## 3 Bulk-Type Acoustic Metamaterials
### Origami-And Kirigami-Based Metamaterials  
[PERSON] et al. utilized the Kresling origami configuration to create a Helmholtz resonator (HR) composed of polypropylene sheets to actively adjust the sound absorption properties to different noise environments, proposing the first tristable origami-inspired acoustic metamaterial (OIAM) [60]. This structure can implement zero-stiffness, bistable, and tristable states by adjusting the folding angle parameters, thereby achieving variable sound absorption characteristics with an absorption rate \(\alpha\) > 0.95 at three frequency ranges (149 Hz, 180 Hz, 275 Hz) by adjusting the structural height. OIAM overcomes the limitations of fixed-frequency absorbers through active structural reconfiguration based on multistable architecture, achieving both broadband low-frequency absorption and load-bearing capability simultaneously. This design has potential applications in industrial and transportation systems. [PERSON] et al. proposed bistable reconfigurable acoustic metamaterials (BRAM) that change shape in response to thermal stimuli, based on combining a Helmholtz resonator and a bistable structure [104]. This structure was fabricated using shape memory polylactic acid (SM-PLA) via 4D printing technology, and it can switch between two stable states at 55 \({}^{\circ}\)C. BRAM exhibits absorption peaks of 130 Hz and 228 Hz in State I, and 156 Hz and 542 Hz in State II, demonstrating multi-band low-frequency absorption. Additionally, it is possible to adjust the absorption band through geometrical parameter tuning via angle modulation, and the shape recovery process shows a linear restoration rate, indicating high controllability. Combined with the idea of this study, they proposed that a manual design based on natural biological structures and an active design method based on the principle of absorption should be adopted to realize lightweight, multifunctional, and intelligent low-frequency noise absorption metamaterials as a gradual revolution to cope with the current limitations of sound absorption metamaterials. [PERSON] et al. developed a reconfigurable acoustic waveguide based on origami-inspired structures [105]. The three-dimensional system, composed of rigid PET plates and flexible hinges, allows for the dynamic reconfiguration of internal acoustic paths through external mechanical deformation, enabling active control over the directionality and propagation of sound. The metamaterial consists of an array of extruded cubes, where \"extruded\" refers to tube-like volumes protruding outward from each face of the cube. As shown in Figure 16, the structure can be transformed into three principal configurations: 1D, 2D, and 3D. Each configuration exhibits distinct acoustic transmission characteristics, which were analyzed through both simulations and experiments. The concept was further extended to structures based on truncated octahedra and hexagonal prisms, demonstrating that the connected tube network can be reversibly formed or disrupted to control the propagation of acoustic energy. To analyze sound propagation, the internal and external air domains were meshed using 10-node quadratic acoustic elements (AC3D10M) in Abaqus 6.12. The structure was modeled as a 4 \(\times\) 4 \(\times\) 4 array of extruded cubes, with the plates treated as rigid reflective boundaries. Nonreflecting boundary conditions were applied to the outer domain. Harmonic pressure was applied at one opening to excite acoustic waves,and steady-state linear responses were computed over a wide frequency range. In the simple single-tube configuration, simulation results showed good quantitative agreement with experimental measurements. For the interconnected configurations, the overall trends matched qualitatively, but geometric imperfections such as gaps between plates caused deviations in the quantitative results. [PERSON] et al.