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This paper presents a theoretical investigation of ultrasonic wave propagation in a fabricated porous aluminum plate whose microstructure consists of columnar pores oriented normal to the plate surface. Attention is restricted to the symmetric modes which are seen to be compatible with a plane wave source whose wavefronts are vertically incident on the plate. The analysis shows the existence of two fundamental modes, a fast mode and a slow mode, which together transmit energy through the microstructure. The reflectivity and impedance properties of the plate are determined, and it is shown how these vary with frequency, pore spacing, and porosity. By varying the latter during the manufacturing process, a material may be fabricated whose acoustic impedance can be set to any desired value between that of water and solid aluminum.