Single or multiple stage blower and nested volute(s) and/or impeller(s) therefor

A positive airway pressure (PAP) device for generating a supply of pressurized breathable gas for treatment of sleep disordered breathing, the PAP device comprising:

• a blower to pressurize gas, the blower including a motor having a shaft and an impeller provided to the shaft,
• the impeller comprising:
• a top shroud having a frusto-conical shape, and the top shroud forming a center opening;
• a bottom shroud having a substantially planar shape;
• and
• a plurality of vanes extending between the top shroud and the bottom shroud,
• each of the plurality of vanes comprising a leading edge and a trailing edge,
• wherein the leading edge extends radially inwardly beyond the top shroud and is exposed through the center opening, and
• wherein the trailing edge comprises surface disturbances structured and arranged to reduce noise.