## 5 Experiments and Discussions
### Experiment Results and Discussions  
Prior to the experiment, the tester conducted preliminary assessment of the environmental noise and performed tests on the hydrophone. The background noise measurements are shown in Figure 11, with the blue curve representing the environmental noise results before the start of the experiment, and the red curve representing the results at the end of the experiment.  
Figure 11a illustrates the results of background noise measurements at 0-500 Hz for the hydrophone, while Figure 11b illustrates the envelope results of the noise spectrum. This consistency suggests that both the experiment environment and the electronic equipment were relatively stable.  
In the experiment, flow noise measurements were conducted on the cylindrical shells with different skin types. The experimental results are illustrated in Figures 12-14, respectively.  
The comparison results between smooth skin and the riblet skin are illustrated in Figure 12, where the blue curve represents the smooth skin and the red curve represents the riblet skin. In the 0-500 Hz range, the riblet skin demonstrated a notable reduction in low-frequency flow noise at 72 Hz, achieving a noise reduction of more than 4 dB. This indicates that flow noise may be influenced by factors such as the structure of the riblet skin.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Operating** & **Linear** & **Low** & **Horizontal** & **Vertical Beam** & **Direct** \\
**Frequency** & **Frequency** & **Frequency** & **Directional-** & **Opening** & **Angle** & **Capacitance** \\
**Range** & **Range** & **Sensitivity** & **ity** & **Angle** & **11** \\ \hline
0 Hz–100 kHz & 20 Hz–80 kHz & \begin{tabular}{c} \(-202\) dB \(\pm\) \\ 1 dB@250 Hz \\ \end{tabular} & \begin{tabular}{c} \(\pm 1.5\) dB \\ @100 kHz \\ \end{tabular} & \
\begin{tabular}{c} \(68^{\circ}\) \(\pm\) 2\({}^{\circ}\) \\ @100 kHz \\ \end{tabular} &
\begin{tabular}{c} \(11\) nF \(\pm\) 1 nF \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}
Table 3: RHCA-14 hydrophone parameters.  
Figure 10: Experiment process diagram.  
Figure 9: Experiment process and objects. (**a**) Experimental process, (**b**) different skins.  
Figure 13 presents the comparison between the smooth skin and the placoid-scale skin. The blue curve represents the smooth skin, while the red curve represents the placoid-scale skin. In the 10-450 Hz range, the placoid-scale skin significantly reduced low-frequency flow noise, with a reduction of 5 dB observed at 150 Hz. However, between 450 and 500 Hz, the placoid-scale skin increased the flow noise.  
In Figure 14, seeking to provide an intuitive comparison between the riblet skin and the placoid-scale skin, it can be observed that riblet skin exhibits superior noise reduction performance below 140 Hz. Conversely, in the frequency range of 140-500 Hz, the flow noise reduction effect of the placoid-scale skin surpasses that of the riblet skin.  
Figure 11: Background noise of the experiment. (**a**) Experimental data, (**b**) envelope diagram.  
Figure 12: Comparison of smooth and riblet skin flow noise. (**a**) Experimental data, (**b**) envelope diagram.  
Figure 13: Comparison of smooth and placoid-scale skin flow noise. (**a**) Experimental data, (**b**) envelope diagram.