A respiratory pressure therapy device for generating a supply of air at positive pressure for the amelioration or treatment of a respiratory disorder, the respiratory pressure therapy device comprising: a device outlet configured to connect to an air circuit to deliver the supply of air to a patient via a patient interface; a controller (4230, 4240) configured to control the respiratory therapy device to provide flow rates and pressures suitable for respiratory therapy; and a pressure generator including an impeller (6000, 6000a, 6000b) for a centrifugal blower, a housing (8004, 8006, 8024) for the impeller (6000, 6000a, 6000b) that includes an inlet and an outlet, and an electric motor configured to rotate the impeller (6000, 6000a, 6000b) to generate the positive pressure, characterized in that the impeller (6000, 6000a, 6000b) comprises: a hub defining an axis of rotation for the impeller (6000, 6000a, 6000b); a plurality of inclined blades, the plurality of inclined blades extending away from the hub, each inclined blade of the plurality of inclined blades including a leading edge and a trailing edge; and a plurality of reverse inclined blades, the plurality of reverse inclined blades extending away from the hub, each reverse inclined blade of the plurality of reversed inclined blades including a leading edge and a trailing edge, wherein each leading edge (6036a, 6036b) of the plurality of inclined blades and the plurality of reverse inclined blades being the most upstream edge of the impeller (6000, 6000a, 6000b) in use; wherein each trailing edge (6038a, 6038b) of the plurality of inclined blades and the plurality of reverse inclined blades being the most downstream edge of the impeller (6000, 6000a, 6000b) in use; wherein at least one of the plurality of inclined blades and/or at least one of the plurality of reverse inclined blades includes serrations on a respective trailing edge (6038a, 6038b); wherein each inclined blade comprises a bottom edge and a top edge, the bottom edge being oriented rotationally rearward in comparison to the top edge; and wherein each reverse inclined blade comprises a bottom edge and a top edge, the bottom edge being oriented rotationally forward in comparison to the top edge. a device outlet configured to connect to an air circuit to deliver the supply of air to a patient via a patient interface; a controller (4230, 4240) configured to control the respiratory therapy device to provide flow rates and pressures suitable for respiratory therapy; and a pressure generator including an impeller (6000, 6000a, 6000b) for a centrifugal blower, a housing (8004, 8006, 8024) for the impeller (6000, 6000a, 6000b) that includes an inlet and an outlet, and an electric motor configured to rotate the impeller (6000, 6000a, 6000b) to generate the positive pressure, a hub defining an axis of rotation for the impeller (6000, 6000a, 6000b); a plurality of inclined blades, the plurality of inclined blades extending away from the hub, each inclined blade of the plurality of inclined blades including a leading edge and a trailing edge; and a plurality of reverse inclined blades, the plurality of reverse inclined blades extending away from the hub, each reverse inclined blade of the plurality of reversed inclined blades including a leading edge and a trailing edge, wherein each leading edge (6036a, 6036b) of the plurality of inclined blades and the plurality of reverse inclined blades being the most upstream edge of the impeller (6000, 6000a, 6000b) in use; wherein each trailing edge (6038a, 6038b) of the plurality of inclined blades and the plurality of reverse inclined blades being the most downstream edge of the impeller (6000, 6000a, 6000b) in use; wherein at least one of the plurality of inclined blades and/or at least one of the plurality of reverse inclined blades includes serrations on a respective trailing edge (6038a, 6038b); wherein each inclined blade comprises a bottom edge and a top edge, the bottom edge being oriented rotationally rearward in comparison to the top edge; and wherein each reverse inclined blade comprises a bottom edge and a top edge, the bottom edge being oriented rotationally forward in comparison to the top edge.