## 3 Bulk-Type Acoustic Metamaterials
### Resonance-Based Metamaterials  
[PERSON] et al. proposed a dielectric elastomer actuator (DEA)-based bending structure inspired by the flowering motion of petals, with the aim of developing a reconstructive acoustic absorber that can actively respond to changing noise environments in real time [97]. The absorber features a multilayer micro-slotted dielectric elastomer bending actuator (MSDEBA), composed of acrylic-based VHB elastomer and a non-stretchable PET film forming a unimorph configuration. Upon high-voltage activation, each triangular petal-like segment undergoes out-of-plane bending, actively increasing the open ratio of a micro-slotted front panel (MSP) and thereby shifting the acoustic resonance frequency. This biomimetic bending mechanism and its integration with the micro-slotted panel are illustrated in Figure 11. This geometry-driven change in the open area alters the acoustic coupling between the slotted panel and the underlying cavity, enabling frequency-selective sound absorption. Electrical actuation (5 kV) enables a peak open ratio of 14.07%, shifting the transmission loss peak from 697 Hz to 1204 Hz (a 72.7% shift), and doubling the 15 dB absorption bandwidth from 435 Hz to 820 Hz. This absorber has the potential to adjust the absorption spectrum in line with real-time noise frequency variations. [PERSON] et al. proposed a flexible acoustic metamaterial using Ion Polymer-Metal Composite (IPMC), a representative Ionic EAP material, to implement electrically adjustable  
Figure 11: (**a,b**) Opening and closing of flower petals in response to light or heat, illustrating the bending motion based on asymmetric expansion. (**c**) Isometric and cross-sectional views of the micro-slotted dielectric elastomer bending actuator (MSDEBA) unit cell in the inactive state. (**d**) Deformed configuration under acoustic frequency-driven high-voltage actuation. (**e**) Exploded view of the tunable acoustic absorber composed of the micro-slotted panel (MSP), MSDEBA, and back cavity. (**f**) Photograph of the fabricated device. Reproduced under a Creative Commons Attribution 4.0 license [97], Communications Engineering, 2024.  
low-frequency acoustic absorption properties [98]. This new structure provides sound absorption characteristics that can be continuously tunable under low voltage. In addition, the combination of IPMC material detection and operation functions allows effective low-frequency sound absorption as well as a wide range of modulation with minimal voltage. Moreover, by attaching an additional MPP, complete sound absorption can be reached at specific frequencies. The IPMC used in the experiment was a Pd-Au electroded IPMC with Nafion-117 and ridge support produced through 3D printing using PLA to provide back-cavity and support structure. These absorbers facilitate real-time low-frequency noise control or the practical use of soft, intelligent material-based acoustic metamaterials. [PERSON] et al. proposed a magneto-mechanical bilayer metamaterial for acoustic wave control through area conservation density adjustment [93]. This structure consists of two layers of magnetic soft material, each of which is designed to have a different magnetization distribution. At this time, the soft material provides high flexibility and reactivity so that the shape can be reconstructed in response to the magnetic field. These double-layer metamaterials can be reconstructed in various forms through the composition of the unit cell while minimizing area change. Figure 12 illustrates both the single-layer and bilayer magneto-mechanical structures used to achieve shape reconfigurability under magnetic actuation. As a result of the experiment, it was confirmed that when the thickness of the lower layer was 2 mm, two acoustic band gaps were formed, 889-1118 Hz and 1580-1958 Hz. In addition, it was verified to maintain the band gap in the sections of 915-1138 Hz and 1654-1785 Hz, even when the lower layer was 4 mm.