I'm aware of some research from my department, though not the lab I work at: https://www.ingentaconnect.com/content/ince/ncej/2013/00000061/00000004/art00001 h

I'm aware of some research from my department, though not the lab I work at: https://www.ingentaconnect.com/content/ince/ncej/2013/00000061/00000004/art00001 https://www.ingentaconnect.com/content/ince/ncej/2013/000000... https://www.sciencedirect.com/science/article/abs/pii/S0022460X15004848?via%3Dihub https://www.sciencedirect.com/science/article/abs/pii/S00224... https://www.sciencedirect.com/science/article/abs/pii/S0022460X13002824?via%3Dihub https://www.sciencedirect.com/science/article/abs/pii/S00224... They place specially shaped "obstructors" in fan ducts which create destructive interference with some of the prominent noise tones.