Excitation of Rayleigh and zero-group-velocity (ZGV) Lamb waves using air-borne N-waves focused by an ellipsoidal reflector

Air-coupled ultrasonic non-destructive testing (NDT) of elastic solids is a challenge due to the large impedance contrast between air and most materials used in industrial and structural applications. However, because air-coupled sensing offers many advantages such as rapid scanning of large areas and the elimination of part immersion for inspection, there remains strong incentive to find unique methods for air-coupled excitation of wave motion in elastic solids. This work presents experimental results of an in-air acoustic source that has been shown to excite wave motion in high impedance elastic solids. The source consists of a spark generator and an ellipsoidal reflector. The spark generator radiates a short-duration, high-amplitude acoustic signal as the result of an electrostatic discharge between two electrodes with high potential difference. Analogous to lithotripter, the spark is located at the near focus and generates an outgoing wave that is then focused at the far focus of the reflector which i...