A flow generator (50) and humidifier (150) construction is described, including a flow generator (50) construction adapted to reduce noise output compared to known flow generators of comparable size. The flow generator (50) includes a chassis forming first (134) and second muffler volumes and a venturi-shaped connection portion (137) , and a metal/polymer composite material blower enclosure which suppresses noise from the blower. The humidifier (150) has a water container (698,699,702) which is adapted for drop in assembly in the humidifier base (503,602) so as to align with air inlet (807) and outlet (662) passages and is retained in sealing relationship with the air flow passages (807,662) by engagement of the humidifier lid (648).