Air-coupled ultrasonic NDE: experiments in the frequency range 750 kHz–2 MHz

Possibilities of air-coupled ultrasonic testing and imaging are examined by means of a measurement system designed for both through-transmission and reflection technique. For that purpose, B- and C-scans have been performed with respect to a wide variety of materials of different acoustic impedance. The observations reported in the paper deal with coating on textile, flaws in an aluminum plate, spot welds on metallic plates, tiny air inclusions in thin castings and ultrasonic reflection on an epoxy plate with a copper layer. The measurements have been performed in either continuous or pulse mode. In the former case, frequency swept sinusoidal signals are applied, in the latter modulated chirps. All results have been obtained at sound frequencies between 0.75 and 2 MHz, produced by air-coupled piezo-based transducers with matching layer.