Differentiating curly birch from silver birch by ultrasound

Curly birch (CB, Betula pendula var. carelica) commands a 10x higher price than silver birch. For thinning purposes it is crucial to differentiate between the two types as early as possible. We studied the possibility to use ultrasound for this differentiation. The propagation velocity of 4 MHz longitudinal ultrasonic 5-cycle bursts transmitted through block samples with a cross-section of 20x20 mm(2) comprising both curly and noncurly birch was determined. The samples originated from southern Finland and were sawn in both longitudinal and radial directions. One sample set consisted of several different sample thicknesses ranging from 2 mm to 12 mm. From time-of-flight measurements (TOF) through the samples the wave velocity under laboratory conditions (R-H = 50%, T = 23 C) was determined from a least-squares-fit. The results indicate a significant difference (t-test 3.2%, velocity difference 24%) in longitudinal direction and a highly significant difference (West 0.1%, velocity difference 22%) in the radial direction. A probability of detection (POD) test was conducted using a larger number of samples originating from both different trunks and from one single trunk from curly and noncurly sections. The results indicate that ultrasound might be useful for differentiation of curly and noncurly birch in young standing specimens.