I have often wondered if larger, shrouded, multi-bladed rotors that can spin at a much lower speed can work. Perhaps uneven blade spacing — or even articulated blades with tunable lead-lag hinges to modulate blade spacing — to spread the noise over a greater frequency range.
See, drones traded moving parts for the annoying whiny motors(but fixed in a rigid frame). If we are going to use moving parts anyway, there's helicopters.
First-hand experience: helicopters are wildly more difficult to fly.
Perhaps the lead-lag hinges can be elastomeric and one could purchase props with different fixed hinge stiffnesses for different amounts of noise dampening or frequency spread but perhaps at the cost of prop-speed change response? There'd probably have to be some compensatory mechanism in the flight- or motor-control software that'd be set for a given prop's hinge elasticity.
Of course, I am just idly speculating here on whether there's any benefit at all to any of my blather.
Complex problem for sure.