# Acoustic Applications of a Foamed Geopolymeric-Architected Metamaterial
## 4 Conclusions  
The paper compares and evaluates the influence of the presence of perforations on the sound absorption ability of the foamed ceramic polymer that, according to dynamic mechanical characterization, behaves as a metamaterial with negative stiffness. In this perspective, the presented data have a simple comparative sense to elucidate the ongoing phenomena leading to SAC improvements, and the test repetitions were carried out in the same experimental condition on differently configurated samples. This preliminary investigation underscores the pivotal role of sustainable building materials in green design and construction, particularly architected metamaterials. Focused on integrating environmentally friendly building materials with exceptional acoustic insulation properties, this research unveils a novel approach through designing and characterising lightweight geo-polymer-based metamaterials. The essence of sustainability is embedded in the selection of materials that not only fulfil structural requirements but also contribute to the overall well-being of the environment. Adopting architected metamaterials represents a promising avenue in this endeavour, as it offers a unique synergy between structural integrity and eco-friendliness. In this context, the foamed geopolymer structure emerges as a noteworthy innovation, embodying an organic-inorganic system augmented by incorporating an oligomeric polysiloxane and aluminium powder as a foaming agent. A standout feature of the developed metamaterial is its remarkably low apparent density of 180 Kg/m\({}^{3}\), coupled with a substantial 78% open-pore foam structure. This achievement contributes to the material's lightweight nature and positions it as an ideal candidate for applicationsdemanding efficient sound insulation. Creating such a structure symbolises a meticulous process combining various elements to yield a \"metamaterial\" with multifaceted benefits. One of the significant contributions of this research lies in its comprehensive characterisation of the metamaterial. The chemical-physical, microstructural, dynamic-mechanical, and sound properties were systematically examined, providing a holistic understanding of its capabilities. We pair acoustic and dynamic mechanical tests with a critical interpretation of the chemical and morphological aspects related to the processing of aluminium powder foamed geopolymers. The lightweight ceramic geopolymeric foams showed unusual viscoelastic properties, such as being highly dissipative at high deformation rates (which is just the opposite of what you can expect from viscoelastic materials) that allowed us to further investigate this foamed system as a potential base for the preparation of acoustic architected metamaterials. The foamed geopolymer's success as an acoustic wave absorption metamaterial is particularly noteworthy, presenting promising prospects for its utilisation in sound insulation applications. The dynamic mechanical viscoelasticity analysis adds another layer of insight, revealing the negative stiffness metamaterial's exceptional energy dissipation characteristics across a spectrum of frequencies. Notably, the material exhibits high energy dissipation at very low frequencies and frequencies exceeding 300 Hz. The shear modulus values of approximately 8.2 MPa in its elastic component and around 20 MPa as its dissipative component. Beyond the laboratory findings, the practical applications of this metamaterial are significant. Its attributes position it as a game-changer in sound absorption, with potential applications ranging from residential buildings to industrial settings where noise control is paramount. The lightweight nature of the material further facilitates ease of handling and installation, making it an attractive option for various construction scenarios. In the broader context of sustainable architecture, integrating architected metamaterials such as the lightweight geopolymer-based foam presented in this study aligns with the principles of environmental responsibility.