## 2 Materials and Methods
### Measurement Method
An exciting signal is sent from the electric generator to the transmitter (T), emitting an ultrasonic wave from the air to the board. The Lamb wave is generated on the board's surface, propagates in the material, and is received by the transducer (R). The acquired ultrasonic signal of this wave is then 150 ied and stored for further offline analysis. The air-borne ultrasonic studies were performed using transducers (emitters) with a central frequency of 30 kHz and 50 kHz (NCG30-D50 ULTRAN GROUP(r), NCG50-D50 ULTRAN GROUP(r), Thorlabs Inc., Newton, NJ, USA). The frequency of the transducers was selected based on the properties of the tested materials, which were determined during preliminary tests. The transmitter was excited by a chirp-codded signal having a linear variable frequency (see Figure 5). For both materials, the same frequency ranges from fmin  
Figure 2: Photographs and diagrams of the position of the simulated delamination defect at the edge of the board placed parallel to its surface (**a**), the crack defect at the edge of the board placed perpendicular to its surface (**b**), and the rupture defect in the middle part of the board (**c**). 21 kHz to f\({}_{\rm max}\)--47 kHz for the 30 kHz transducers were applied (in the Supplementary Materials, the results for the 50 kHz transducer are shown and, in this case, the range of frequency of the chirp was set from f\({}_{\rm min}\)--35 kHz to f\({}_{\rm max}\)--65 kHz). The time duration of the transmitted signal was 300 ms and the transmitter stimulation voltage, obtained by a custom-made power amplifier, was about 150 V (gain of about 45 dB). The Lamb waves, leaking from the board, were received by a wideband (40 kHz-2.25 MHz) Bat-1 (@Microaocustics, MicroAcoustic Instruments Inc., Ottawa, ON, Canada) transducer, which works together with a transimpedance preamplifier Q-Amp TM (MicroAcooustic Instruments Inc., Ottawa, ON, Canada) dedicated to the Bat-1 transducer. The amplifier is characterized by a bandwidth ranging from 10 kHz to 5 MHz, a sensitivity of 950 mV/pC, and the possibility of polarizing the membrane or not depending on the type of the transmitting transducer used (capacitive transducer or piezoelectric transducer). The signals were sent directly from the amplifier to the oscilloscope. Then, the signals were sent from the receiver to the oscilloscope for further analysis. The recorded signals were correlated with the reference signal to obtain the compressed signal [38; 39; 40]. Such a well-known procedure allows for receiving signals with a relatively high signal-to-noise ratio [38]. The ultrasonic experiments were performed for both types of boards using 30 kHz and 50 kHz transducers. Qualitative agreement was observed in the amplitudes measured at these two frequencies (see Figures S1 and S2 in Supplementary Materials). However, due to the high attenuation of the materials, the signal-to-noise for 30 kHz was higher than for 50 kHz and, in the following sections, only the results for 30 kHz will be presented. To measure the maximum amplitude of transmitted signals, we specially selected the positioning angles between the transducers (transmitter and receiver) and the surface of the boards, the ranges of the applied frequencies, and the power of generated signals. The measured signals were averaged three times. Then, the signal processing procedure was carried out in the following three steps: (i) acquisition of the transmitted signals (A-scans), (ii) calculation of the B scan, and (iii) evaluation of the maximum amplitudes of the signals as a function of position. Figure 6 shows schematically the signal analysis procedure. The red boxes in Figure 6 represent the area where defects were generated. In the middle panel (B-scan), boxes are close to the board edge and, in the middle part of the board, while in the right panel (amplitudes vs. position), the boxes are shown at the board edges only. The same procedure was first applied to reference boards (with no defects) of both types.