Normalized Frequency-Domain Acoustic Features for Same-Vehicle Load-State Difference Analysis

Roadside acoustic sensing may provide a low-cost approach to vehicle load-state monitoring. This study evaluates a normalized frequency-domain descriptor for controlled same-vehicle comparison. Vehicle-pass signals underwent resampling, pre-emphasis, framing, and windowing; event-level power spectral density from 500 to 5000 Hz was normalized and partitioned into 500–1000, 1000–3000, and 3000–5000 Hz bands. Field measurements comprised 60 passes of one 4 × 2 box truck at 20, 30, and 40 km/h with payloads of approximately 1000 and 4000 kg. Heavy load consistently increased the low-frequency proportion and decreased the high-frequency proportion. Two-way analysis of the additive log-ratio ln(PL/PH) identified significant speed, load, and speed × load effects, with heavy–light separation decreasing as speed increased. A CarSim template was reparameterized at total masses of 9540 and 12,540 kg and a wheelbase of 7.15 m. Wheel-force impulses were convolved with a mass-dependent modal response to generate uncalibrated relative acoustic signals. None of the 48 sensitivity evaluations reproduced the full field direction. Thus, the descriptor supports same-vehicle comparison under controlled conditions, whereas the current dynamics-to-acoustic model is a reproducible discrepancy analysis rather than a calibrated representation of field sound radiation.