Acoustic characterization of hybrid structural materials

Conventional acoustic barriers using foams, fabrics, fiberglass, or polymers have limited effectiveness to mitigate low-frequency acoustic waves (<~1000 Hz) owing to impractically large mass or volume requirements. It is also desirable that acoustic barriers be more amenable to multifunctional integration. With current additive and hybrid fabrication processes attaining critical commercial maturity in recent times, it is an opportune time to investigate hybrid structural materials (HSM) with novel combinations of materials having local engineered configurations to address these aspects. In this study, acoustic characterization of nanoparticle-impregnated Kevlar fabric and meta-acoustic barriers (inspired by metamaterials) having different types of inclusions is carried out. A colloid-based treatment method is used to impregnate dry nanoparticles into the fabric. A fast, full-field, non-destructive, and affordable inspection technique based on digital image analytics (DIA) is developed to assess treatment level and uniformity of nanoparticle impregnation in the fabric and validated via scanning electron microscopy and energy dispersive X-ray spectroscopy imaging. Normal-incidence impedance tube tests are conducted to obtain the absorption coefficient and transmission loss (TL) for neat and treated fabric. The influence of parameters such as number of layers, percentage by weight of nanoparticle addition, spacing between layers, and residual porosity is examined. Influence of nanoparticle material, shape, size, and treatment level on acoustic performance is also examined. It is found that while absorption decreases with increase in nanoparticle addition for frequencies above ~2500 Hz, increasing the number of layers shifts peak absorption to lower frequencies. Residual porosity was found to contribute to about 30 – 50 % of the total absorption. Above 1500 Hz, 3 – 5 dB of TL increase is obtained for the treated samples vis-à-vis the neat samples. With increase in the weight of nanoparticle addition, measured TL tends to be closer to mass law predictions. For the ranges considered, variations in nanoparticle size and shape show only a slight influence on the acoustic performance. A coupled acoustic-poroelastic model, calibrated using parameter extraction for Kevlar samples with nanoparticle treatment, is implemented in FEM simulations to predict the TL of neat and treated samples under normal incidence, with verification against benchmark cases. Further, meta-acoustic barriers incorporating various local oscillator inclusions that offer opportunities to defy mass law-driven TL seen in conventional barriers are analyzed using their effective-mass models. Resonant, damped and inertant inclusions and combinations thereof that result in negative and complex effective-mass are considered. The influence of the inclusion’s orientation, oblique incidence, and the presence of multiple oscillators on TL is examined using nondimensionalized metrics and compared to those of statistically-equivalent limp mass and double wall barriers. Due to their anomalous effective-mass, meta-acoustic barriers are shown to have tunable broadband frequency ranges of TL increase over their conventional counterparts even encompassing the low-frequency regime. Comparisons of the effective-mass model against FEM simulations and experimental data, using Helmholtz resonator (HR) configurations along ducts, confirms its applicability. Finally, application of HSMs in a prototypical lightweight acoustic and ballistic barrier cladding (LABBCLAD), integrating nanoparticle-impregnated Kevlar with metamaterials- and bio-inspired design innovations for combined acoustic and ballistic enhancements is demonstrated experimentally.