## 4 Summary and Conclusions  
The review findings highlight that soft acoustic metamaterials broaden the design space of acoustic control by merging wave physics with the adaptability of soft materials. Across the spectrum of implementations, there is a clear distinction yet complementarity between metasurface-type and bulk-type SAMs. Planar metasurfaces excel at precise wave  
Figure 16: A reconfigurable three-dimensional acoustic waveguide using the origami principle. Reproduced under a Creative Commons CC BY-NC 4.0 license [105], Science Advances, 2016.  
front engineering on surfaces, such as generating targeted phase delays to focus or steer sound. In contrast, bulk three-dimensional metamaterials allow volumetric interaction with sound, enabling phenomena like omnidirectional attenuation and multi-band acoustic filtering within a single material. These bulk three-dimensional metamaterials can be applied to advanced features such as vibration blocking and concealment, and small, low-frequency sound absorption devices. They are also used in smart and multifunctional materials such as soft robotics, 3D electronics, and noise mitigation in architectural, industrial, and transportation systems. A key advancement in this field is the integration of soft, deformable materials into acoustic metamaterial design, which has enabled levels of tunability and reconfigurability that were not possible with conventional rigid structures. Many recent SAM systems have evolved toward active or adaptive operation by incorporating elements that respond to external stimuli and allow on-demand adjustment of acoustic properties. Notable implementations include systems that use magnetic fields to modify the stiffness of resonators, structures that deform in response to electric fields, and architectures that can be tuned by pneumatic pressure. The range of materials used in SAMs has expanded beyond hard plastics and metals to include soft matter such as silicones, hydrogels, shape-memory polymers, and other stimuli-responsive substances. This material diversity has allowed the development of metamaterials capable of altering their acoustic state in response to magnetic, electric, thermal, or mechanical triggers, thereby achieving adaptive functionalities such as switchable absorption or tunable bandgaps with real-time responsiveness. Despite these advances, several challenges remain in transitioning SAMs from laboratory-scale demonstrations to widespread practical applications. Fabricating large-scale, structurally robust soft metamaterials with consistent acoustic performance remains a complex task, as subwavelength structuring of soft materials often involves intricate processes and tight control over material uniformity across extended areas or volumes. Furthermore, the smooth integration of sensors, actuators, and control logic is required to realize real-time reconfigurability, which raises additional challenges related to power supply, feedback stability, and system complexity. Active SAMs have different required voltages and power densities depending on how they are driven, resulting in inherent trade-offs such as response speed, energy efficiency, and system integration in the process of commercialization. For example, dielectric actuator (DEA) based systems require a high voltage of about 5 kV, which leads to reduced power conversion efficiency, complexity in high voltage circuit design, difficulties in wiring and insulation when implementing large-area arrays [45]. On the other hand, graphene or ITO-based electrical drive metamaterials are capable of low-power operation but have limited functional implementation due to their limited strain or modulation range [37]. These low-voltage and low-power drives are advantageous for integration and energy efficiency but have limited modulation width and response speed. In contrast, high voltage and high power drives provide a wide range of control and fast response, but there are barriers in heat generation, reliability, and large-area scalability.