Gravity damper for installation in square or rectangular air ducts. For air extraction because of overpressure. The frame is made by galvanized sheet steel. With blades made by anodized aluminium. The blades in the length they have seals in rubber for noise reduction when the damper is closing. If the damper has three or more blades then they are bridged for smoother operation. The blades are parallel to the first mentioned dimension.