Ultrasonic velocity measurements in the thickness direction of paper

Instruments and techniques for measuring the shear and longitudinal ultrasound velocity through paper in the Z-direction (perpendicular to the plane of the sheet) are described. The fundamental problems in determining elastic properties with ultrasound on thin, rough, fibrous samples are analyzed. Limits are set on sample characteristics, which will assure that the Z-direction velocity measurements are reasonable indicators of Z-direction elastic properties. Data taken over a range of frequencies, pressures, and surface characteristics are presented and analyzed in terms of our concepts of Z-direction wave propagation in paper.