# Bio-Inspired Aerodynamic Noise Control: A Bibliographic Review
## 2 Biological Studies of the Owl's Silent Flight  
The aeronautical community's interest in the silent flight of the owl is due to the fact that only the owl can fly almost silently in all the flying creatures and man-made aircraft [22]. Even for human observers, they cannot perceive the existence of a \"silent\" owl when the owl is gliding beyond a distance of approximately 3 m [23; 24]. Fossil records show that the owl lived together with prehistoric animals, such as pterosaurs, about 75 million years ago. After a long period of evolution, the owl evolved the silent flight ability approximately 20 million years ago. From the viewpoint of natural selection and biological evolution, on the one hand, the owl prey on nocturnal creatures (such as mice and other small animals) for a living [25], thus their vision degrades while hearing enhances, which make the owl rely mainly on the weak voice to locate and track their potential prey. This requires the owl to be able to fly silently to avoid the sound of themselves to cover that of their prey, which in turn would disturb their ability to aurally locate the prey. On the other hand, typical prey of the owl has very high alertness, even a small  
Figure 2: An overview of the studies on bio-inspired aerodynamic noise control: (**a**) by categories; and (**b**) by years (LES and TES denote leading edge serrations and trailing edge serrations, respectively). dangerous signal would let them take evasive action and escape capture quickly. This also requires the owl to be able to identify and fly to the location of prey quickly and quietly. The hearing of the owl's prey is acute in the frequency ranges of 2-20 kHz. However, the owl only generate noise at frequencies below 2 kHz, no matter in gliding flight or flapping flight. Therefore, the flight of the owl is almost totally silent to their prey [22]. The first published flight noise measurements, which were conducted by [PERSON] and [PERSON] [26; 27], revealed that many species of owl (five species of small owls, three species of medium-sized owls and two species of large owls) do not generate ultrasonic noise components (relating to frequency ranges above 15 kHz) during the flapping flight phase, compared with other birds. In fact, [PERSON] et al. [24] found that a significant part of the acoustical noise spectrum of the owl was shifted strongly towards the low frequency ranges below the hearing sensitive range of humans and their typical prey. However, no comparison of the measured flight noise spectrum of the owl to other non-silently flying birds was made in this study. [PERSON] et al. [28] presented a comparison of the flight noise between the tawny owl and the mallard duck. The acoustical results gave a proof that the flight noise of the former was indeed lower than that of the latter. The mallard duck generates noise in the range of approximately 3-5 kHz with its peak at around 4 kHz, while the tawny owl's flight noise has noticeable low-frequency characteristics, which ranges from 50 Hz to 1.5 kHz, with its maximum being between 200 Hz and 700 Hz. However, the measured flight noise was recorded in a somewhat uncommon manner: the flying mallard duck was recorded outdoors while the tawny owl was measured in a large gym since the flyover noise of the owl was too low to be recorded in an outdoor environment. Moreover, no correction for different flight speeds of the duck and the owl was performed within their study, thus it was impossible to determine whether the silent flight of the owl mainly originates from low-speed flight or its evolved special morphological structures [29; 30]. Recently, a horizontal 92-channel microphone array and two high-speed cameras were used by [PERSON] et al. [31; 32] to measure the flight noise and speeds of a barn owl and two non-silently flying species (common kestrel and Harris hawk) in an outdoor environment. During the measurement, the birds glided over the microphone array under natural flying conditions, according to their natural behavior.