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There's some interesting research around toroidal propellers which move the same amount of air, but create far less turbulence and so make a less annoying noise

There's some interesting research around toroidal propellers which move the same amount of air, but create far less turbulence and so make a less annoying noise. However, they're not going to get significantly quieter until the energy density of the batteries gets significantly better (a lot of what you're moving with a drone is your energy storage). 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.

jcims 3y ago
Zipline has built an interesting asymmetric design that's remarkably quiet. Mark Rober featured it in one of his recent videos. https://www.youtube.com/watch?v=DOWDNBu9DkU&t=831s https://www.youtube.com/watch?v=DOWDNBu9DkU&t=831s
- pwatsonwailes 3y ago
  It does a similar sort of thing - asymmetric offset propellors shunt the frequency of what gets emitted way down. It still emits a similar amount of energy, but the direction of emission changes, and with the lower frequency, there's a lower perceived sound. Toroidal props actually have less sound coming off them, and at a lower frequency. I've got some ideas about how you'd potentially then have a greater spread of frequency by creating an offset toroidal, but I suspect the efficiency goes down, and I've got better things to do with my time than model sound profiles of weird, hard to manufacture (other than printing) prop geometries. 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)
  - robotguy 3y ago
    >I can get really boring on applications of acoustic design I found that fascinating. TIL about the car tire noise. I think you've got the right kind of audience here.
    - jcims 3y ago
      100%
    - lazycouchpotato 3y ago
      There's a great video by Andrew Lam on YouTube about the engineering that goes behind making a tire. https://www.youtube.com/watch?v=C9aX0vohM_8 https://www.youtube.com/watch?v=C9aX0vohM_8
    - schrectacular 3y ago
      Agree, GP there is an an audience for your pontificating, if you ever feel like it.

jcims 3y ago

Zipline has built an interesting asymmetric design that's remarkably quiet. Mark Rober featured it in one of his recent videos. https://www.youtube.com/watch?v=DOWDNBu9DkU&t=831s https://www.youtube.com/watch?v=DOWDNBu9DkU&t=831s

pwatsonwailes 3y ago
It does a similar sort of thing - asymmetric offset propellors shunt the frequency of what gets emitted way down. It still emits a similar amount of energy, but the direction of emission changes, and with the lower frequency, there's a lower perceived sound. Toroidal props actually have less sound coming off them, and at a lower frequency. I've got some ideas about how you'd potentially then have a greater spread of frequency by creating an offset toroidal, but I suspect the efficiency goes down, and I've got better things to do with my time than model sound profiles of weird, hard to manufacture (other than printing) prop geometries. 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)
- robotguy 3y ago
  >I can get really boring on applications of acoustic design I found that fascinating. TIL about the car tire noise. I think you've got the right kind of audience here.
  - jcims 3y ago
    100%
  - lazycouchpotato 3y ago
    There's a great video by Andrew Lam on YouTube about the engineering that goes behind making a tire. https://www.youtube.com/watch?v=C9aX0vohM_8 https://www.youtube.com/watch?v=C9aX0vohM_8
  - schrectacular 3y ago
    Agree, GP there is an an audience for your pontificating, if you ever feel like it.