Detection of rot in standing trees by means of an acoustic technique

A technique is presented to allow the possible prediction of decay in the stem of standing trees. This acoustic technique is widely used to evaluate the ‘Reverberation Time’ of rooms, a factor which is inversely proportional to the amount of absorbing material in the room. The notion of reverberation time may then be extended to any vibrating system. The investigation of the application of this technique to trees was inspired from woodmens' operation of ‘sounding’ trees. This is an operation which consists of analysing by the ear the response of a tree when struck by a hammer on its trunk. In this respect, the function of main concern is the vibrational response of the tree trunk. Firstly the technique was tested in the laboratory where studies were conducted on freshly hewn logs of spruce. Then it was implemented in a small field study on standing trees to predict the presence of decay in their stems. A tree is considered as a mechanical system with proper vibration characteristics. A shorter reverberation time for a decayed tree would be expected compared to that of a tree with sound wood. Instead, the reverberation time was found to be longer for a decayed tree, contradicting the results found from the studies on the wood logs. Moreover, the reverberation time for trees was found to be relatively short and other indications point out to the existence of some coupling between the vibrational modes of the tree. It was also found that more information on the health status of the tree could be obtained from its frequency response.