Are all Drones so loud? Is there any possible way to make the quieter? Even silent? Looking through the videos and reading a bit it sounds like an issue is with the gears or motor producing the majority of the sound.
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.
I'm assuming you're referring to fixed-pitch hovering drones. There, most of the noise is made by the propellers. For reference, running a quadcopter at full blast with no props on is pretty quiet.
There are some optimizations you could theoretically do by moving to variable pitch, and trading off pitch for RPM (or the other way around, I don't fly helis myself)
After researching the blades a little further is seems a company called eurocopter did find a way to reduce the noise made by the propellers [1]. Heres a video of the difference between the two blade types [2]. Do you think this would be applicable to the blades at the drone level?
Helicopter propellers are typically even louder, since the tips are going supersonic (due to the diameter of the propellers). The trademark "chop chop chop" sounds, as far as I know, are caused by sonic booms. The tech you're linking to is about minimizing the source of sound for the sonic boom.
Airplane and drone props are entirely subsonic. They make a lot of noise when the pitch is significantly different than the speed that the airplane "should" going at. In other words: if you imagine a propeller cutting through butter, there is a certain distance it would go in one rotation. Propellers are loud when they're going significantly over or under that distance. Also - I'm just a hobbyist, so my explanation may not be correct.
This effect is especially noticeable with fixed-pitch propellers used for hovering.
For reference, check out these videos. Note that airplanes have 1D variable pitch.
