It is well-known that many species of owl have the unique ability to fly silently, which can be attributed to their distinctive and special feather adaptations. Inspired by the owls, researchers attempted to reduce the aerodynamic noise of aircraft and other structures by learning their noise reduction features from different viewpoints and then using the gained knowledge to develop a number of innovative noise reduction solutions. Although fruitful results have been achieved in the bio-inspired aerodynamic noise control, as far as the authors know, comparatively little work has been done to summarize the main findings and progresses in this area. In this bibliographic survey, we systematically review the progresses and trends of the bio-inspired aerodynamic noise control, including the macroscopic and microscopic morphological characteristics of the owl wing feathers, the noise measurements on both flying birds in the field and prepared wings in the wind tunnel, as well as theoretical, numerical and experimental studies that explored the feasibility, parameter influence, aerodynamic effects and underlying mechanisms of the four main bio-inspired noise reduction techniques, i.e., leading edge serrations, trailing edge serrations, fringe-type trailing edge extensions and porous material inspired noise reduction. Finally, we also give some suggestions for future work.

The low frequency and broadband reduction of vibration and sound from damping treatment is one of the major concerns in research and engineering. In this paper, the combination of artificial metamaterial and damping design is used to improve the reduction effect. The vibration and sound radiation properties of the bare, free layered damping and metamaterial damping panels are compared and studied. Via the comparison of various panels, the vibration and sound radiation reduction and the enhancement from metamaterial damping are analysed. The effects of the material parameters of metamaterial damping elements are studied also. The numerical results indicate that the vibration reduction is significantly improved from the metamaterial damping in quite wide frequency ranges from low to high frequencies; the sound radiation is suppressed effectively also, especially the peaks are reduced obviously. A better reduction effect can be obtained via metamaterial design even with the same added mass as traditional damping treatment, especially, at the middle to low frequencies. The effects from the material parameters of metamaterial damping design are different from those of free layered damping design. And the manipulation and control of the damping effects from metamaterial design will be flexible and easy to realize. Further, an experimental system is built, and the experimental study and verification are carried out. The significant reduction and obvious enhancement from metamaterial damping design are observed from the experimental results.