They glide silently through the night, a blur of noctilucent plumage. Unlike most birds, owls make virtually no noise when they fly, allowing them the element of surprise while hunting their prey. Several species, in fact, have developed specialized plumage to minimize aerodynamic noise from their wings.
According to a study published in The Journal of Sound and Vibration, owl’s feathers move through the air at inaudible frequencies above 1.6 kilohertz. Three plumage factors make this possible. A stiff comb on the leading edge of the wing breaks turbulence into small currents and reduces sound. An uneven fringe at the wing’s trailing edge further muffles noise. But perhaps the most notable mechanism is the soft, velvety down that covers the owl’s feathers.
The down consists of hairs that grow perpendicular to the feather’s surface but bend over in the flow direction to form a forest-like thatched canopy of interlocking fibers.
By studying the sound-mitigating mechanisms of owl wings, Justin W. Jaworski, assistant professor of mechanical engineering and mechanics at Lehigh’s P.C. Rossin College of Engineering and Applied Science and a team of researchers are learning the effects of surface textures on a blade’s acoustic signature. Jaworski and his team hope that their efforts will lead to improvements in the aerodynamic design of mechanical wings.
The team recently used the downy feathers as a model for a 3-D printed wing attachment. According to Physics.org, the prototype reduced wind turbine noise by 10 decibels without impacting aerodynamics. “The canopy of the owl wing surface pushes off the noisy flow,” Jaworski said. “Our design mimics that but without the cross fibers, creating a unidirectional fence—essentially going one better than the owl.”
According to researchers, the implications of this study may be far-reaching. The acoustic insights could inspire new sound absorption techniques that incorporate flexible roughness to help decrease noise and vibrations created by aircraft wings, wind turbines, and engines. What other applications can you envision?