Abstract: A support structure for stiffening one or more vibrating plates of an acoustic stringed instrument: at least one sheet-like piece supported at least partly at a distance from the plate by means of one or more formed pieces arranged between the sheet-like piece and the plate to be stiffened. The entity formed by the sheet-like piece and the plate to be stiffened, together, is the shape of a lens thinning towards the edges in cross-section.