## 3 Bulk-Type Acoustic Metamaterials
### Modular Metamaterials  
The modular soft structure has the advantage of being able to adjust the acoustic properties by combining individual modules and being expandable, easy to maintain, and optimized for conditions. [PERSON] et al. proposed a modularized pneumatic soft acoustic metamaterial [101]. Through casting molding, they designed air-pressure-based soft scatterers in various types, such as squares, hexagons, and circles. And they performed experiments to compare their performance. The sound absorption mechanism works by adjusting the air pressure to deform the scatterers and adjust the bandgap, accordingly, blocking sound within specific frequency ranges. Their study proved that increasing the air pressure enlarges the bandgap, enhancing the sound attenuation effect, with an average transmission reduction of approximately 20.2 dB over a maximum bandgap spanning 3209.7-4639.7 Hz. [PERSON] et al. designed a pneumatic soft phononic crystal to realize a tunable bandgap [102]. Their research uses the principle that in the absence of air pressure, the soft phononic crystal does not show a bandgap. However, when air pressure is applied, the scatterers of the soft phononic crystal are deformed, inducing a reversible bandgap. The bandgap width can be adjusted by varying the applied air pressure within a range of 0 to 120 kPa. Furthermore, they studied how the scatterers' geometric parameters--such as diameter, height, and thickness affect the evolution of the phononic crystal's bandgap. This demonstrates the applicability of the combination of pneumatic soft robots and tunable acoustic devices.  
In the case of a 3D printing-based reconfigurable structure among modular soft structures, it has the advantage of being able to manufacture complex and diverse-shaped modules quickly and at low cost, and can be customized and applied to applications. [PERSON] et al. proposed a novel technique for controlling the alignment of rigid scatterer columns consisting of acrylonitrile-butadiene-styrene (ABS) by introducing mechanical deformation on flexible and stretchable soft substrates with the aim of actively regulating acoustic waves in a particular frequency band [103]. This deformation process leads to localized resonance effects, which in turn modify the propagation of sound waves through the material. The  
Figure 15: Fabrication of Soft-Stiff hybridized polymeric film and photograph of a 3D structure self-morphed from a PNIPAM/SU-8 film with morphing units. Reprinted with permission from [100]. Copyright 2020, American Chemical Society.  
soft substrate used in the study was fabricated from hyperelastic Ecoflex, chosen for its ability to withstand mechanical strain without compromising its structural integrity. By applying tensile strain \(\varepsilon\) to the substrate, they observed variations in the width of the bandgap. Especially, the study identified a bandgap frequency of 3.021 kHz at a maximum tensile strain of \(\varepsilon\) = 0.3, demonstrating the potential of this method to manipulate sound transmission properties in a controlled manner. They mentioned the potential for a precise manipulation of sound switching, as well as reducing noise and improving the acoustic quality of the environment.