The propeller was invented 200 years ago. I think the aerodynamics for silent ones have been known for 70 years (laminar flow). The rest is just politics - tho

The propeller was invented 200 years ago. I think the aerodynamics for silent ones have been known for 70 years (laminar flow). The rest is just politics - those who pay for the conditioning system are not the ones who suffer from the noise - and even if they were, they have no competence to differentiate. Maybe if apartments had noise ratings, and they could charge neighboring buildings for the measurable loss of worth from noise. Maybe there are precedents from airfield politics.