## 3 Experimental Setup Testing and Analysis Results
### Concrete Object Crack Detection
#### 3.2.3 Concrete Object with Cracks (Object 3) Testing  
Object 3 was 50 cm in thickness with a crack 12 cm deep in the concrete. The impact-echo device struck one side of the studied object. The microphone was utilized to collect the sounds, and the sound card processed the recording and produced the audio file to be imported into MATLAB for simulation, resulting in the waveform figure shown in Figure 13. Figure 13a depicts the incident impact point wave graph. Figure 13b presents a waveform graph of the first reflected wave. The t\({}_{1}\) is the impact time (243.3 \(\upmu\)s), whereas t\({}_{2}\) is the reflected wave receiving time (256.7 \(\upmu\)s). The time lapse is expressed as t\({}_{2}-\)t\({}_{1}\) =13.4 \(\upmu\)s. Because the distance between the impact device and receiver was 5 cm, the velocity of the P-wave was 3748 m/s. Figure 13c presents the overall reflective wave from the MATLAB simulation. Figure 13d reveals two conspicuous frequencies, signal A at 3.8 KHz and signal  
Figure 12: Impact wave graph of object 2 with a crack depth of 15 cm: (**a**) incident impact point wave graph; (**b**) first reflected wave waveform graph; (**c**) MATLAB simulation graph; (**d**) spectrogram.  
B at 13.7 KHz obtained using FFT. A thickness of 49.3 cm and a crack depth of 13.6 cm were obtained in this study calculated by the wave velocity of 3748 m/s. In addition, at 7.3 KHz, the amplitude was the same as that of Signal B at 13.7 KHz, at a depth of approximately 23.9 cm, possibly resulting from an internal defect at the bottom of the concrete. A total of five crack depth measurements were performed in this studied object, and the measured values were 12.7 cm, 14.1 cm, 13.9 cm, 13.6 cm, and 13.3 cm. The average value of the crack depth matched the actual crack depth of the studied object that was originally manufactured by the concrete industry.  
The testing results from the regular concrete object (Object 1) and cracked concrete objects (Object 2 and 3) were organized by their thickness, P-wave velocity, main frequency, measured crack depth, and error in Table 2.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Type**} & \multirow{2}{*}{
\begin{tabular}{c} **Object** \\ **Thickness/Crack** \\ **Depth (cm)** \\ \end{tabular} } & \multirow{2}{*}{30/NO} & \multirow{2}{*}{3742} & \multirow{2}{*}{6.4} & \multirow{2}{*}{28.1/NO} & \multirow{2}{*}{6.3\%/0} \\  & & & & & & \\ \hline \multirow{2}{*}{Regular} & Object 1 & \multirow{2}{*}{30/NO} & \multirow{2}{*}{3742} & \multirow{2}{*}{6.1} & \multirow{2}{*}{29.5/NO} & \multirow{2}{*}{1.6\%/0} \\  & & & & & & \\ \cline{1-1} \cline{5-7}  & & & & & & \\ \cline{1-1} \cline{5-7}  & Object 2 & \multirow{2}{*}{30/15} & \multirow{2}{*}{3748} & \multirow{2}{*}{12.4} & \multirow{2}{*}{29.5/14.5} & \multirow{2}{*}{1.6\%/3.3\%} \\ Cracked & & & & & & \\ \cline{1-1} \cline{5-7}  & Object 3 & \multirow{2}{*}{50/12} & \multirow{2}{*}{3748} & \multirow{2}{*}{13.7} & \multirow{2}{*}{49.3/13.6} & \multirow{2}{*}{1.4\%/13\%} \\ \cline{1-1} \cline{5-7}  & & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Data of testing objects.  
Figure 13: Impact wave graph of Object 3 with a crack depth of 12 cm: (**a**) incident impact point wave graph; (**b**) first reflected wave waveform graph; (**c**) MATLAB simulation graph; (**d**) spectrogram.