Perforated liners are used in many applications to absorb sound. Liners can be placed inside the nacelle of an aircraft engine and throughout the bypass duct to absorb noise propagating forward and aft. Similar liners can be used in low Mach number applications, for instance, to improve wind tunnel acoustics by diminishing wall reflections. These perforated acoustic absorbers can even be used to improve room acoustics in still air, although, these designs needn't account for mean flow over the surface. Our group has investigated the performance of complex multi-layer liners constructed with an absorbing material, i.e. foam or bulk absorber, topped by a perforated sheet and wire or fabric cloth. These liners are designed for low-drag and self-noise in low Mach number (M<0.3) applications.
We are currently investigating the use of acoustic metamaterials for acoustic sensing. These engineered surfaces can be used in turbulent environments to sense the direction of external acoustic sources.