## 1 Introduction  
Piezoelectric ultrasonic transducers have become indispensable in the field of non-destructive material testing, since [PERSON] used continuous ultrasonic waves to identify material defects [1]. Those are usually used in a liquid medium due to the impedance matching of the transducer and the ambient medium. However, there are some materials that must not be immersed in a liquid, not even for testing. These include composite materials, like wood-based materials, that absorb the liquid and thus change its properties. Due to the acoustic impedance of an air/wood interface, only 0.1% of the initial energy can be transmitted [2]. However, powerful ultrasonic transducer and sensitive receiver were developed in recent years to address this [3]. Piezoelectric ultrasonic transducers can also be designed more easily if they do not require extra protection against liquids. So, for such kind of materials, air-coupled ultrasound (ACU) has established itself in many areas of non-destructive testing (NDT) after the high impedance difference between solid and gas could be overcome with new developments (e.g., application of a matching layer for impedance matching of piezoceramics to air) [4]. Wood products, in particular, are of great interest today and those cannot be tested in a liquid medium. The large, well-known furniture manufacturers want to be able to offer their furniture at ever lower prices. One step towards this is to make the particleboards used thinner and thinner. This saves material and money. However, they have to make sure that these pieces of furniture still withstand the loads. This is only possible if the suppliers can ensure that the particleboards are free of defects. Now, during the production process of a particleboard after gluing, pressing, and out gassing, air inclusions and imperfectly glued regions in the particleboard can strongly influence the load capacity [5]. Suppliers and/or manufacturers must, therefore, test their wood products non-destructively to meet the high demands of the furniture industry. The use of NDT with ultrasound can be sorted into three groups: the pulse-echo technique, where flaws can be detected by the echo of a transmitted pulse wave and, with the known of the time of flight and the speed of sound in the test object, the depth of the flaw can be determined [6; 7]; the surface-wave technique, where the surface near flaws can be detected, like the pulse-echo method, which is a one-side method, where the transmitter and the receiver is only at one side of the test object [8]; and the through-transmission technique, which is also used in this publication. In the through-transmission technique, the transmitter is at one side of the test object, and the receiver is at the opposite side [9; 10]. This has the advantage compared to the pulse-echo technique in that the sound wave must transmit the test object only once; therefore, thicker material can be tested [3]. If the transmitter is stationary, and the receiver is moved in steps, tomographic measurements can be made. The transmitter emits a pulse-like sound wave via the coupling medium to the test object, where it is largely reflected and, to a small extent, transmitted. In the test object, the sound wave propagates until it reaches the next boundary of the test object to the ambient medium, where it is reflected and transmitted once more. The transmitted signal is recorded by receivers. Flaws in the material could change the impedance, consequently resulting in a decreased measurable pressure amplitude. The smaller acoustic pressure presents as a shadow in the pressure field and so can be detected. These measurements can be transferred to different forms of visualization.