The use of air-coupled ultrasound to test paper

Capacitance transducers containing a thin polymer membrane have been used t o transmit ultrasonic signals with frequencies in excess of 1 MHz through v arious paper products such as paper and cardboard. At normal incidence, a r esonance was visible in thicker samples, the frequency of which could be co rrelated to parameters such as the thickness of the paper sample and the mo isture content. It has also been demonstrated that images can be obtained o f changes in structure across paper and card samples.