## 2 Background Information
### Generation Mechanism of Hydrodynamic Noise
#### 2.1.4 Mechanical Noise  
Mechanical noise is defined as sound produced by the operation of mechanical equipment, the interaction between its components, and the movement of the equipment structure. The genesis of mechanical noise is typically attributed to factors such as mechanical vibration, friction, impact, and the imbalance of rotating parts within the equipment [47; 48]. The sources of mechanical noise are extensive and include pumps, turbines, fans, gearboxes, engines, and other fluid machinery and power equipment [49]. While the generation of mechanical noise is not inherently associated with fluid flow, mechanical vibrations can excite the surrounding fluid, thereby propagating sound waves in the water, which subsequently become an integral component of hydrodynamic noise. Consequently, the mitigation of mechanical noise is predominantly contingent upon the design of vibration isolation and the optimization of equipment structure [50; 51]. The frequency range of mechanical noise is typically broad, encompassing numerous frequency bands, from low to medium and high frequencies. The spectral characteristics of mechanical noise are closely related to the operational conditions of the equipment, the design of its components, friction properties, and other factors. The following are some common mechanical noise spectral characteristics:  
Low-frequency noise is characterized by its ability to propagate over long distances and its tendency to be more audible at lower frequencies. Such noise typically originates from substantial components within equipment, such as large gears or bearings, and its frequency range typically extends from 10 Hz to 1000 Hz. This particular type of noise is often characterized by a pronounced vibration component, manifesting as a low-frequency rumbling or creaking sound [52]. Medium- to high-frequency noise: This noise is often generated by the rapid oscillation of miniature components, such as gears and blades, typically within the frequency range of 1000 Hz to 10 kHz. This type of noise is generally characterized by its sharpness and the manifestation of a high-frequency acoustic emission [53]. Broadband noise is defined as a type of noise that exhibits a wide spectrum of frequencies. The mechanical noise of certain equipment, such as pumps and turbines, has been observed to manifest as broadband noise, characterized by a spectrum encompassing multiple frequency bands. This noise typically comprises both low- and high-frequency components and may be attributed to a combination of mechanical factors [54]. The phenomenon of mechanical noise should not be exclusively regarded as a form of noise pollution; it is generally associated with the operational effectiveness of the equipment. Excessive vibration and noise have been demonstrated to accelerate the wear and fatigue of mechanical components, thereby reducing the service life and efficiency of equipment [55]. Prolonged exposure to mechanical noise and vibration can potentially lead to fatigue damage of mechanical parts, and may even result in equipment malfunction or sudden failure, including common problems such as bearing wear and gear tooth damage. Moreover, the impact of mechanical noise on both the working environment and the external environment should not be overlooked. The impact of mechanical noise on the health and well-being of operators is well-documented, and there is increasing evidence that it can also have adverse effects on surrounding residents and wildlife [56; 57]. The sound power level \(L\) of mechanical noise is usually expressed in logarithmic form, and the common calculation Formula (6) is as follows:  
\[L\ =\ 10\log(\frac{P}{P_{0}}) \tag{6}\]  
where  
\(L\): Sound power level of noise (unit: dB),  
\(P\): Sound power generated by the noise source (unit: W),  
\(P_{0}\): Reference sound power (usually \(10^{-12}\) W). Mechanical noise is a pervasive phenomenon in industrial, transportation, and household equipment.