US Patent 7992674 Dipole flow driven resonators for fan noise mitigation

A fan system includes a rotor supported for rotation about a fan axis. The rotor has a central hub and a plurality of blades each extending outwardly from the hub to a tip. The rotor blades define a rotor plane perpendicular to the fan axis. A first acoustic resonator has an opening disposed on a first side of the rotor plane and a second acoustic resonator has an opening disposed on a second side of the rotor plane. The acoustic resonators are configured to provide a dipole resonator system operable to at least partially reduce a blade pass frequency tone in an upstream and a downstream direction simultaneously.