Most birds are audible in flight because their wings shed turbulent eddies that radiate sound, especially at the trailing edge. Owls suppress this at the source: leading-edge serrations fragment incoming turbulence into smaller structures, the fringed trailing edge lets airflow leave gradually instead of snapping off a sharp edge, and the velvety upper surface damps the high-frequency components that would otherwise carry.
The result is flight noise with energy concentrated at low frequencies and sharply reduced above roughly 2 kHz — under the hearing range of the rodents owls hunt. Researchers have confirmed the effect with microphone arrays in anechoic chambers, and those same measurements hand engineers a target: shift or absorb sound where the listener cannot hear it, rather than trying to eliminate turbulence altogether.