# Defect Detection in Solid Timber Panels Using Air-Coupled Ultrasonic Imaging Techniques
## 1 Introduction  
Due to the growing emphasis on environmental protection and sustainable development, the use of timber as a construction material is increasing. Thus, the need for quality inspection of timber is also becoming more important. Timber panels are the most common raw material used for engineered wood products such as multiplex. It can be obtained by peeling off the trunk of a tree with sharp blades or by slicing rectangular blocks of wood. Reliable quality control of timber panels is necessary to detect any timber flaws, improve wood product safety, and expand their service life. Defects occur during both the production process and the use phase of timber panels. In the production process, they can arise during pressing, gluing, drying, and curing. During long-term use, the deterioration of timber structures occurs due to both biological (decay, fungi, termites, etc.) and physical (acid rain, sunshine, worms, etc.) factors. Considering the engineering applications, defects such as delaminations, knots, wormholes, and cracks are often of great concern [1]. Many nondestructive testing (NDT) methods have been developed to detect the aforementioned defects [2; 3; 4; 5]. Machine vision techniques are promising methods [6; 7], but they have two major drawbacks. Firstly, they can only detect surface defects but not inner defects. Secondly, timber panels are unique even if they originate from the same tree, whichrequires the application of advanced pattern recognition methods for defect detection [8]. Some techniques, such as those using sonic stress waves, electrical resistivity, or ultrasound, need direct contact or a coupling agent between the transducers and the sample, which makes the inspection process cumbersome and time-consuming. Furthermore, coupling agents, such as gel, liquid, or glue, may contaminate the sample. Therefore, the air-coupled ultrasound (ACU) method is preferred because it overcomes these drawbacks by avoiding direct contact with the sample [9; 10; 11]. Furthermore, transducers mounted on brackets can scan a large area within a short time, the measurement process can be fully automated, and the defects can be visualized using the C-scan method [12]. All these advantages make the ACU method suitable for automated industrial timber production, especially for continuous manufacturing [13; 14; 15; 16; 17]. The ACU techniques are often divided into two modes: through-transmission mode and reflection-transmission (echo) mode. In the through-transmission mode, the transmitter and the receiver are located on both sides of the sample. The receiver receives the pulses which travel through the sample. In the reflection-transmission mode, the transmitter and the receiver are located on the same side of the sample and the receiver receives the reflected pulses. In both cases, the time of flight (TOF) and attenuation of the signal are the two main parameters used to detect the defects present in the specimen [18]. In this paper, attenuation and TOF are picked for defect detection. The most significant challenge of the ACU techniques versus contact-type ones is the significant impedance mismatch between the air and solids. At every boundary (transducer/air, air/solid, solid/air, and air/receiver), the majority of energy is reflected and only less than 0.5% of the energy of the transmitter can be captured by the receiver [19]. Thus, the research on the ACU testing technology mainly focuses on the following two aspects: (1) the development of high-performance ultrasonic transducers, and (2) efficient signal processing of weak ultrasonic signals. Various studies have been published on the two subjects. [PERSON] and [PERSON] [20] proposed that composite ultrasonic transducers with a high enough ceramic column and small column spacing would have a significantly better performance. [PERSON] et al. [21] added a quarter wavelength thickness impedance matching layer to the outer surface of traditional sensors to reduce the attenuation of ultrasonic waves. [PERSON] et al.