Abstract
This work recasts the conductance of free-standing and surface-bonded PWAS actuator, subject to free-fixed-free boundary conditions, into the Airy form of a lossy, half-length Fabry-Pérot resonator (FPR), demonstrating that the EMI conductance is the fringe spectrum of an FPR cavity whose resonance is restricted to odd spatial harmonics. Building upon this theoretical framework, semi-analytical simulations establish the dual-mode longitudinal propagation characteristic of the surface-bonded PWAS. Time-frequency analysis isolates Mode I Fabry-Pérot interference as the physical origin of the multi-peak conductance structure, with Mode II contributing only a localized amplitude near its cut-off. A modal dominance index derived from the Airy denominator quantifies the spectral contributions of the propagating Mode I and the evanescent-to-propagating Mode II. Leveraging this modal separation, fringe-frequency analysis retrieves the dominant-mode group velocity directly from the spectral fringe pattern. Near-integer fringe ratios obtained across the full bandwidth transition to non-integer deviations within localized windows, quantifying dispersive interference. This shift mirrors the interaction between host longitudinal and flexural modes, a wave-mechanics analogy validated experimentally through the recovery of near-ideal fringe patterns and wave speeds under single-mode conditions. Consequently, simultaneous mode contamination yields non-integer fringe ratios and broadened peaks that diagnose mode-mixing, rendering conductance-based extraction unreliable. To resolve multi-mode contamination where conductance-based extraction is unreliable, time-gated pitch-catch sensing restores the integer fringe structure required for group velocity recovery. Non-integer ratios and broadened peaks instead diagnose mode-mixing directly from the fringe spectrum. The FPR framework thus unifies EMI for modal diagnostics and pitch-catch for velocity extraction in complex environments.
