Characterization and propagation of acoustic emission signals in woody plants: towards an improved acoustic emission counter

The physics of ultrasonic acoustic emissions ( A E s ) was investigated for AE transmission through w o o d and transducers. The physical properties measured were velocity, attenuation and frequeney composition ol" AEs produced by two sources: cavitation events in xylem and peticil lead breaks. T h e authors also measured the relative sensitivity of var ious combinations of ultrasound transducets and ampliflers to aid in the selection of a measuring system optitnized for cavitation detection iti woody plan ts . Some of the authois' conclusions are; (1) Softwoods {Thuja, Pinus) attenuate AEs more rapidly t h a n hardwoods (maple, birch), (2) The velocity of A E s in wood exceeds that tneasured by others in w a t e r so the main medium of AE transmission tnust be the cellulose, (3) The strongest frequencies of AEs are in the range of 100-300 kHz, (4) Cavitationinduced AEs tend to shift to higher frequency as w o o d dehydration progtesses. (5) One cannot determine the locus of origin of AEs from its frequency cotnposition, (6) The frequency composition of the acoustic emissions probably canno t be determined at all with the sensors used because of their tendency to 'ring'. The data collected in this paper were used to aid in the design of an improved AE counter having a seven-fold increase in signal to noise ratio compared to counters previously used in our laboratory. The improved counter, tnodel 4615 Drought Stress Monitot, is now cotntnetcially available frotn Physical Acoustics Corp,, Princeton, N J , U,S,A, Key-words: acoustic emission; xylem cavitation; .icer saeehaiimi: Thuja oecidentalis: Pitius strobus.