Low noise fan

Claim 1

A fan comprising: a hub rotatable on an axis; blades secured to said hub, each blade extending radially outward from said hub to a blade tip; an inner ring coaxial with said axis having a portion axially offset from said hub;. a shroud disposed around said blade tips;. stators extending radially from said inner ring to said shroud, a first one of the stators being angularly spaced from a second one of the stators so that a first acoustic wave generated by one of the blades passing over the first one of the stators is substantially cancelled by a second acoustic wave generated by said one of the blades passing over the second one of said stators, wherein said stators comprise pluralities of paired first and second stators, each pair comprising one of the plurality of first stators and a corresponding one of the plurality of second stators, the plurality of first stators being leading stators and the plurality of second stators being trailing stators, the leading and trailing stator of each pair angularly spaced from each other by a first distance, the trailing stator of each pair of stators being angularly spaced by a second distance from a leading stator of an adjacent stator pair, said first and second distances being different from one another and selected so that acoustic waves generated by a fan blade passing over the leading and trailing stators of each pair have a phase difference of 180°.