## 3 Bulk-Type Acoustic Metamaterials  
Three-dimensional (3D) acoustic metamaterials (AMs) have bulk structures and are characterized by controlling acoustic characteristics within the entire space. Due to these characteristics, it can be used in a wide range of applications and has a high degree of freedom in design. However, sophisticated manufacturing processes are essential along with complex designs, and technical difficulties exist, especially in implementing large-area bulk structures based on soft materials. For example, 3D printing, which is mainly used, is utilized in various complex structure fabrication processes, but mass production is difficult due to high cost and slow processing speed [57]. Also, the method of molding a large-area  
Figure 10: Schematic of a dielectric elastomer actuator-based multifunctional smart window that can regulate light transmittance and sound absorption. Reproduced under a Creative Commons Attribution (CC BY) 4.0 license [40], Actuator, 2021. Figure 9: (**a**) Diagrams of various parts of an active acoustic material film and (**b**) composite structure of the metasurface model. Reproduced under a Creative Commons Attribution License (CC BY) [46], Frontiers, 2022.  
structure by molding/injection molding can be mass-produced with high repeatability and product consistency, but there are limitations in implementing complex microstructures, and materials have limitations due to durability issues in practical applications [58]. In addition, in metal-insulator-metal (MIM)-based metamaterials, continuous large-area patterning at tens of centimeters through roll-to-roll nanoimprint (R2R NIL) has been successfully performed, but its application to soft acoustic metamaterials is still in its infancy [59]. In addition, for example, reconfigurable structures based on Origami or Kirigami require manual assembly, which limits consistent quality and automated production [60]. Accordingly, it is pointed out that the practical use of soft-based three-dimensional (3D) acoustic metamaterials (AMs) requires the introduction of high-throughput methods such as roll-to-roll processes, modular automatic assembly systems, and 4D printing to overcome the existing throughput limitations [61]. Meanwhile, three-dimensional (3D) acoustic metamaterials (AMs) can be classified into composite/hybrid structures [62; 63], modular structures [64; 65; 66; 67], and reconfigurable structures [68; 69] depending on the structural design. The composite/hybrid structure is a structure in which soft material with high flexibility and reactivity and rigid material with excellent mechanical stability are combined to complement each other by utilizing each other's advantages [39; 70]. Details of these structures include the mechanism of soft-rigid layered structure, adhesive multiphase structure [71], and topologically designed structure [72; 73; 74]. Among the classifications, reconfigurable structures are systems that reversibly alter their shape in response to external stimuli, enabling real-time modulation of various acoustic responses within a single configuration [19]. This type of structural reconfiguration is implemented through diverse mechanical deformation mechanisms. Notable approaches involve folding structures based on origami and Kirigami [75; 76; 77; 78; 79; 80], actuator integration on flexible substrates, and compliant deformation using dielectric elastomers (DEs) [81; 82]. These approaches offer broader tunability than conventional AMs, particularly in frequency response, damping performance, and directional control. Reconfigurable AMs are capable of performing multiple functions within a single structure. This adaptability enables applications such as acoustic cloaking [83; 84], tunable waveguides [85], and multifunctional sensor systems [86; 87; 88] by allowing real-time responses to environmental changes. To complement the structural and functional classifications, Table 4 highlights selected bulk-type AMs and their acoustic performance metrics, including absorption levels and frequency ranges.