Design and Realisation of Thin Acoustic Metamaterials, and Ventilated Metamaterials for Aerospace Applications

The increasing demand for lightweight, compact, and efficient noise-control solutions in aerospace, automotive, industrial, and architectural applications has motivated the development of advanced acoustic materials capable of overcoming the limitations of conventional absorbers. Traditional sound-absorbing materials often require large thicknesses, particularly for low-frequency noise attenuation, making them unsuitable for many space-constrained engineering applications. Acoustic meta-materials offer a promising alternative by enabling extraordinary manipulation of sound waves through engineered subwavelength structures. This thesis investigates the design, analysis, optimization, fabrication, and experimental validation of novel acoustic metamaterials for low-frequency sound absorption and noise control. Several innovative metamaterial architectures are proposed, including tunable resonant metamaterial panels, fractal acoustic metamaterials, cross micro-perforated fractal hybrid metamaterials, one-dimensional ventilated acoustic metamaterials, and broadband ventilated acoustic metamaterial absorbers. Analytical modeling based on acoustic impedance and transfer matrix methods, finite element simulations, and experimental measurements using impedance tube techniques are employed to evaluate their acoustic performance. A tunable acoustic metamaterial inspired by the Ashok Chakra geometry is developed to achieve high absorption efficiency through geometric tailoring of resonant characteristics. Fractal-based metamaterials are subsequently introduced to increase the effective acoustic path length within compact volumes, enabling enhanced low-frequency absorption without increasing structural thickness. The integration of fractal cavities with cross micro-perforated panels further improves broadband absorption performance and provides additional flexibility in acoustic tuning. To address the growing need for simultaneous ventilation and noise reduction, a series of one-dimensional ventilated acoustic metamaterials are proposed and investigated. These structures enable airflow while maintaining excellent sound absorption performance through engineered resonant channels and internal acoustic resonators. The developed one-dimensional ventilated metamaterial absorber demonstrates efficient absorption in the low-frequency range without requiring rigid backing reflectors, allowing operation in both waveguide environments and free space. Furthermore, broadband ventilated metamaterial concepts based on spiral and rainbow-trapping resonator configurations are introduced to extend the absorption bandwidth while preserving ventilation capability. The outcomes of this research demonstrate that carefully engineered acoustic meta-materials can achieve significant low-frequency sound absorption, ventilation compatibility, and compactness beyond the limitations imposed by conventional acoustic treatments. The proposed designs provide new opportunities for next-generation passive noise-control systems in aerospace cabins, ventilation ducts, transportation systems, industrial facilities, and architectural