Silent Owl Flight: Bird Flyover Noise Measurements

Mostgeneraofowls(Strigiformes)havetheabilityto flysilently.Themechanismsofthesilent flightoftheowlhave been the subject of scientific interest for many decades. The results from studies in the past are discussed in detail in thispaperandtherationaleforthepresentresearchisgiven,whichincluded flyovernoisemeasurementsondifferent 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 thatofotherbirds,evenif flyingatthesamespeed.Thisnoisereductionissignificantatfrequenciesabove1.6kHz.At frequencies above 6.3 kHz the noise from the owl remains too quiet to be measured.