Integrated air-coupled ultrasonic transducer with front-end preamplification for structural health monitoring

Air-coupled ultrasonic testing provides non-contact structural evaluation for scenarios in which liquid or gel couplants are impractical. However, its field deployment is limited by severe acoustic impedance mismatch, microvolt-scale received signals, electromagnetic interference, and cable-induced signal degradation. To address these coupled limitations, this study developed an integrated air-coupled ultrasonic sensing-head that combines a manufacturable hollow-glass-microsphere epoxy matching layer, a conformal conductive shielding layer with controlled grounding, and a closely coupled low-noise front-end preamplifier. The integrated architecture was evaluated through long-range weak-signal reception experiments and a preliminary wall-climbing robotic tile-debonding inspection. Under shielded conditions, integrated front-end preamplification improved the signal-to-noise ratio by 23.70 to 34.80 dB relative to the unamplified baseline over propagation distances of 400 to 2000 mm. It also outperformed downstream external preamplification by 10.43 to 12.92 dB. After tapping-based zoning of the inspection regions, the robotic experiment showed that the sensing-head could acquire non-contact through-transmission signals and distinguish region-level feature differences under mobile inspection conditions. These results support a compact sensing-head strategy that improves acoustic coupling, EMI immunity, and weak-signal preservation for air-coupled ultrasonic inspection.