A method is provided for forming an add-on component for an aerofoil which enable the structure of the aerofoil to be tuned in order to reduce the amplitude of sound produced at frequency fpeak when air flows in a flow direction from the leading edge over the trailing edge of an aerofoil. The method applies both to add-on components having a slitted formation and a Double-Rooted Trailing Edge Serration (abbreviated to “DRooTES”).