US Patent 7946804 Axial fan unit having reduced noise generation

An axial fan unit includes two or more impellers accommodated in a housing. Each impeller has a plurality of blades disposed about a center axis and is rotated by an associated motor. The rotation of the impellers creates a flow of air generally flowing along the center axis. A plurality of ribs are disposed between the impellers and connected to the housing. The ribs support an associated motor. At least one of an air-inlet side edge and an air-outlet side edge of each rib is inclined with respect to a direction that is perpendicular or substantially perpendicular to the center axis, such that it gets closer to one of an air-inlet side end and an air-outlet side end of the axial fan unit as it moves away from the center axis.