An impeller shroud for a mechanical system has a hyperboloid shape with a rim at the air-inlet end. The face of the inlet rim is perpendicular to the flow direction. A plurality of slots through which air is allowed to pass is disposed symmetrically about the rim. A plurality of solid areas extends along the circumference of the rim between the adjacent slots. The ratio of the sum of the plurality of slots in degrees and the sum of the solid areas in degrees disposed about the rim is between 3:1 and 11:1.