Transportation noise has become a major concern for residents in urban environments, giving rise to stacked legislation at various levels. Current noise reduction technologies do not seem to be sufficient to achieve the targeted reduction. Noise abatement classical procedures add additional weight. Active technologies are mainly applied to downscaled models components and have a moderate technology readiness level. To avoid the introduction of massive components and to improve performance, especially in the low frequency range, layouts of micro-slits or micro-perforated panels have been investigated. Despite their advantages, Micro-Perforate based solutions are still moderate for real problems. They are mostly intended for room acoustic applications if used as absorbers. Knowledge about their acoustic behavior for problems in the transportation industry is still ongoing research and the broadband absorption of finite-thin partitions is still unsolved. The general goal of this project is to create bio-inspired meta-structures able to efficiently dissipate flow-induced noise without requiring increasing area or added weight. Inspired by the silent flight of certain species of night bird predators like the owl, our research team has proposed a single-layer micro-perforated coating placed over the floor of a shallow cavity that can provide up to 10dB reduction in the tonal noise induced by the acoustic resonances of the cavity at low subsonic Mach number. It has been also shown that a suitable choice of the constitutive parameters of multi-layered fibrous anisotropic materials, when shielded by an optimised micro-perforated flexible canopy, could dramatically enhance the absorption of low-speed flow-induced noise at mid frequencies.
These findings about the combined effects of surface roughness and internal anisotropy on the reduction of airframe noise provide the key hypothesis underlying the current project: extraordinary low-frequency absorption of flow-induced noise can only be achieved at sub-wavelength scales from the development of bio-inspired hierarchical meta-materials, combining thin micro-perforated inclusions periodically assembled across multiple length scales in a soft anisotropic matrix of fibrous material. This hierarchical micro-structure with a double property of self-similarity and periodicity at each level of the hierarchy is the building block on which will be designed the materials studied in the current project. Although it is anticipated that these structures will enhance band gap nucleation and so filtering and dissipation of acoustical energy over a wide frequency range, wave propagation into these materials is still unclear and calls for new research, especially when coupled to an external flow.