## 2 Materials and Methods
### Experimental Setup  
In the experiment, air was used as the coupling medium between the transmitter and the particleboard and the receiver. As a transmitter, an Airmar pulse ultrasonic transducer was used. The used model AT50 has an active diameter of 45 mm and operates at 50 kHz. Therefore, the wavelength in air at 50 kHz is approx. \(\lambda=6.9\) mm. With an amplitude of 80 V, the transmitter was sending 10 sinusoidal oscillations with a repeating frequency of 1 Hz. The distance between the transmitter and the particleboard was always 280 mm and, therefore, greater than the near field length, which is, according to [PERSON] and [PERSON] [45], approximately 72 mm, of the transmitter in air at this frequency. During the whole measurement, the transmitter was stationary. The particleboard was a commercially available wooden medium-density fiberboard (MDF). The dimensions of the particleboard measured at height = 800 mm, width = 800 mm, and depth = 20 mm. For wooden particleboards, the choice for suitable flaw imitations are pieces of paper ([PERSON] et al. [19; 21], [PERSON] et al. [22; 23]) instead of Teflon tape which is historically used for CFRP and GFRP (Fahr [46]). This is through the assumption that paper has nearly the same material properties as the particleboard. Here, the thin air film between the piece of paper and the particleboard serves as flaw imitation. The air film causes a similar impedance change in the material properties like a real air inclusion at the top of the particleboard. The pieces of paper used as flaw imitations were circular with a diameter of \(\omega\)20 mm, which is larger than the wavelength, and \(\omega\)4 mm, which is smaller than the wavelength. The wooden particleboard was also immovable. A Knowles microphone, SPU0410 LR5H-QB, was used as a receiver, due to its recording frequency up to 80 kHz and its small dimension with the height = 1.1 mm, width = 3 mm, length = 3.76 mm, and an acoustic port of \(\omega\)0.25 mm. Since both transmitter and particleboard were immobile, the receiver must move to obtain the information for a C-scan. It was attached to an XYZ traversing stage from RoboCylinder with the maximum traversing path of \(x\) = 0-400 mm, \(y\) = 0-200 mm, and \(z\) = 0-200 mm. During the whole measurement, the XYZ traversing stage did not move in the \(z\) direction. The entire system was controlled by a standard computer. Via USB, the XYZ stage unit was connected to the computer and controlled by a LabView program. With a National Instruments (NI) system, the transmitter was controlled and the signal of the microphone recorded. The NI system consisted of a chassis PXIe-1085, with an inserted card for the communication with the computer (PXIe-8398), a card to record the measurement data (PXIe-5171R with a maximum sampling frequency of 250 MHz and 8 channels), and a card (PXIe-5423) as a frequency generator to control the transmitter. For the amplification of the control signal of the transmitter, the power amplifier 1040L of the company E&I was used to generate the required amplitude of 80 V. Figure 1 depicts the experimental setup. The experimental setup is identical to that in [PERSON] et al. [22], for completeness it is explained here. The only difference is that in the present study, only the experiments with a flaw imitation e20 mm \(>\lambda\) and a flaw imitation e4 mm \(<\lambda\) and a measurement from a flawless particleboard are performed.