# Bio-Inspired Aerodynamic Noise Control: A Bibliographic Review
## 2 Biological Studies of the Owl's Silent Flight
Noise measurement results of 50 successful flyovers showed that the barn owl produces flight noise that is 3-8 dB below that of other birds, even under similar flight speeds. The third-octave-band sound pressure level spectra of the barn owl are significantly lower than both the common kestrel and the Harris hawk at frequencies above 1.6 kHz (in fact, too low to be measured above 6.3 kHz). Since the mean flight speeds of different birds are not too different in the flying noise measurement (see Table 1), the authors drew the conclusion that \"the silent flight of the owl is not only an outcome of its low flight speed but is also a direct consequence of its plumage adaptation\". [PERSON] et al. [29; 30; 34] carried out aerodynamic noise measurements on the prepared wings of two species of silently flying owls (barn owl and tawny owl) and three non-silently flying birds (buzzard, sparrowhawk and pigeon) in an aeroacoustic open-jet wind tunnel [35] using a 56-channel microphone array and high resolution multichannel data acquisition systems. The advantage of this study is that the noise level can be compared at exactly the same speed. The advantages and disadvantages of the noise measurements on flying birds and prepared wings are presented in Table 2, which shows that they are complementary approaches. The results show that both owl species have a significantly lower gliding flight noise than other birds for frequency bands from 800 Hz to 16 kHz, over the whole \begin{table}
\begin{tabular}{l c c c c c} \hline \hline \multirow{2}{*}{**Species**} & \multirow{2}{*}{**Mass (g)**} & \multirow{2}{*}{**Number of Flights**} & \multicolumn{2}{c}{**Mean**} & \multirow{2}{*}{**Max**} & \multirow{2}{*}{**Min**} \\ \cline{3-4}  & & & & **Speed (m/s)** & \\ \hline Common kestrel & 198 & 31 & 5.2 & 6.2 & 3.8 \\ Harris hawk & 660 & 5 & 5.3 & 6.4 & 4.2 \\ Barn owl & 298 & 14 & 5.4 & 6.7 & 4.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of the flight noise test conducted by [PERSON] et al. [31; 32]. range of flow speeds between approximately 7 m/s and 20 m/s. Moreover, the observation of 3D sound maps revealed that the noise sources of the buzzard were located near the wing tip, while they were distributed on the wing surface of the tawny owl. These studies reaffirmed the speculation of [PERSON] et al. [31; 32] that the silent flight ability of the owl is indeed related to the special wings and feathers characteristics, not only a consequence of the low flight speed. The special wings and feather structures evolved by the owl, which result in silent flight, first attracted the attention of ornithologists. In 1934, [PERSON] [20] qualitatively compared the wings and feathers of the silently flying owls with that of non-silently flying birds, and then he first reported three distinctive and unique characteristics of the owl's wing, which were held responsible for the silent flight: (1) the leading edge serrations, as also observed by [PERSON] [36]; (2) the soft fringes at the trailing edge of the main wings and of some primary feathers; and (3) a fluffy down-like upper surface and a velvety lower surface on the owls' wings, as well as thick down on their legs. In the early 1970s, [PERSON] et al. [24] built a 240 m\({}^{3}\) reverberation chamber and spent more than nine months training one specimen of the Florida barred owl to fly along a certain trajectory from the ground towards forwards food in a reproducible way. In the conducted flyover measurements, the flight speeds, as well as the emitted noise of the owl at different heights from the ground, were measured. This study confirmed [PERSON]'s conclusions since the owl would emit sounds as strong as other flying birds if the leading edge serrations and the trailing edge fringes were removed. Moreover, the flying characteristics would also be severely damaged without these devices. [PERSON] et al.