Creative Commons CC-BY-NC licence https://creativecommons.org/licenses/by-nc/4.0/ Three-dimensional Sound Field Computation and Optimization of the Delamination Detection based on the Re-Radiation Torben Marhenke 1 , Sergio J. Sanabria 2 , Jens Twiefel 1 , Roman Furrer 3 , Jürg Neuenschwander 3 , Jörg Wallaschek 1 1 Leibniz University Hannover, Institute of Dynamics and Vibration Research (IDS), Hannover, Germany, marhenke@ids.uni-hannover.de 2 ETH Zürich, Computer Vision Laboratory (CVL), Zürich, Switzerland 3 Swiss Federal Laboratories for Materials Science and Technology (Empa) Dübendorf, Switzerland Abstract Air-coupled ultrasound (ACU) is increasingly being used for non-destructive material testing, for instance, to assess cracks, delamination or mechanical properties. The advantage of this method is that no coupling medium has to be used between transducers and probe, which provides high reproducibility and allows for continuous scanning. However, for testing in production, a minimum distance must be often kept between the material and the receiver to avoid sample contamination or transducer damage. As a consequence, the pressure fields diffract around the defects and the ACU images become blurred and show lower defect sensitivity with increasing separation between probe and transducer. Therefore, the minimum detectable delamination size is severely limited. It is of great interest to develop methods that compensate for these phenomena. In this paper a re-radiation method is applied to improve the quality of ACU NDT images. The method combines the Rayleigh-Sommerfeld integral and time- reversal mirrors to determine three-dimensional sound field characteristics with a low measurement effort. The input data is the ACU pressure distribution measured in only one plane arbitrarily separated from the sample. By means of the re-radiation algorithm, it is possible to quantitatively reconstruct the pressure field directly on the probe surface, which compensates for undesired diffraction phenomena and significantly improves lateral resolution and sensitivity. We tested the method on a 22 mm thick medium density fibreboard (MDF) and a distance of 160 mm between the sample an the receiver. This improves the detectable delamination size from 25 mm to under 5 mm and the sensitivity for a 38 mm delamination from -11 dB to -20 dB. 1. Introduction A standard procedure for the non-destructive quality assurance of board composites is the examination for delamination by means of air-coupled ultrasound (ACU) (1–3). With the help of this method it is possible to assess for example wood-based materials without coupling medium during ongoing production. The excitation is done by an ACU transducer, whereby the distance between transducer and board corresponds to the near-field length. This position is selected because, due to reflections (4), only a small part of the sound energy propagates through the board. If the board is located at the near