## 5 Experiments and Discussions
### Sensitivity Measurement Experiment Results and Discussion  
In Section 5.2, it was observed that both the riblet skin and the placoid-scale skin exhibit superior performance in flow noise reduction compared to the smooth skin. To further determine whether the flow noise measurement results of the experiment stemmed from the structural design, the sensitivity measurement experiment for the hydrophones was devised.  
In this experiment, different hydrophone sensitivities are measured by replacing the cylindrical shell with smooth skin, riblet skin, and placoid-scale skin, respectively. The sensitivity measurement experiment was conducted at Zhejiang University, with sketches and some physical objects depicted in Figure 15.  
In this experiment, the paper presents the results for the representative frequencies of 111 Hz and 313 Hz. These results are illustrated in Figures 16-18.  
In the sensitivity measurement experiment, the frequencies of 111 Hz and 313 Hz were observed under power supplies of 10 mV and 50 mV. Using a standard smooth-skin cylindrical shell as reference, it was noted that at 111 Hz and 313 Hz, the rates of change for the riblet skin surface cylindrical shells were 11.82% and 14.49%, respectively. In contrast, the rates of change for the placoid-scale skin cylindrical shells were 2.08% and 0.69%. This observation leads to the conclusion that the covering material of the riblet skin cylindrical shells has a much more detrimental effect on the sound pressure level.  
By combining the flow noise experiment and the sensitivity measurement experiment, the effects of flow noise reduction for the different types of skin can be obtained. The results of the flow noise measurement experiment and the sensitivity measurement experiment for the different skin types are presented in Table 4.  
Figure 14: Comparison of riblet and placoid-scale skin flow noise. (**a**) Experimental data, (**b**) envelope diagram.  
Figure 15: Sensitivity calibration experiment sketch.  
Consequently, the analysis of reducing flow noise by riblet skin cylindrical shells in Section 4 is deemed inaccurate. The primary factor contributing to flow noise reduction during the experiment is not solely attributed to a decrease in surface turbulence intensity. In fact, in the experimental study, the results of reducing flow noise by riblet skin are influenced by the material properties or structural characteristics of the riblet skin. This provides useful insights for reducing flow noise.  
In contrast, the material (high-temperature resins) of the placoid-scale skin cylindrical shells exhibits favorable sound transmission characteristics. This finding substantiates  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Type**} & **Reduction** & **Reduction** & **Measuring Error** & **Measuring Error** \\  & **Range (Hz)** & **Capability (dB)** & **(111 Hz)** & **(313 Hz)** \\ \hline Smooth Skin & / & / & / & / \\ Riblet Skin & 0–500 & 4 & 11.82\% & 14.49\% \\ Placoid-Scale Skin & 0–450 & 5 & 2.08\% & 0.69\% \\ \hline \hline \end{tabular}
\end{table}
Table 4: Different skin flow noise and sensitivity measurements.  
Figure 16: Sensitivity measurement for smooth skin at 111 Hz and 313 Hz.  
Figure 17: Sensitivity measurement for riblet skin at 111 Hz and 313 Hz.  
Figure 18: Sensitivity measurement for placoid-scale skin at 111 Hz and 313 Hz.  
that, compared to smooth skin, placoid-scale skin can effectively decrease low-frequency flow noise by targeting a boundary layer turbulence intensity reduction. In the process of flow noise reduction by placoid-scale skin, the numerical simulation results showed that the placoid scales can further reduce the intensity of the turbulent boundary layer by weakening the pressure vortex, thereby reducing the flow noise of AUVs. This conclusion is verified by the experimental results for the placoid-scale skin.