Observation of interference effects in air-coupled ultrasonic inspection of wood-based panels

The air-coupled ultrasonic inspection method is a widely applied non-destructive measuring technique in the wood-based panel industry. The technology is mainly applied to detect panel delaminations by analyzing the transmitted signal. Recent research deals with the use of ultrasonic techniques not only for the qualitative but also for the quantitative characterization of wood-based panels. To achieve a fundamental understanding of the behavior of ultrasonic waves in wooden panels, it is necessary to study the mechanisms that affect ultrasonic transmission and velocity during testing. Impedance and attenuation effects have been examined in previous studies. This article focuses on the interferences of ultrasonic waves. The interferences can be detected in experiments where the ultrasonic transmission is tested against the panel thickness. The results are verified with a mathematical model that explains the interferences due to multiple reflections inside the tested panels. By fitting the experimental data to the model predictions, the ultrasonic velocity and attenuation can be determined. So far, interference effects have not been considered for the non-destructive testing of wood-based panels. This research is a contribution to a better understanding of the mechanisms influencing the air-coupled ultrasonic methods.