A vibration system for concrete pipe making machines using the dry cast method and which employ adjustable molds comprised of corner panels joined to intermediate panels. Two coupled vertically spaced-apart vibrators are mounted in two opposite corner panels of the mold core. The vibrators are mounted on shelves welded to the interior of the walls that form the mold surface. The shelves are also welded to vertical end walls that extend inwardly to form the box-like corner panel. The corner panels and intermediate panels are secured together by removable fasteners that join the vertical end walls of the corner panels to the corresponding vertical end walls of the side panels. The joined corner panels and side panels provide a rigid core structure similar to a solid core and allow the entire core to become the vibration structure.