Ultrasonic Acoustic Emissions from the Sapwood of Thuja occidentalis Measured inside a Pressure Bomb

Abstract

An improved method of counting acoustic emission (AE) events from water-stressed stems of cedar (T. occidentalis L.) is presented. Amplified AE are analyzed on a real time basis by a microcomputer. The instrumentation counts AE events in a fashion nearly analogous to scintillation counting of radioactive materials. The technique was applied to measuring ultrasonic AE from the stems of cedar inside a pressure bomb. The shoots were originally fully hydrated. When the shoots are dehydrated in the bomb by application of an overpressure very few AE were detected. When the bomb pressure is reduced after dehydration of the shoot, AE events could be detected. Ultrasonic AE are apparently caused by cavitation events (= structural breakdown of water columns in the tracheids of cedar) and not by the breaking of cellulose fibers in the wood.