The propeller was invented 200 years ago. I think the aerodynamics for silent ones have been known for 70 years (laminar flow).<p>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.<p>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.