surface 16 is that side which faces toward the center core 12 whereas the distal surface 18 faces in a direction away from the center core 12 a plurality of outwardly directed tangs 20 extend away from the distal surface 18 of each perforated core 14 in a somewhat similar fashion a plurality of inwardly directed tangs 22 extend from the proximal surface 16 of each perforated core 14 the tangs 20 22 may take the form of punch tabs or pieces bent integrally from the sheet like membrane or stock in a roll forming or other type of manufacturing operation in the preferred embodiment of this invention the tangs 20 22 are formed simultaneously by passing raw sheet stock through the nip of punch rollers 24 as shown in fig8 of course other forming techniques are possible fig7 shows one exemplary form taken by the tangs 20 22 here one inwardly directed tang 22 opposes each outwardly directed tang 20 such that they appear in something of a nose to nose fashion the bending of each tang 20 22 away from the base stock material of the perforated cores 14 creates a void or opening in the stock material thereby establishing its perforated characteristics in practice the outwardly directed tangs 20 are longer than the inwardly directed tangs 22 with both being bent away from the general plane of the stock membrane by an equal but opposite acute angle of course other angular orientations are possible and it is not necessary that the inwardly directed tangs 22 be bent away from the plane of the stock membrane by an equal but opposite angle to that of the outwardly directed tangs 20 in other words it is possible that the.