Detection of decay in logs through measuring the dampening of bending vibrations by means of a room acoustical technique

Abstract

In this paper a technique is presented to possibly predict the presence of decay in logs. This technique is used in room acoustics to evaluate the Reverberation Time of rooms, a quantity that describes the rate of decay of the sound level, and which is inversely proportional to the amount of absorption in the room. However, as sound is caused by the vibration of air particles, the use of the notion of Reverberation Time may then be extended to any vibrating system. The investigation of the potential application of this technique to logs was first inspired from woodsmen's operation of sounding trees, an operation consisting in judging by ear the response of a tree when struck on its trunk with a hammer. In this respect, the function of main concern is the vibrational response of the log, the impulse response, which, for instance, may be recorded by an accelerometer. In general, the impulse response of a system, be it mechanical, electrical or acoustical, is a sort of signature of the system from which several quantities of interest may be processed (in acoustics, some of these quantities are purposely chosen as quantitative descriptors of subjective impressions). In this study, a log is considered as a mechanical system with its proper vibration characteristics. Hence, the presence of decay in its material, which in general increases the damping properties of wood, is in analogy with a more furnished room expected to be translated by a shorter Reverberation Time as compared to a sound log. The results of this study reveal this fact, and this may open new possibilities for the process of strength grading of wood elements in sawmills.