Equivalent gradient properties of porous material with macro void and panel structures

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Abstract
In order to reduce the interface sound reflection, best sound absorption may be achieved by having a low acoustic impedance at the air-absorber interface and a gradient property is used inside the absorber. This study explores how such gradient properties may be realized by a distribution of voids and/or perforated panel structures. Finite-element simulation for the basic module is conducted to reveal the relationship between the design and equivalent fluid properties. The gradient of various properties are then identified. Genetic algorithm is used to optimize the geometrical parameters for best absorption performance under the constraint of a given thickness and a set of boundary conditions. The optimized design is then considered as parallel-series arrangement of homogeneous layers each with different acoustic impedance. For a chosen practical application, a feasible design is determined and its performance is compared with the traditional absorber of optimized, uniform properties. Gains in broadband sound absorption performance is analyzed with a single-digit descriptor relevant to the application.