# Multifunctional Integrated Underwater Sound Absorption Materials: A Review
## 4 Summary of Research on USAMs with Hydrostatic Pressure Resistance and Corrosion Resistance
### Research Challenges and Methods of Improvement  
One of the main challenges in the development of these materials is achieving a balance between sound absorption performance, hydrostatic pressure resistance, and corrosion resistance. The following are some of the challenges of current research:  
Limited sound absorption performance: Most of the materials with good hydrostatic pressure and corrosion resistance do not have good sound absorption properties. Normally, ceramic- and metallic-based materials possess excellent mechanical properties, but they have lower acoustic absorption coefficients compared to polymer-based materials. However, polymer-based materials have a poorer corrosion resistance than ceramic materials.  
Complex fabrication processes: The fabrication processes for these materials are often complex and require specialized equipment, which makes them difficult to manufacture on a large scale. For example, the fabrication of metallic foams requires multiple manufacturing steps, such as melting, foaming, solidification, and post-processing, which often involve complex processes. The fabrication of metal-organic frameworks involves multiple synthetic steps, including solvothermal synthesis, post-synthesis activation, and shaping, which make them challenging to scale up.  
The cost of these materials is often high due to the complex fabrication processes and the use of expensive materials. Metal matrix composites are expensive due to the cost of the reinforcing materials, the complexity of the manufacturing process, and the need for specialized and expensive equipment.  
In summary, the main challenges in this field are incorporating additives, optimizing microstructure, using innovative fabrication techniques, and using low-cost materials.  
Methods for addressing the above challenges are as follows:  
Composite materials consist of a matrix material, such as rubber or polymer, reinforced with filler materials, such as carbon nanotubes or graphene oxide. Additive materials can provide the necessary strength and stiffness to resist hydrostatic pressure, while the matrix material can provide corrosion resistance.  
The optimization of microstructures is a crucial aspect of enhancing the sound absorption performance of materials. This can be achieved by controlling various factors such as pore size, density, porosity, and thickness. The simultaneous control of these factors can help to create an optimal microstructure that enhances sound absorption performance.  
Innovative techniques such as 3D printing and electrospinning can be used to fabricate these materials with complex structures and high precision. Nanoparticles, such as metal  
Figure 8: Preparation process of multifunctional USAMs.  
oxides and carbon nanotubes, can be added to the matrix material to improve its mechanical and acoustic properties.  
The use of low-cost materials such as natural fibers and recycled materials can help to reduce the cost of these materials while maintaining their properties.