The use of air-coupled ultrasound to test paper

Capacitance transducers containing a thin polymer membrane have been used to transmit ultrasonic signals with frequencies in excess of 1 MHz through various paper products such as paper and cardboard. At normal incidence, a resonance was visible in thicker samples, the frequency of which could be correlated to parameters such as the thickness of the paper sample and the moisture content. It has also been demonstrated that images can be obtained of changes in structure across paper and card samples.