Application of Air-Coupled Ultrasound to Noncontact Evaluation of Paper Surface Roughness

An approach for characterizing paper surface quality by ultrasound as an alternative non-contact method is presented. In this work, an air-coupled ultrasound at frequency range from 0.3 MHz to 4.2 MHz has been applied to surface roughness characterization, where a series of sandpapers and pure papers having random and relatively wide range of root-mean-square of roughness Rq from 2.0 to 92.8 are employed as specimens. The amplitude of reflected wave from each specimen is measured with pulse-echo configuration at normal incidence. A Kirchhoff-based scattering model is used to express the scattering phenomena from random rough surfaces and the relations between the normalized amplitude of the reflected wave and surface roughness parameters are then examined. It has been shown through the experiments that high frequency air-coupled ultrasound up to 4 MHz is useful to characterize surface roughness in the order of few microns of Rq. In addition, it has been suggested that an irregularity of paper surface geometry such as skewness could be characterized from the deviation of the normalized amplitude.