I've not actually measured the volume of various props - I can, and should! - but based on my memory I don't think it's quite as simple as "smaller props make m

I've not actually measured the volume of various props - I can, and should! - but based on my memory I don't think it's quite as simple as "smaller props make more noise". 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.