Why can a ragged owl-wing trailing edge be quieter than a clean sharp edge?

Answer: Breaks up edge vortices

Adds more lifting area — Extra area is not the point. A trailing-edge fringe can be thin and wispy, yet still change how the two air streams behind the wing meet. If the goal were just more area, a broad smooth flap would do better. The noise lesson is subtler: the edge shape can change vortex formation without adding much lifting surface.

Breaks up edge vortices ✓ — Right. A sharp clean edge can shed organized vortices, which become a strong source of broadband edge noise. Owl-like fringes and porous edges can make that interaction weaker and less coherent. Reviews and simulations describe trailing fringes as changing how air streams mix at the edge. So the messy-looking edge is a flow-control feature, not damage.

Absorbs sound like foam — Foam absorption is a plausible everyday sound idea, but the trailing edge is not mainly a sponge. A moving wing makes noise by unsteady flow at the edge, so the owl-like fringe works before the sound is fully born. It changes how air streams and vortices meet, rather than simply soaking up finished sound waves.