What kind of 'drones'? Tri-/Quad-/Hex-/Octa- etc copters are typically direct-drive, with unshrouded conventional propellors. You could certainly make them quie

What kind of 'drones'? Tri-/Quad-/Hex-/Octa- etc copters are typically direct-drive, with unshrouded conventional propellors. You could certainly make them quieter by optimising the pop design, putting the props inside shrouds, etc. Just using a bigger, slower spinning prop, on a bigger (so the props are further apart) chassis can go a long way, in terms of noise. For an electric helicopter, most of the noise comes from the pressure waves coming off the tail blades interfering with the waves coming off the main rotors. Full size helicopters minimise this by switching to NOTAR and fenestron designs. Gear noise can be substantial on older designs of model helicopter, direct-drive is extremely rare, I think only Robbe ever made a direct drive design, cheap, strait cut gears are more common. Belt drive designs are available now though, and at low RPM, reasonable quiet. If you're really concerned about noise though, you'd just build around a motor glider. So long as its not going so fast that the whole thing whistle's it'll be effectively silent.