Air-coupled ultrasound and rapid elastic property characterization using focused acoustic beams

Plate-wave dispersion spectra have been inferred directly in a single air or water-coupled coordinate scan using mirror-focused broadband air-coupled acoustic beams on both transmitting and receiving transducers. The Lamb mode spectra are extracted in a two-dimensional FFT of the measured broadband focused air-coupled signal. To enhance the typically weak air-coupled signal, we have employed a novel signal coding scheme using a random-phase analog burst 200 μsec in length. Subsequent correlation yields the system impulse response, from which we obtain the processed results as a wavenumber-versus-frequency plot. The experiment is modeled simply using a single complex transducer point beam for generation and one for detection. Comparison of the inferred dispersion curves with calculated spectra using nominal stiffness values, shows excellent mode separation and good agreement with predictions. Experimental results are presented for isotropic media such as Lucite and aluminum, and for anisotropic plates such as composite laminates and wood. Results using water-coupling are also presented, including a graphic demonstration of electromechanical reciprocity.