Silent Owl Flight: Bird Flyover Noise Measurements

Most genera of owls (Strigiformes) have the ability to fly silently. The mechanisms of the silent flight of the owl have been the subject of scientific interest for many decades. The results from studies in the past are discussed in detail in this paper and the rationale for the present research is given, which included flyover noise measurements on different species of birds. Successful acoustic measurements were made on a Common Kestrel, a Harris Hawk, and a Barn Owl. Measurements on three other birds did not lead to reliable results. The setup and procedure used for the outdoor measurements are discussed. These include the estimation of the trajectory from dual video camera recordings and microphone-array measurements with a moving-focus beamforming technique. The main result from the 50 successful flyovers is that the owl flight produces aerodynamic noise that is indeed a few decibels below that of other birds, even if flying at the same speed. This noise reduction is significant at frequencies above 1.6 kHz. At frequencies above 6.3 kHz the noise from the owl remains too quiet to be measured. [PUBLICATION ABSTRACT]