Recent observations with air-coupled NDE in the frequency range of 650 kHz to 1.2 mHz.

An air-coupled NDE-system has been used for the inspection and imaging of discontinuities and inhomogenities in different kinds of materials, such as coating variations on tissue, spot welds on steel, and air inclusions in metal plates and welds. The measurement system operates in either continuous or pulse mode at sound frequencies between 0.65 and 1.2 MHz. Air-coupled piezo-based transducers with matching layers are used to overcome the acoustic impedance gap with air.