Abstract: An air injection device including a housing defining an air duct therein and a valve associated with the air duct for controlling the flow of air therethrough from a pressure source (55). The valve has a sheet (59) of flexible material positioned within the air duct, the sheet (59) being mounted on a face (60) of the air duct with the ends of the sheet (59) restricted from moving in a direction substantially orthogonal to the face (60) but free to move in a direction parallel to the face (60). The sheet (59) is operably connected to control, such as a solenoid valve (62), for controlling the contour of the sheet (59), within an air flow passing through the air duct. In one embodiment a second sheet is positioned on the inside top face of the duct opposite the first sheet (59). The air injection device is particularly useful for application with a sheet separator-feeder.