# 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
#### 2.2.4 Metamaterials  
In recent years, acoustic metamaterials have become a popular topic for researchers in the field of acoustics. The purpose of these materials is to design structures with unique properties that are not present in natural materials [52]. The concept of metamaterials originated in the field of electromagnetics. [PERSON] discovered that the dielectric properties and the permeability of electromagnetic materials could have negative values [53]. This is distinct from conventional materials with positive material parameters, and thus the concept of electromagnetic metamaterials was theoretically proposed. Later on, this theory was applied to the study of optical metamaterials. In the case of natural materials, material parameters such as mass density, Young's modulus, and Poisson's ratio are positive [54]. However, with artificial construction, it is possible to achieve effective material parameters that are negative within specific frequency ranges. Acoustic metamaterials can manipulate and control sound waves by modulation of their structures, which can result in a zero, or even negative, refractive index for the control of sound at subwavelength scales [55]. Numerous reports have shown the influence of various factors, such as the structure of materials (e.g., conical cavity [56], hole-shape materials [57], and locally resonant phononic woodpiles [58]), geometric and material parameters [59; 60], and localized resonances [61] on acoustic absorption. [PERSON], [PERSON] et al., have successfully achieved invisibility cloaking in acoustic waves through the use of conductivity equations. These equations are derived from the similar concept of \"transformation media,\" which was originally applied to manipulating electromagnetic waves to achieve invisibility cloaking [62; 63]. [PERSON], [PERSON] et al., conducted a study to investigate the sound attenuation of an acoustic barrier made with metamaterial. In their study, the authors constructed a 1:10 scale model consisting of cylindrical wooden bars, measuring 30 cm high and 1.5 cm in diameter. Different alternating rows of cylindrical bars were arranged with an empty space to form regular geometries. The results suggest that barriers made with metamaterials can be effective substitutes for traditional barriers in mitigating noise [64]. However, there have been relatively few reports on the use of metamaterials for underwater sound absorption. [PERSON], [PERSON] et al., designed a sound absorption slab that utilized locally resonant acoustic metamaterials, resulting in broadband (800-2500 Hz) absorption of the underwater sound [10]. [PERSON], [PERSON] et al., designed an underwater honeycomb-type acoustic metamaterial (AM) plate composite. Its structure is shown in Figure 7a, which had a thickness of 20.25 mm, and achieved an underwater sound transmission loss (STL) over 20 dB higher than that of homogeneous materials with the same area density. Under a hydrostatic pressure of 3 MPa, this structure still achieved an average STL of 10 dB in the frequency range from 2 kHz to 10 kHz [65]. Using several detuned multi-order micro-perforated panels with a backing cavity (MPPB) basis, an 11-unit meta-structure with a thickness of only 75 mm was developed, as shown in Figure 7b, achieving almost a 100% maximum absorption coefficient and an average absorption coefficient of 80% at frequencies ranging from 1380 to 3150 Hz. This is caused by the precise adjustment of the parameters of micro-perforated panels, which are arranged in parallel with a backing cavity [66]. This method also achieved a high underwater sound absorption coefficients of over 0.9 in the range of 12\(-\)30 kHz, with a compressive strength of over 5 MPa in \"phononic glass\", the model simplification process of phononic glass is shown in Figure 7c [67].