Effect of moisture content on ultrasonic velocity and attenuation in woods

Abstract

Ultrasonic velocity and attenuation are measured at 1 MHz in several kinds of wood as a function of the moisture content U. The results are interpreted in terms of water absorption mechanism of woods. The velocity decreases rapidly with increasing U up to a critical value U 1 at which the curve shows a sharp bend, and the decrease is very slow for U> U 1. This critical point U 1 is in the range 20–50% depending on the species of wood and is regarded as the fibre saturation point of the wood, the moisture content at which the wood substance is saturated with absorbed water. Ultrasonic measurements thus provide us with an accurate way to determine the fibre saturation point. The attenuation curve also shows a critical change at U = U 2, which is slightly lower than U 1. The attenuation shows no change from U = 0 up to U 2, but begins to increase rapidly thereafter. This point U 2 indicates the moisture content at which free water begins to enter the vacant space of wood cells.