What lets a barn owl glide slowly and quietly, apart from its fringed, velvety feathers?

Answer: Its wings are large for its weight, so it can fly slowly with gentle flaps.

Its small body keeps the noise down, since a lighter bird makes softer wingbeats. — Small size helps a little, but it is not the main reason the flight stays quiet. A small bird that has to flap hard still makes sharp wingbeat noise. A barn owl's wings are large for its weight, so each part of the wing does less work and the air around it stays calmer. The fringes and velvet then only have to smooth an airflow that is already gentle.

Its wings are large for its weight, so it can fly slowly with gentle flaps. ✓ — Right. Large wings carrying little weight let the owl stay airborne at low speed, so it does not need hard, fast flaps. That matters because aerodynamic noise rises quickly with speed and with rough separation of the air. The fringes and velvet come second, working on an airflow the wings have already calmed.

Its wings are stiff and pointed, so they slice through the air without noise. — This sounds aerodynamic, but it names the wrong first layer. Stiff, pointed wings suit fast flight rather than slow quiet gliding. A barn owl's quiet start is broad wings that carry little load, which is why it can fly slowly without stalling. The feather microstructures then work on an airflow that is already quieter.