The signal-to-noise penalty of air or gas-coupled ultrasonics has so far limited its application principally to feasibility demonstration experiments and qualitative assessments of material condition [1–3]. In most cases to date, use of air-coupled ultrasound to estimate material properties has been performed by observing the phase-matched transmitted signal amplitude and correlating this occurrence with the plate wavespeed. While this approach has approximate validity, other experimental conditions, including geometry and frequency, as we have shown [4], can significantly alter the estimate of material properties made only on the basis of phase-matched amplitude extrema. In addition, since the acoustic impedance mismatch is so large in air-coupled measurements, material damping plays a greatly enhanced role in determining the nature of the transmitted or reflected field.
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