Silent PPG project

Looks like near silence is possible!

See https://phys.org/news/2019-03-acoustic-metamaterial-cancels.html

3D-printed “ringlike structure, created to mathematically perfect specifications, for cutting out sounds while maintaining airflow”

“The basic premise is that the metamaterial needs to be shaped in such a way that it sends incoming sounds back to where they came from”

"By comparing sound levels with and without the metamaterial fastened in place, the team found that they could silence nearly all—94 percent to be exact—of the noise, making the sounds emanating from the loudspeaker imperceptible to the human ear.

“Drones are a very hot topic… people are complaining about the potential noise.”

“The culprit is the upward-moving fan motion,” Ghaffarivardavagh says. “If we can put sound-silencing open structures beneath the drone fans, we can cancel out the sound radiating toward the ground.”

Source paper from https://journals.aps.org/prb/abstract/10.1103/PhysRevB.99.024302 attached

PhysRevB.99.024302.pdf (1.2 MB)

So I propose a simple one-piece 3D-printed frame with a single pair of ~31.5" counter rotating triple props – similiar to one-half of @RogerThat 's design.

Need someone to do the acoustic impedance calculations and design… hopefully the rim doesn’t have to be 6-inches thick.

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I suppose the helical silencer would be placed after the props, on the outflow, so it could have the same outer diameter, just with a smaller outlet (perhaps 60%). Would this affect thrust much? Perhaps not. Might not need to increase rim length much either.

Only looked at the vid - they put it in a tube at the end, is that part of their tuning?
I dont see how it wold work for putting around a prop.

Single piece 30" 3D print - thats a heck of a big print and they have printers that can do it but I dont see it being cost effective.

Cheers

No the tube in the video is not part of the tuning. The tech can be used in many applications including walls. In one of the photos you can see the guy blocking noise to his ears. It works by redirecting the sound waves. The included pdf gives more details. I see it as like kind of like a mute on a trumpet. So if the two counter rotating props have a wide carbon rim, perhaps it could be glued on to the outflow end. As it is mostly hollow, it could probably be very light. Perhaps if can be shaped to aid flow.

Cosine Additive can print 1100x900x800mm polycarbonate with 50% carbon fill. A previous large quote worked out to about $32/lb. Not sure if that would be strong enough for the entire frame, perhaps. If not, 3D printing continuous carbon fiber with PEEK plastic via robotic arm is a thing now too. Would be cool to do the whole unit in one piece. Might only cost a few hundred bucks. Easy to modify the design at each iteration.

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This works by redirecting the sound waves back toward their source… I don’t think anyone really wants that who’s flying their paraglider around :wink: It’s possible that the sound waves being reflected will be canceled by the sound waves being emitted so that it is a nicer flying experience, but all that energy has to go somewhere… In the drone, they shoot it up into the sky where no one cares…

Maybe we could mount two… one after the props, and one before the pilot… then we just figure out how to harness all that energy building in the center :wink:

Yes, silencers on both ends.