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The Silent Flight of Owls
https://www.knowablemagazine.org/article/technology/2020/how-owls-fly-without-making-a-sound [knowablemagazine.org] describes an interesting new look at owl flight, and a recent attempt to figure out why they are so much quieter than other birds, and quieter than just about any other flying thing. Based on this paper, https://www.annualreviews.org/doi/10.1146/annurev-fluid-010518-040436 [annualreviews.org]
Laboratory measurements have shown that the slight swoosh made by a barn owl is below the threshold of human hearing until the owl is about three feet away — a feat of stealth that biologists and engineers are far from completely understanding. But researchers from both disciplines are working to solve the riddle of silent flight — some with the aim of designing quieter fans, turbine blades and airplane wings.
Such owl-inspired innovations can reduce noise by as much as 10 decibels, similar to the difference in noise between a passing truck and a passing car...
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First, Graham [1934] pointed out an unusual structure called the “comb,” which literally looks like a comb projecting forward from the wing’s leading edge. Second, he noted that most of the owl wing is covered with a soft layer of velvety feathers. Finally, he observed that the feathers on the trailing edge of the wing form a ragged fringe.Most researchers still agree that the comb, the velvet and the fringe combine in some way to reduce noise, but the owl may have more tricks up its sleeve. “When all is said and done, I think we’ll have a number of mechanisms, including Graham’s,” says Clark.
The article goes on to quantify the sound of an owl relative to human hearing. Also shows some CFD that purports to show tiny vortices shed off an owl wing, these rotate both ways and there is the possibility of destructive interference.
Once, sitting around a dying campfire in the north woods with a few friends, we all noticed a presence overhead, the barest of swishes, could have only been an owl. It was a pitch black night, we never saw a thing.
