Ultrasonic Acoustic Emissions from the Sapwood of Cedar and Hemlock

Abstract

Measurements are reported of ultrasonic acoustic emissions (AE) measured from sapwood samples of T. occidentalis L. and Tsuga canadensis (L.) Carr. during air dehydration. The measurements were undertaken to test the following 3 hypotheses: each cavitation event produces 1 ultrasonic AE; large tracheids are more likely to cavitate than small tracheids and when stem water potentials are > -0.4 MPa [megapascal], a significant fraction of the water content of sapwood is held by capillary forces. The last 2 hypotheses were recently discussed at length by Zimmermann. Experimental evidence consistent with all 3 hypotheses was obtained. The evidence for each hypothesis, respectively, is: the cumulative number of AE nearly equals the number of tracheids in small samples; more water is lost per AE event at the beginning of the dehydration process than at the end; and sapwood samples dehydrated from an initial water potential of 0 MPa lost significantly more water before AE started than was lost by samples dehydrated from an initial water potential of about -0.4 MPa. The extra water held by fully hydrated sapwood samples may have been capillary water as defined by Zimmermann. An improved method for the measurement of the intensity of ultrasonic AE is reported. Intensity is defined here as the area under the positive spikes of the AE signal (plotted as voltage vs. time). This method was applied to produce a frequency histogram of the number of AE vs. intensity. A large fraction of the total number of AE were of low intensity even in small samples (4 mm diameter by 10 mm length). This suggests that the effective listening distance for most AE was < 5-10 mm.