## 1 Introduction  
Flat panels made from plastics are crucial structural components in various industrial sectors. They often serve as substitutes for metal boards when the use of metal is impractical due to factors such as chemical resistance or weight requirements. These panels are frequently employed in vital elements of mechanical systems and production lines, including medium- to large-capacity tanks, sliding mechanism components, and chemical-resistant ventilation systems. However, detecting various imperfections, such as cracks, delaminations, or local inhomogeneity, during the manufacturing process of such materials has been a challenge. This research presents a novel approach using noncontact ultrasound for nondestructive detection of these defects, offering a potential solution to this industry-wide issue, marking a significant advancement in the field and bringing hope for improved quality control in the industry. Quality control of prefabricated panels used as structural or facade elements exposed to outdoor conditions is one of the most critical stages of the production process. It is mainly performed by rough (visual) assessment of the product quality or destructive testing of selected elements of a given production run. The board's producers usually perform quality control, while in subsequent stages, the panels are transported to wholesalers and sales points and may not be appropriately protected and stored [1; 2; 3; 4]. In Industry 4.0 and Industry 5.0, quality control is a continuous, data-driven process integrated at all stages of production [5; 6; 7; 8; 9]. Ultrasonic techniques can be used to assess the quality of these types of materials and detect structural inconsistencies. From the point of view of ultrasonic tests, the difficulties in studies of polymer-like materials result from their relatively high attenuation (about 20 dB/m) compared to classical solids (metals) 0.17 \(\div\) 0.25 dB/m for the 30 kHz. In addition, they are often available in relatively large formats; thus, measuring entire boards (for distances of about 1 m and even more) is challenging. Ultrasound is a well-known and accepted method for nondestructive integrity testing of various materials, including polymers and polymer-matrix composites. One of the drawbacks of the classical ultrasonic method is that it requires the application of liquid couplings (like ultrasonic gels, oil, and water) for better transmission of the wave energy from the transducer to the studied material. Recent developments allow the application of airborne ultrasound, a relatively new noncontact testing technique that works without any liquid or gel couplant [10]. Yet another advantage of the method is that the test can be performed automatically for each manufactured board without stopping the production line [11]. One of the first studies using air-borne ultrasonics in polymer materials was carried out by [PERSON] et al. [10; 12]. They applied a through-transmission method using the capacitance transducers for defect detection and reported wave velocity dispersion and the possibility of detecting Lamb waves in the material [10]. Airborne pulse-echo ultrasonics can be easily applied to imaging surface topography based on the measurements of the amplitudes of reflected signals. In turn, the measured time of flight (TOF) of the reflected signals (assuming the sound velocity in the air is known) can be converted to a 2D elevation profile [12; 13; 14]. Another modality relies on transmitted signals through the samples using a separate transmitter and receiver on adjacent sides of the samples. Such a technique, based on the signal amplitudes (C-scans), allows for the control of material homogeneity, damage detection, monitoring of degradation processes, and integrity of the materials [12; 15; 16]. Specific, resonant-based methods can also be applied to measurements of sound velocity or thickness [12; 17; 18].