Review of the use of air-coupled ultrasonic technologies for nondestructive testing of wood and wood products

Air-coupled ultrasonic (ACU) is a contactless ultrasonic measurement method which has become increasingly popular for material characterization. This is due to a growing number of advanced materials which cannot be contaminated during the testing processes by coupling agents utilized in conventional ultrasonic testing. This paper provides a review of the applications of ACU to wood and wood products. The ACU fundamentals, including principles, working modes and commercial transducers used for this purpose, is briefly described. The emphasis of this paper is on approaches of inspection and characterization. The applications of ACU to wood characterization with reference to wood quality aspects are summarized. Correlations between the ACU parameters (i. e. amplitude, velocity, and spectrum) and the wood properties (i.e. density, moisture content, strength, and stiffness) as well as the wood defects (i. e. knots, cracks, decay, insect damage, and delamination) are dealt with in detail. Finally, a discussion of apparent future research directions completes this review.