Presented here is the development of a predictive model for impedance of single-degree-of-freedom (SDOF) and two-degree-of-freedom (2DOF) acoustic linings that is suitable for the design stage of suppression of inlet noise for turbo-fan engines. It is required that over a probable range of lining physical parameters and operating conditions the impedance spectrum is predicted with accuracy sufficient to support a lining design process and assessment of achievable attenuation. The starting point is a published impedance model for SDOF linings that primarily focuses on the transfer impedance of conventional and micro-perforate face sheets with grazing flow. This is expanded here to include 2DOF linings, introducing new issues related to transfer impedance of the inserted septum. Problems addressed are related to the septum insertion process that can change thickness, hole diameter and open area ratio of the uninstalled septum, and introduce blockage. Required empiricism is discussed and models for face sheet and septum-in-core transfer impedance are derived, applicable to a specific range of sheet thickness, hole diameter, and open area ratio. Manufacturing processes considered are mechanical drilling in the case of the carbon fiber laminate face sheet that is conventional perforate, and laser drilling in the case of the epoxy film micro-perforate septum material. Benchmarking is carried out by comparison of acoustic field predictions, using the proposed lining model in an FEM propagation code, with measured data from a grazing flow duct facility. Test samples include SDOF and 2DOF linings, including cases with three segments, each with distinct physical properties. Example results of comparisons are shown to highlight the fidelity of the impedance model over a frequency range compatible with the grazing flow duct geometry.

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Additive manufacturing allows the cost-effective fabrication of cellular porous structures with tailorable geometries. Structures with small pore sizes offer improved acoustical performance; however, they often result in increased structural weights and are difficult to produce using low-cost additive manufacturing methods. Here, we leverage our recent work demonstrating the fabrication of fibrous structures using 3D printing, to develop multifunctional porous structures with enhanced acoustical properties. Our method allows the addition of fibers to a structural host, which helps improve the acoustical performance without a significant weight or flow-reduction penalty. In this presentation, we outline the fabrication method and study the effect of adding fibers to the acoustical performance of porous structures with periodic gyroid unit cells. The effect of fibers on the acoustical impedance and flow resistance is studied using an impedance tube and a flow bench, respectively. We then use an inverse characterization method to analyze the effects of changing fiber and structural parameters on the acoustical performance. Our results show that adding fibers to porous host structures can drastically improve their broadband noise reduction potential.