| Abstract: | A system for detecting and counting ultrasonic acoustic emissions(AEs) from woody stems in the waveband of 0.1–1.0 MHzis described. AEs from small pieces of stems of Mains sylvestrisand Chamaecyparis lawsoniana occurred up to a rate of 6000 min–1,and could be related to the water loss as the stems dried. AEsdid not occur randomly, but were aggregated. Each event is interpretedas a single cavitation occurring in the water within a tracheidor fibre. AEs could be temporarily stopped by rewetting thetissue or cutting it to release tension. From knowledge of thenumber of cells in the stems, the fraction of total counts pernumber of tracheids or fibres present in the stem was calculated.This was found to be less than 0.16. One reason why this fractionis so low is that AEs may occur simultaneously; it is also probablethat some AEs emit energy outside the waveband studied. Eventhough the fraction of AEs recorded is low, this instrumentcould prove to be a very powerful tool for investigating theoccurrence of cavitation in woody stems. Key words: Ultrasound, Cavitation, Wood |
