Hot dipped galvanized sheets undergo roll forming that indents multiple beads down the length of the sheet starting at 6″ off the edge and then on one foot centers. The bead acts to strengthen the finished duct and according to SMACNA can be a substitute for cross breaking the duct. In addition to making the sheet stronger, the bead reduces deflection of the duct and therefore acts as a noise dampener (reduces oil canning) when the duct system is running.