The sound must be deafening, as if you had four hair-dryers at the same time. The Impeller drone invented by Royal College of Art student Marcus Kung is unfortunately not the quietest, but has a big advantage: there is no propeller to be found. Well impellers. I did not know the word either, but an impeller is a centrifugal fan. You know them, because they are also in, for example, hair dryers.
The principle is basically the same: the air is pulled in from the back, but not pressed out at the front, but on the side. If you then put four of those things as wheels on a drone and tilt them 90 degrees, you can create an upward pressure and the Impeller Drone can fly without one moving part.
Not having propellers has a number of advantages besides the disadvantage of the enormous noise. The biggest thing is that much less force is needed to get the drone higher in the air, so for projects where noise is less of an issue but energy consumption, this would be a very suitable design. It also ensures that the drone is much more resistant to collisions with other objects, because there is no propeller that can break off and you can actually give the entire device a bumper to absorb shocks and collisions.
That makes an impeller drone very suitable for dangerous chores, or in places where the elements can play a part as a pilot. Then a little extra noise is easy to oversee, is not it? Unfortunately, it is still a design that is not in production, so if you want to have one you will really need to contact Marcus himself.