I found an old JPL document detailing "Aerodynamic Noise from Rotors, Propellers, and Lift Fans" (http://cafefoundation.org/v2/pdf_tech/Noise.Technologies/NASA.

I found an old JPL document detailing "Aerodynamic Noise from Rotors, Propellers, and Lift Fans" (http://cafefoundation.org/v2/pdf_tech/Noise.Technologies/NASA.1970.Prop.Noise.Review.pdf http://cafefoundation.org/v2/pdf_tech/Noise.Technologies/NAS...) which has some interesting details. A 300hp hub, spinning a 9ft diameter propeller at 1584 RPM should be roughly 72dB as measured from 1,000 feet. From a mile away, it will be more like 57dB or about TV volume from 6 feet away. -- Then again, these are spinning far faster than you'd expect from a wind turbine.