Paper characterization by measurement of thickness and plate resonances using air-coupled ultrasound

Measurements of different types of paper (surface density from 80 gr/m² to 300 gr/m²) using air-coupled, wide frequency band ultrasound and analysis in the frequency domain is presented. The working frequency range is 0.6-2.2 MHz. At normal incidence, thickness resonances in the paper samples are excited and sensed. From the analysis in the frequency domain, of such resonances, density, longitudinal velocity and attenuation of sound waves in the paper samples are obtained. At oblique incidence, Lamb waves are observed by use of Cremer´s coincidence rule. The velocity of different Lamb modes is measured.