# Multifunctional Integrated Underwater Sound Absorption Materials: A Review
## 2 Underwater Acoustic Absorption Mechanisms and Research Advancements in This Area
### Research Advancements in Underwater Sound Absorption Materials
#### 2.2.1 Polymer-Based  
Polymer-based absorption materials offer several advantages, including a low cost, easy fabrication, and a close acoustic impedance (1,568,970 Rays) to water [32]. Some polymer-based materials also exhibit damping properties, making them ideal for use in USAMs. The loss factor \(\delta\), defined as the ratio of the imaginary part to the real part, is critical for determining the sound absorption coefficient in underwater environments. The equations for calculating the sound absorption coefficient are provided below:  
\[\tan\delta=\frac{E^{\prime\prime}}{E^{\prime}} \tag{2}\]  
\[E^{*}=E^{\prime}+E^{\prime\prime}j \tag{3}\]  
Herein, \(E^{\prime}\) is storage modulus, \(E^{\prime\prime}\) is loss modulus, and \(E^{*}\) is the complex dynamic modulus. \(E^{\prime}\) is a measure of elastic energy stored, and \(E^{\prime\prime}\) represents the equivalent energy loss during the collision [1; 36]. The dynamic mechanical property \(E^{*}\) of a material can be evaluated via its damping and absorption capability. A higher loss factor \(\tan\delta\) indicates better damping properties. When sound propagates through polymer-based materials, the polymer chains are alternately compressed and elongated on a micro/nanoscale [1]. This phenomenon has led to the development of polymer-based underwater absorption materials, which are known for their better damping properties and mechanical strength. These include polymer-based USAMs, such as foam plastics, polyurethane foam, polyethylene, polystyrene, polycarbonate, and polypropylene [5]. Of these materials, polyurethane (PU) foam is the most widely used [37] due to its good sound absorption performance and mechanical properties. Polymers with an impedance match have limited performance for underwater sound absorption because of their low loss factor [1]. To address this, a new method was developed that combines two or more polymers into a physical network connected by a cross-link [38; 39]. For example, [PERSON] prepared composites of _Euromnia ulmoides_ gum (BUG)/styrene-butadiene rubber (SBR). The diagrammatic morphology of the sample is shown in Figure 4a, which have excellent pressure resistance and sound absorption coefficients in the low-frequency range from 3 kHz to 8 kHz. The SBR/BUG (70/30) has an absorption coefficient 24.23% higher than that of SBR alone [40]. [PERSON] et al., developed a hybrid material composed of carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs) in a polydimethylsiloxane (PDMS) matrix. The SEM images of the sample is shown in Figure 4b. The GNPs formed junctions with air micro-voids, and the material showed a significant increase in hydrostatic pressure from 0.4 MPa to 1.0 MPa in a frequency range from 1500 Hz to 7000 Hz, compared to materials containing only CNTs or GNPs [41]. To overcome the limitations of homogeneous materials, gradient impedance polymers have been proposed [7]. However, the thickness of the wedge material is typically large, limiting their applications and increasing their cost. To address this problem, nanofibrous membranes with a high porosity and high airflow resistivity were manufactured, and carboxyl functionalized multi-walled carbon nanotubes (MWCNT-COOH) were added to polydimethylsiloxane (PDMS) with a dispersant to enhance its underwater acoustic properties. The picture of sample is shown in Figure 4c. The addition of MWCNT-COOH and surfactant improved the sound absorption coefficient from 1500 Hz to 7000 Hz under normal pressure (0 MPa) to above 0.75 [42]. [PERSON] et al., prepared nanocomposite films for underwater sound absorption applications via spin coating. The addition of 2 wt% of MWCNT-COOH to the PDMS matrix significantly improved the elastic modulus by 48% compared to pure PDMS, and the PDMS/MWCNT-COOH/surfactant nanocomposite exhibited a sound absorption coefficient of 0.25, which was much higher than that of pure PDMS.