I tend to think of noise as an undesirable waste of energy because the generator was not designed for that purpose. But that doesnt mean this waste can be avoided.
So I wonder how much of this noise is due to flawed design and how much of it is just impossible to get rid of without reducing the plane performance (speed) altogether.
That is, 1. is it possible to make an object fly at low altitude, say 1 kilometer, and still at high speed (say 700 kph), without producing any sound hearable at ground level, and 2. Would that necessarily entail sound reduction techniques that augment energy expenditure?
This is a theoretical question that doesnt require the technology to preexist but still should not violate any known law of nature.
Definitions: the flying object, whatever its shape and method of propulsion should still be able to carry passengers, so e.g. it cannot be flat like a pancake.