There's always a hope that someone might invent a really silent propellor, and there's some funky stuff you can do there with trailing edge profiles, but I don't think we'll see a huge leap beyond toroidals for a while.
Related note - the offset bit is similar to how car tyres work by having patterns in the tyre which spread the frequency response generated by each, so they don't have a multiplicative effect.
(I can get really boring on applications of acoustic design but I'll cut off there)
I found that fascinating. TIL about the car tire noise.
I think you've got the right kind of audience here.
The natural extent of this progression is to have one or two very large 2-4 blade rotor (for low disc loading, high endurance, lower rotor speed and noise), with a small auxiliary rotor for yaw control if only using one lift rotor.
Yes, the natural endpoint is in fact just a helicopter. Only reason we don't see more of that (edit: in photo drones specifically) is the mechanical complexity.
For FPV stuff, I've flown everything from 25mm to 9" (prop size). The ones that seemed quietest were 2.5"-3", very light, and designed for performance.
I think it has to do with the pitch of the noise and the "idle point" of your motors.
Smaller props turn faster, and the faster they're turning the higher the pitch of the noise they produce. At some point it borders on inaudible. Even if it's technically producing more decibels (It might be! I've not tested that.), it isn't as disruptive. Larger props turn slower, producing less noise and at a lower pitch.
"Idle point" is what I'm calling how much throttle you have to give it to hover. For my favorite 5" freestyle FPV quad, that's 18% (+/- 2% depending on temperature and humidity). It's acceptably quiet if I'm just hovering or slowly moving around, but sounds like an angry weedeater if I'm flying aggressively. The idle point for my 2.5"-3" quads is probably around 25%; my 7" long range build is 12%.
The more throttle you give it, the faster the props spin, the louder and higher pitched it is.
Currently, only the video is usable from drone footage, which is a huge limitation. All drone footage either has music or an audio track from an external source overlaid on top of it.
I wonder if the propeller and wind noise could be cancelled or removed with some fancy algorithms. It wouldn't eliminate the produced noise, but it would at least make the audio track usable.
For what use cases? Outdoor concerts? The acoustics at 100ft in the air are unlikely to be great anyways.
I wouldn't call it a huge limitation.
The only case that comes to mind where it was really successful IMO is if you're using an FPV quad to chase a vehicle: race cars, dirt bikes, monster trucks, etc. All of those things are loud enough that they're clearly audible over the prop and wind noise. If anything, for dynamic chase footage, the sound of the quad's motors spinning up adds interest.
Otherwise, you're not going to hear anything worthwhile from the air.
A silent drone could be much creepier...
In my experience, a wild deer can hear a photography quadcopter from ~200' away. They "spook" if I approach closer than ~150'.