ok, I think the confusion is coming from the fact that I’m using the word fin to describe the wrong part. In your diagram, it looks like “fin” is used to describe the well walls.
So I’ll reword. What is the purpose of the well walls? And what sort of difference does it make to not have them?
oh I see, I think you mean diffusers with circular (tuboid) holes or polar diffusers.
they offer diffusion pattern that follows a polar 3 dimensional (solid angle) geometry. They are good for places where audience is sitting in a 3dimensional pattern, as in multiple tiers with side VIP balconies.
The skyline offers 2 dimensional diffusion pattern, vertical and horizontal and a basic cavity based QRD diffuser offers diffusion on 1 dimension (horizontal or vertical) based on how it is mounted.
Horizontal diffusion pattern if preferred if there only going to be 1 or 2 people in the studio. Since no one is hanging upside down from a ceiling of studio, vertical diffusion is not super beneficial. Which is why most of the times cheaper 1 dimensional QRD cavity based diffusers are used more.
Maybe a picture will be better.
Well walls (fins?).
No well walls.
With well walls
Without well walls
ah that makes it much easier. The fins themselves dont effect the reflective bounding box as much. Its an attempt at catching more oblique reflections coming into the diffuser at larger angles of incidences, but those are quite subtle .
the fins could technically be removed as a construction aesthetic or as a measure to save material cost. A non-fin version would still diffuse and be quite predictable at the same frequency ranges but the operation of the non-fin would be limited to a certain ‘incidence span’ pi - 2 ArcTan (d^-1). Theoritically fin versions are better but only very slightly. The cost of materials could be omitted. It should still have almost the same coverage