There are plenty of examples scattered throughout build threads but I don't have them catalogued in that way. The fabric type doesn't really matter, they are all transparent enough at low frequencies.
How to do the holes is a little more complicated but practically speaking more and larger is better. A few smaller holes along the wall will turn the soffit into a tuned Helmholtz resonant absorber... very effective over a narrow frequency band. Figuring out what frequency of absorption you'll get with a certain soffit and hole combination isn't trivial, and you (probably) don't want to attack just a narrow frequency band anyway. If you were really determined you could theoretically internally divide the soffit into a few segments along its length and tune each to attack a specific measured in room problem nodal frequency. But if the holes become large and numerous enough the whole soffit becomes a broadband absorber, which isn't as effective as the resonant absorber at those tuned frequencies, but remains moderately and uniformly effective over a wide frequency band.
Carrying that idea to its end, you can just leave one or both faces of the soffit (horizontal, vertical) open framed and use cloth instead of drywall to "close it in" visually, trimmed appropriately. That has been done many times.
Last edited by Bigus; 04-29-2019 at 08:07 AM.