An apparatus comprising: a plurality of cells that are open and arranged with parallel longitudinal axes to form a core ; a set of channels through a number of cell interfaces between cells of the plurality of cells in which the set of channels allows air to flow between the cells of the plurality of cells, wherein each of the number of cell interfaces is formed by one or more cell walls of the plurality of cells; a porous face sheet coupled to a first portion of the plurality of cells of the core that allows air to flow through the porous face sheet into the plurality of cells, the porous face sheet coupled to the first portion of the plurality of cells of the core such that open spaces are defined between the cells and the porous face sheet; and an impervious face sheet coupled to a second portion of the plurality of cells of the core that causes the air, and thereby acoustic waves flowing through the plurality of cells, to reflect off the impervious face sheet back into the plurality of cells, the impervious face sheet coupled to the second portion of the plurality of cells of the core such that open spaces are defined between the cells and the impervious face sheet; wherein the porous face sheet and the impervious face sheet are arranged parallel to the longitudinal axes of the plurality of cells; and wherein the apparatus is arranged such that air that flows into the core through the porous face sheet may flow (i) into and between the cells, (ii) into the open spaces between the cells and the porous face sheet, and (iii) into the open spaces between the cells and the impervious face sheet. a plurality of cells that are open and arranged with parallel longitudinal axes to form a core ; a set of channels through a number of cell interfaces between cells of the plurality of cells in which the set of channels allows air to flow between the cells of the plurality of cells, wherein each of the number of cell interfaces is formed by one or more cell walls of the plurality of cells; a porous face sheet coupled to a first portion of the plurality of cells of the core that allows air to flow through the porous face sheet into the plurality of cells, the porous face sheet coupled to the first portion of the plurality of cells of the core such that open spaces are defined between the cells and the porous face sheet; and an impervious face sheet coupled to a second portion of the plurality of cells of the core that causes the air, and thereby acoustic waves flowing through the plurality of cells, to reflect off the impervious face sheet back into the plurality of cells, the impervious face sheet coupled to the second portion of the plurality of cells of the core such that open spaces are defined between the cells and the impervious face sheet; wherein the porous face sheet and the impervious face sheet are arranged parallel to the longitudinal axes of the plurality of cells; and wherein the apparatus is arranged such that air that flows into the core through the porous face sheet may flow (i) into and between the cells, (ii) into the open spaces between the cells and the porous face sheet, and (iii) into the open spaces between the cells and the impervious face sheet. <EOS>