Disclosed is a CPAP or breathing machine with a fan blower impeller where the fan blade shroud has a wave shaped perimeter or circumference to minimise noise to improve the responsiveness of the fan. The impeller for use in a blower for providing a supply of air at positive pressure comprises a plurality of vanes extending from a disk-like shroud, each of the plurality of vanes extending from a starting point at an inner diameter of the shroud to an outer tip at an outer diameter of the shroud. The impeller has a single shroud having a plurality of blades extending from a surface of the shroud into an upstream direction relative to the direction of flow of the pressurized gas. The shroud also has a substantially wavy shaped outer circumference or perimeter and an inertia of less than about 3.2 g cm2 and preferably is about 2.2 g cm2. The wavy shaped outer diameter extends between a minimum outer diameter and a maximum outer diameter, wherein the difference between the minimum and maximum diameter is about 4 to 22mm. The maximum outer diameter is provided in a vicinity of the outer tips of each of the plurality of vanes. The maximum outer diameter is provided adjacent the radially outer tips of each of the plurality of vanes. The minimum outer diameter is provided between each of two adjacent vanes of the plurality of vanes. The minimum outer diameter is in the range of about 24 to 32 mm. The maximum outer diameter is in the range about 38 to 46 mm. Each of the plurality of vanes has a portion with constant height and a portion of varying height.
