## 3 Experimental Setup Testing and Analysis Results
### Concrete Object Crack Detection
#### 3.2.1 Regular Concrete Object (Object 1) Test  
The characteristics tested were thickness and internal defections. Thickness testing was performed on both sides of the object, whereas internal defect testing was performed on only one side.  
(1) Thickness test: The impact-echo device was used to strike one side of the studied object. The microphone collected the sounds, and the sound card processed the sound to form an audio file for MATLAB to conduct simulations. The obtained waveform graph is presented in Figure 10. Figure 10 presents the waveform of the incident impact point. Figure 10 depicts the waveform of the first reflected wave. It can be seen from the figure that t\({}_{1}\) is the impact time (243.3 \(\upmu\)s), whereas t\({}_{2}\) is the reflected wave receiving time (257.7 \(\upmu\)s). The time lapse is t\({}_{2}\) - t\({}_{1}\) = 14.4 \(\upmu\)s. Because the distance between the impact device and receiver was 5 cm, the velocity of the P-wave was 3742 m/s. Figure 10 shows the overall reflected wave, which was simulated using MATLAB, and Figure 10 presents the wave frequency, which was obtained using FFT and indicates that the main frequency was 6.4 KHz. After calculation, dividing the wave velocity by twice the frequency revealed that the thickness of the object was approximately 28.1 cm, which was almost the same as the actual thickness of 30 cm. To verify the feasibility of this system, we also strike the other side of the studied object. Figure 11 presents the waveform of the incident impact point. Figure 11 shows the waveform of the first reflected wave. It can be seen from the figure that t\({}_{1}\) was the impact time (243.3 \(\upmu\)s), whereas t\({}_{2}\) was the reflected wave receiving time (257.7 \(\upmu\)s). The time lapse was obtained by subtracting t\({}_{1}\) from t\({}_{2}\) (14.4 \(\upmu\)s). Because the distance between the impact device and receiver was 5 cm, the velocity of the P-wave was 3742 m/s. Figure 11 shows the MATLAB simulation of the reflected wave. Figure 11 presents the wave frequency graph obtained from FFT and main frequency was 6.1 KHz. The same calculation method revealed that the studied object thickness was approximately 29.5 cm, which was similar to the actual thickness of 30 cm. This object has performed a total of 10 measurements, which are measured five times on each side. The measured values on one side are 28.1 cm, 27.7 cm, 27.4 cm, 27.9 cm, and 28.9 cm. The other side of the measured values is 28.7 cm, 29.1 cm, 29.6 cm, 29.5 cm, and 29.8 cm. The average value of each side is close to 30 cm, which is the actual thickness of the studied object. (2) Internal defect testing: This test was conducted on a regular concrete object without internal cracks (Object 1). The frequency domain waveforms in Figures 10 and 11 reveal the absence of continuous high-frequency reflected waves after the highest main frequency, so this Object was considered to contain no internal cracks.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Type** & **Thickness (cm)** & **Crack Depth (cm)** \\ \hline entry 1 & Object 1 & 30 & N/A \\ \hline entry 2 & Object 2 & 30 & 15 \\  & Object 3 & 50 & 12 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Testing object characteristics.  
Figure 11: Regular concrete impact wave graph from striking the other side of the object: (**a**) incident impact point wave graph; (**b**) first reflected wave waveform graph; (**c**) MATLAB simulation graph; (**d**) spectrogram.  
Figure 10: Regular concrete impact wave graph: (**a**) incident impact point wave graph; (**b**) first reflected wave waveform graph; (**c**) MATLAB simulation graph; (**d**) spectrogram.