The enormous variation in wood stiffness both within and amongst trees, results in the production of low-grade solid wood products from some trees. Accordingly, it would be highly desirable to segregate logs to ensure that only those logs with predominantly high stiffness wood are processed into structural timber products. This study examined whether sound flight velocity (km/s) could be used as a measure of wood stiffness to allow such segregation. Butt logs were cross-cut from 316 Pinus radiata D. Don trees and measured with three non-destructive acoustic devices, before and after harvest, to establish whether there was a relationship between stress wave velocity along the wood grain and the machine stress-grades of boards sawn from those logs. The wave velocity along the grain of logs was closely correlated with wood stiffness, whilst tree size and basic density estimated from depth of pin penetration of a Pilodyn were only moderately related. The outcomes of this study indicate that non-destructive acoustics tools offer a means of sorting logs according to wood stiffness prior to milling. A highly significant and positive relationship was found for acoustic resonance measurements made on logs; a weaker, but still significant relationship was obtained for time of flight measurements from standing trees. The challenge now is to develop a non-destructive tool that is able to more