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 configured to control the respiratory therapy device to provide flow rates and pressures suitable for respiratory therapy; and a pressure generator including an impeller for a centrifugal blower, a housing for the impeller that includes an inlet and an outlet , and an electric motor configured to rotate the impeller to generate the positive pressure, characterized in that the impeller comprises: a hub defining an axis of rotation for the impeller ; 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 of the plurality of inclined blades and the plurality of reverse inclined blades being the most upstream edge of the impeller in use; wherein each trailing edge of the plurality of inclined blades and the plurality of reverse inclined blades being the most downstream edge of the impeller 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 ; 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 configured to control the respiratory therapy device to provide flow rates and pressures suitable for respiratory therapy; and a pressure generator including an impeller for a centrifugal blower, a housing for the impeller that includes an inlet and an outlet , and an electric motor configured to rotate the impeller to generate the positive pressure, a hub defining an axis of rotation for the impeller ; 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 of the plurality of inclined blades and the plurality of reverse inclined blades being the most upstream edge of the impeller in use; wherein each trailing edge of the plurality of inclined blades and the plurality of reverse inclined blades being the most downstream edge of the impeller 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 ; 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 . <EOS>