Blower for a respiratory apparatus for providing a supply of breathable gas at positive pressure comprising: a breathable gas inlet; and a breathable gas outlet; a rotating portion comprising a shaft and an impeller configured to accelerate the breathable gas entering the blower at the breathable gas inlet; a stationary portion; the impeller being characterized by: a shroud having a substantially wavy or tooth shaped outer circumference, wherein an outer diameter of the shroud varies between a maximum outer diameter (Dmax) and a minimum outer diameter (Dmin); and a plurality of vanes extending from the shroud, wherein, vis-à-vis the vanes, the shroud is located further distanced from the breathable gas inlet; wherein the maximum outer diameter (Dmax) of the shroud is reached in a vicinity of or adjacent radially outside tips of the vanes while the minimum outer diameter (Dmin) is reached between two adjacent vanes; wherein the vanes are radially arranged and extend from an inner diameter to an outer diameter; wherein the vanes have a substantially uniform height from their starting point at the inner diameter until a first intermediate diameter (Dint1), and a decreasing height from said first intermediate diameter (Dint1) towards their end at their outer diameter, the first intermediate (Dint1) diameter lying between the inner and outer diameters; and wherein the vanes are substantially straight from their starting point at the inner diameter until a second intermediate diameter (Dint2) and are curved from the second intermediate diameter (Dint2) towards their end at the outer diameter, the second intermediate diameter (Dint2) lying between the inner diameter and the outer diameter, and the curvature of the vanes being positive, i.e., towards a direction of rotation. a breathable gas inlet; and a breathable gas outlet; a rotating portion comprising a shaft and an impeller configured to accelerate the breathable gas entering the blower at the breathable gas inlet; a stationary portion; the impeller being characterized by: a shroud having a substantially wavy or tooth shaped outer circumference, wherein an outer diameter of the shroud varies between a maximum outer diameter (Dmax) and a minimum outer diameter (Dmin); and a plurality of vanes extending from the shroud, wherein, vis-à-vis the vanes, the shroud is located further distanced from the breathable gas inlet; wherein the maximum outer diameter (Dmax) of the shroud is reached in a vicinity of or adjacent radially outside tips of the vanes while the minimum outer diameter (Dmin) is reached between two adjacent vanes; wherein the vanes are radially arranged and extend from an inner diameter to an outer diameter; wherein the vanes have a substantially uniform height from their starting point at the inner diameter until a first intermediate diameter (Dint1), and a decreasing height from said first intermediate diameter (Dint1) towards their end at their outer diameter, the first intermediate (Dint1) diameter lying between the inner and outer diameters; and wherein the vanes are substantially straight from their starting point at the inner diameter until a second intermediate diameter (Dint2) and are curved from the second intermediate diameter (Dint2) towards their end at the outer diameter, the second intermediate diameter (Dint2) lying between the inner diameter and the outer diameter, and the curvature of the vanes being positive, i.e., towards a direction of rotation. a shroud having a substantially wavy or tooth shaped outer circumference, wherein an outer diameter of the shroud varies between a maximum outer diameter (Dmax) and a minimum outer diameter (Dmin); and a plurality of vanes extending from the shroud, wherein, vis-à-vis the vanes, the shroud is located further distanced from the breathable gas inlet; wherein the maximum outer diameter (Dmax) of the shroud is reached in a vicinity of or adjacent radially outside tips of the vanes while the minimum outer diameter (Dmin) is reached between two adjacent vanes; wherein the vanes are radially arranged and extend from an inner diameter to an outer diameter; wherein the vanes have a substantially uniform height from their starting point at the inner diameter until a first intermediate diameter (Dint1), and a decreasing height from said first intermediate diameter (Dint1) towards their end at their outer diameter, the first intermediate (Dint1) diameter lying between the inner and outer diameters; and wherein the vanes are substantially straight from their starting point at the inner diameter until a second intermediate diameter (Dint2) and are curved from the second intermediate diameter (Dint2) towards their end at the outer diameter, the second intermediate diameter (Dint2) lying between the inner diameter and the outer diameter, and the curvature of the vanes being positive, i.e., towards a direction of rotation.