Patent ID: 8777560

Claim:
An axial flow fan comprising: a rotor rotatable about an axis, the rotor having a number of blades and a radius, the number of blades generating a number of tonal noises when the rotor is rotating in a non-uniform flow, the number of tonal noises each having a phase and a magnitude; and at least one obstruction being positioned at a first distance radially away from the axis and at a second distance axially away from the rotor, the first distance being less than the radius, the at least one obstruction being positioned around the axis such that the at least one obstruction generates a second noise, when in the non-uniform flow, having a phase that is out of phase with the phase of one of the number of tonal noises, the second distance being selected such that a magnitude of the second noise is substantially equal to the magnitude of the one of the number of tonal noises, the at least one obstruction being shaped such that an interaction of the at least one obstruction with the rotor has a low harmonic content rate, and the at least one obstruction being a sinusoidal obstruction forming a ring, the sinusoidal obstruction having a number of lobes.