I'm sceptical that that's true. We can put mufflers on gas engines to make them quieter. The prop, though, is moving a lot of air, and there's no way to get around that and have it still work.
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.
