Central Basics of Sonic Tree Tomography

By Frank Rinn INTRODUCTION Sonic measurements have been used for strength classification of utility poles since the 1960’s. Tapping with a hammer on one side of the pole induces a sound wave. A sensor at the other end of the pole detects when the stress wave arrives. The distance between tapping point and detector location is then divided by the measured time of flight, delivering a sonic speed. The higher this speed, the higher the modulus of elasticity, a good measure of wood quality and load carrying capacity. Starting in 1990, this principle was used on standing trees in order to identify internal damages; however this approach was found to be quite limited in exactness and reliability. The principles working well while measuring lengthwise to the fibers do not apply in the same ways when measuring perpendicularly for finding internal damages in cross-sections (Niemz 2001).