D. Ouis, Detection of decay in logs through measuring the dampening of bending vibrations by means of a room acoustical technique, WOOD SCI TE, 34(3), 2000, pp. 221-236
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 Rev
erberation Time of rooms, a quantity that describes the rate of decay of th
e sound level, and which is inversely proportional to the amount of absorpt
ion in the room. However, as sound is caused by the vibration of air partic
les, the use of the notion of Reverberation Time may then be extended to an
y 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 whe
n 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 impuls
e response of a system, be it mechanical, electrical or acoustical, is a so
rt 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 lo
g is considered as a mechanical system with its proper vibration characteri
stics, Hence, the presence of decay in its material, which in general incre
ases the damping properties of wood, is in analogy with a more furnished ro
om 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.