-driven benchmark without temperature for the nonlinear measures (0.50), but not for the conventional indices (0.18). In-sample fits overstated predictive skill (median R2 0.81 and 0.36), so held-out prediction against surrogate covariates provides a practical test for acoustic indicators in single experiments. Since the buckets replicate the measurement, not the process, replication across batches and cycles is needed.