Looks like near silence is possible!
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)
Need someone to do the acoustic impedance calculations and design… hopefully the rim doesn’t have to be 6-inches thick.