# Multifunctional Integrated Underwater Sound Absorption Materials: A Review
## 2 Underwater Acoustic Absorption Mechanisms and Research Advancements in This Area  
### Underwater Acoustic Absorption Mechanism and Impact Factors  
The Schematic of sound propagation as shown in Figure 2. The sound absorption coefficient \(\alpha\) is defined as the ratio of absorbed sound energy to total incident sound energy, which is a critical parameter for quantifying the sound absorption capability of materials [6]. \(\alpha\) is:  
\[\alpha=\frac{E_{\alpha}}{E_{i}}=\frac{E_{i}-E_{r}-E_{t}}{E_{i}}=1-\frac{E_{r}+ E_{t}}{E_{i}}=1-\left|\frac{Z_{s}-\rho c}{Z_{s}+\rho c}\right|^{2} \tag{1}\]  
The absorbed sound energy is denoted by \(E_{\alpha}\), the total incident sound energy is denoted by \(E_{i}\), \(E_{r}\) is the reflected sound energy, and \(E_{t}\) refers to the transmitted sound energy. \(Z_{s}\) represents the surface impedance, and \(\rho\) and \(c\) denote the density and speed of sound, respectively.  
In general, the sound absorption coefficient is a measure of sound energy consumption. As underwater acoustic materials continue to develop, there are three methods that can be utilized to enhance the sound absorption coefficient. The first principle involves the conversion of sound energy to heat energy, which is then dissipated through the vibration and friction of molecules in the porous sound absorption materials with pore walls when sound waves penetrate these materials. The second principle relies on matching the surface impedance of the materials with the water impedance to increase the absorption coefficient through the dissipation of sound energy at the interface of different media. The third principle involves local resonance sound absorption [11; 12; 13]. For porous  
Figure 1: Schematic of (**a**) polymer-based composite USAAM (the carbon fiber honeycomb is incorporated into the sound-absorbing periodically arrayed structure) and sound absorption performance [7]; (**b**) metal-based USAAM (porous aluminum) and sound absorption performance [8]; (**c**) ceramic-based USAAM (SiC foam) and sound absorption performance [9]; and (**d**) underwater sound absorption metamaterials (host rubber) and sound absorption performance [10].  
Figure 2: Schematic of sound propagation: I represents the incident wave, r is the reflected wave, t is the transmitted wave, and I represents sound intensity [6].  
materials, the main factors that influence sound absorption are flow resistance, porosity, curvature, the thermal characteristic length, the sticky feature length, [PERSON]'s modulus, bulk density, Poisson's ratio, loss factor, thickness, shape, layered structure, and boundary features [14; 15; 16; 17]. Generally, the JCA model and Biot-Allard model are utilized to study these factors [18; 19]. Furthermore, in the case of viscoelastic polymer materials, the sound absorption performance of USAMs is primarily influenced by interface characteristics of the element components and internal structure [1; 20; 21; 22; 23; 24; 25]. The principal calculation methods typically utilized in this field are the finite element method, transfer matrices, and the finite difference time domain [26; 27; 28; 29; 30; 31; 32]. Furthermore, based on different requirements and purposes, the underwater sound absorption properties can be obtained through various testing methods: full sea trial, acoustic water tank, dynamic mechanical test, compressive test [1], transmission method [33], waveguide method [34; 35], et al.