Some producers solve it by framing the blades:
I have another offer:
The blades 1 have wingtips (like modern airplanes) 2 which blended with circle 3. This circle may carry some extra small blades 4 (could be more than two) which located firther from the center and create relatively better lift.
The blades are safer, the lift force is stronger - slower speed, less turbulence = less noise.
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