Silent Owl Flight: Comparative Acoustic Wind Tunnel Measurements on Prepared Wings

The wings and feathers of most genera of owls show unique properties that are held responsible for the silent flight of the owls. This ability to fly silently has long been of interest for engineers, with the aim to transfer the basic noise reducing mechanisms to technical applications such as blades of fans and propellers. The present paper describes acoustic and aerodynamic wind tunnel measurements on prepared bird wings of different species, among them two silently flying species of owls, the barn owl (Tyto alba) and the tawny owl (Strix aluco). The different wings are characterized in the study as technical airfoils in terms of their acoustic and aerodynamic performance. The experiments took place in an aeroacoustic open jet wind tunnel using microphone array measurement technique and deconvolution beamforming algorithms. Simultaneously to the acoustic measurements, the aerodynamic forces of the wings were captured using a six-component balance. This study, which is a complementary study to the approach of performing flyover measurements on birds flying in gliding flight, further confirms experimentally that the silent owl flight is a consequence of the special wing and plumage adaptations of the owls and not a consequence of their lower flight speed only.