Abstract: An inlet orifice (12) and an inlet orifice (12) and centrifugal fan (21) assembly that promotes fan efficiency and reduces radiated noise. The orifice has an elliptical entry portion (13) and an elliptical diffuser portion (14) that meet smoothly at a throat (16). The major axes of the two ellipses are parallel to the axis of rotation of the fan (12). The relationship between the major and minor axes of the ellipse defining the entry portion (13) and the relationship between the major and minor axes of the ellipse defining the diffuser portion (14) may be the same or different. The major axes of both ellipses may be equal in length or different. Preferred relationships are disclosed. The fan (21) has an outer envelope (22) that is generally cylindrical but with an entry portion (23) that, when assembled with the orifice (12), extends into the diffuser portion (14) of the orifice (12) and has an outer contour that conforms to the contour of the diffuser portion (14).