## 4 Experiments of Impact Source Localization in Stiffened Panel  
### Impact Signal Generation and Wave Velocity Measurement  
In the impact source locating experiment, the impact source needed to meet the following requirements: the impact source energy must be stable with good repeatability; the impact point position must be precisely controllable; the personnel safety must be ensured. In this paper, a steel projectile is launched by a launcher. The diameter of the projectile was 7.5 mm and the weight was 1.73 g. The initial velocity of the projectile is shown in Table 2. The average velocity was 21.0 m/s.  
The wave velocities in 10 directions are measured at intervals of 10\({}^{\circ}\) in the range of 0 to 90\({}^{\circ}\). On the flat side of the panel, four sensors were placed at intervals of 100 mm in each direction, with vacuum silicone grease as the couplant. The steel projectile hit the flat side to generate an impact signal, and five experiments were performed in each direction. The Softland Times company's acoustic emission instrument (DS2-16B) was used to collect data at a sampling rate of 3 MHz, and the receiving sensor was nano 30 produced by the PAC company. The diagram of the experiment is shown in Figure 9.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Number of experiments** & 1 & 2 & 3 & 4 & 5 & 6 & 7 & 8 & 9 & 10 \\ \hline
**Velocity (m/s)** & 21.3 & 21.4 & 21.4 & 20.9 & 21.2 & 21.3 & 19.4 & 21.1 & 21.3 & 20.9 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Steel projectile initial velocity.  
Figure 8: Energy factor curves: (**a**) point 1; (**b**) point 2; (**c**) point 3; (**d**) point 4; (**e**) point 5; (**f**) point 6; (**g**) point 7.  
When calculating the velocity in a certain direction, the signal arrival time of each sensor was obtained by artificially setting the threshold interception waveform, and the velocity was obtained by dividing the distance between two adjacent sensors by the arrival time difference of the received corresponding signal. The two sets of velocities can be calculated in each direction, and the average of the 10 sets of velocities of the five experiments was taken as the wave velocity in that direction. The average value of the wave velocity in each direction is calculated as shown in Table 3.  
It can be seen from the table that the measured wave velocities in each direction was basically the same. The overall average velocity was 5.43 km/s, which coincides with the simulation. The experimental results were slightly slower than the simulation results due to the delay effect of the switching device. Therefore, the wave velocity used for impact source localization was 5.43 km/s.  
The impact signal received by the sensor was a broadband signal. For the part whose frequency did not exceed 500 kHz, the sampling rate of 3 MHz was sufficient. But the sampling resolution for the part with higher frequency was low, thereby generating errors such that the calculated wave velocity in each direction was not exactly equal. However, the high-frequency portion of the impact signal had a low energy content, and as the propagation distance increased, the attenuation was severe and the energy was further reduced. At the same time, whether the sampling rate matched the spacing of the sensor directly affected the accuracy of the measurement result. The closer the sensors were, the higher the sampling rate was required [48]. The setting in this paper is reasonable. The distance between the adjacent receiving sensors in the same direction was 100 mm, and the distance was large enough. At this time, the sampling rate was 3 MHz, the difference in arrival time of the received signals of any two sensors in the time domain was sufficiently large and can be clearly identified. For the above two reasons, the wave velocity measurement error was small, and the influence on the locating result can be neglected.