Many species of owl, including the barn and barred owl, use both visual and bi-aural location to search for prey around dusk and at night. Their bi-aural location system has a maximum sensitivity between 3-6kHz although the hearing of the owl has an upper limit in excess of 20kHz. Its prey, typically voles and mice, squeak and and squeal in the frequency range of 3-6kHz and this range of frequency includes the rustling of leaves made by prey. The hearing of these prey is acute between 2-20kHz. The owl in both gliding and flapping flight generates noise at low frequencies below 2kHz, but is almost totally silent at frequencies above 2kHz. Hence the flight of the owl is almost silent to its prey. When an owl attacks its prey from its perch from a height of 6-10m the prey are unaware of its approach before they are captured by its long talons.The noise suppression devices developed by owls during their long evolution period of millions of years have been identified. They comprise (i) leading edge feathers in the form of a comb, (ii) trailing edge feathers in the form of a fringe, and (iii) fluffy down on the wings and legs.The paper discusses the aerodynamic characteristics of each of these devices and suggests tentative explanations as to how these flow characteristics lead to a large reduction in the noise generated by the owl in flight and especially in its critical range of frequencies above 2kHz.

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Three model problems were examined to assess the di culties involved in using a hybrid scheme coupling ow computation with the the Ffowcs Williams and Hawkings equation to predict noise generated by v ortices passing over a sharp edge. The results indicate that the Ffowcs Williams and Hawkings equation correctly propagates the acoustic signals when provided with accurate ow information on the integration surface. The most di cult of the model problems investigated inviscid ow o ve r a t wodimensional thin NACA airfoil with a blu -body vortex generator positioned at 98 percent c hord. Vortices rolled up downstream of the blu body. The shed vortices possessed similarities to large coherent eddies in boundary layers. They interacted and occasionally paired as they convected past the sharp trailing edge of the airfoil. The calculations showed acoustic waves emanating from the airfoil trailing edge. Acoustic directivity and Mach n umber scaling are shown.

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