Experimental demonstration of a three-dimensional omnidirectional and broadband acoustic concentrator using an anisotropic metamaterial

We report the theoretical design and experimental demonstration of a three-dimensional (3D) omnidirectional and broadband metamaterial-based concentrator for airborne sound. The proposed mechanism uses a homogeneous anisotropic acoustic metamaterial with an ellipsoidal equifrequency contour to efficiently redirect the acoustic energy impinging on its outer surface into the central region, regardless of the incident direction. A design of the metamaterial unit cell is proposed as a practical implementation of our strategy, which is simply realized by perforating a solid spherical shell with a linearly shrinking cross section in the radial direction. We analytically and numerically prove that the non-resonant anisotropic effective acoustic parameters required for building the concentrator are produced with such a design. Good agreement is observed between the theoretical predictions and experimental measurements. An effective concentration of the incident acoustic energy is observed within a broadband that ranges 1000–1600 Hz. The experimental realization of this 3D acoustic concentrator with a simple design, low energy loss, replaceable constituent material, and omnidirectional and broadband functionality offers new possibilities for acoustic manipulations and may have important applications in a plethora of scenarios ranging from energy harvesting to noise mitigation.