The Measurement and Interpretation of Ultrasound from Woody Stems

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Abstract
A system for detecting and counting ultrasonic acoustic emissions (AEs) from woody stems in the waveband of 0.1–1.0 MHz is described. AEs from small pieces of stems of Mains sylvestris and Chamaecyparis lawsoniana occurred up to a rate of 6000 min−1, and could be related to the water loss as the stems dried. AEs did not occur randomly, but were aggregated. Each event is interpreted as a single cavitation occurring in the water within a tracheid or fibre. AEs could be temporarily stopped by rewetting the tissue or cutting it to release tension. From knowledge of the number of cells in the stems, the fraction of total counts per number of tracheids or fibres present in the stem was calculated. This was found to be less than 0.16. One reason why this fraction is so low is that AEs may occur simultaneously; it is also probable that some AEs emit energy outside the waveband studied. Even though the fraction of AEs recorded is low, this instrument could prove to be a very powerful tool for investigating the occurrence of cavitation in woody stems.