## 2 Background Information
### Generation Mechanism of Hydrodynamic Noise
#### 2.1.3. Cavitation Noise  
Cavitation is when the pressure of a liquid drops below its vapor pressure in a localized region, resulting in the formation of bubbles or vapors in the liquid (as shown in Figure 3). The subsequent rupture of these cavitation bubbles leads to the release of a substantial amount of energy and the generation of robust sound waves, a phenomenon referred to as cavitation noise [37]. This phenomenon has been observed in various fluid mechanical equipment, including pumps, turbines, propellers, and submarine propellers, particularly under conditions of high flow rates and low pressure [38; 39; 40]. The consequences of cavitation noise are twofold: it can adversely affect the performance of the equipment and potentially cause damage to the equipment itself. Cavitation noise has been demonstrated to have a detrimental effect on equipment performance and has the potential to trigger cavitation, which can result in equipment damage. Consequently, the research and control of cavitation noise is of significant theoretical and engineering importance [41]. Figure 3: \(Cont.\)  
The intensity of cavitation noise is closely related to the generation and collapse processes of cavitation bubbles, as well as the prevailing hydrodynamic conditions [43]. Specifically, the sound power level of cavitation noise is influenced by the number of bubbles, the size of the bubbles, and the energy released upon their collapse. These factors, in turn, are closely related to the fluid's flow rate, pressure field, and viscosity [44]--especially in low-pressure regions, where cavitation becomes more pronounced. To quantify the intensity of cavitation noise, researchers frequently employ theoretical models based on the process of bubble growth and collapse. Assuming that the formation and collapse of bubbles follow a specific law, the sound power level of cavitation noise can be predicted by the following expression (5):  
\[L_{cav}\ =\ 10\log(\frac{P_{cav}}{P_{0}}) \tag{5}\]  
where  
\(L_{cav}\): sound power level of cavitation noise in dB,  
\(P_{cav}\): sound power generated by cavitation,  
\(P_{0}\): reference sound power (usually \(10^{-12}\) W). In order to estimate cavitation noise with greater precision, researchers frequently employ a combination of hydrodynamic models and experimental data for analysis. Given the propensity of cavitation noise to be easily interfered with and polluted by ambient noise, accurately distinguishing it from background noise becomes paramount to research endeavors. [PERSON], XY [45] developed a Venturi test bed experimental platform to simulate the cavitation phenomenon and its generated noise in a Venturi flow field. The separation and analysis of cavitation noise and background noise was achieved through wavelet transform, followed by a comparative analysis of the differences between them to study the characteristics of cavitation noise. Additionally, [PERSON] [46] employed wavelet spectral analysis to experimentally investigate the time-frequency characteristics of cavitation noise under different cavitation states and explored the relationship between the cavitation noise and the cavitation process. The experimental findings indicated that the spectrum of cavi  
Figure 3: Cavitation forms under different back pressure conditions at an inlet pressure of 4 MPa [42]: (**a**) back pressure 0.6 MPa; (**b**) back pressure 0.7 MPa; (**c**) back pressure 0.8 MPa; (**d**) back pressure 0.9 MPa; (**e**) back pressure 1.0 MPa; (**f**) back pressure 1.1 MPa; (**g**) back pressure 1.2 MPa; and (**h**) back pressure 1.3 MPa. tation noise typically manifests as broadband frequency and exhibits discernible impulse characteristics. Furthermore, the noise observed in varying cavitation stages demonstrated unique time-frequency characteristics and compositions. Additionally, a substantial discrepancy was identified between the cavitation noise and the background noise in terms of frequency characteristics, thereby enabling effective distinction of cavitation noise from environmental noise.