Optimized acoustic properties of cellular solids

The optimized cell size and shape, cell size variations, sample thickness, and air cavity depth behind the sample for best sound absorption performance of air-filled porous materials having simple cell morphologies are studied in this paper. The focus is on cellular foams that are rigidly framed, e.g., aluminum alloy foams and honeycombs. The governing equations of wave propagation are solved by using the point-matching method, and the predictions are compared with known analytical solutions. The effects of cell size variations are studied for Voronoi polygons. A domain-matching method is introduced to obtain the optimal combination of cell size and shape, sample thickness, and cavity depth for selected ranges of frequency. At given porosity, the effect of cell shape on sound absorption is small. The optimized cell size for best sound absorbers is on the order of similar to 0.1 mm for practical combinations of sample thickness, cavity depth, and porosity. A random distribution of cell sizes tends to tighten the region where combinations of sample thickness and cavity depth achieve high sound absorption coefficient. (C) 1999 Acoustical Society of America. [S0001-4966(99)06408-5].