Quadcopters can now navigate fairly well indoors. One could imagine many possible uses, including telepresence and security, but which are limited because of the loud noise.
Assuming zero 'motor noise' what are the theoretical limits to how silent a quadcopter could be, with rotors between 1 and 30 cm diameter. Also assume no 'buoyancy' (attached helium balloon) and 1kg mass.
What are the factors and what is the optimal configuration for lowest sound output from a perspective of the drone being at 1m altitude 1m away, over a flat surface in a 10mx10mx2m room with nothing else?