A broadband low-reflection bending waveguide for airborne sound

We design a bending waveguide capable to transmit broadband airborne sound with high efficiency while maintaining the wavefront undisturbed. Based on coordinate transformation, analytical formulae are derived to predict the parameter distribution of the required constituent materials composing the waveguide. A practical implementation is presented by employing acoustic metafluids that are formed with periodically arranged slabs of subwavelength dimensions in air-filled acoustic chambers. By studying the acoustic properties of the unit structures in the quasi-static limit, it is demonstrated that the effective mass density and bulk modulus of the proposed metamaterial can be modulated independently by tuning the geometry parameters and the temperature in the chamber. By virtue of the free-modulated features, the range of realizable effective parameters with metafluid are substantially broadened, and the corresponding acoustic impedance can be perfectly matched to the background medium. The performance of the bending waveguide is verified by demonstrating the low-reflection transmission of broadband sound and the ability of keeping the wavefront undisturbed. According to the effective medium theory, our scheme offers the flexibility to further raise the upper limit of the operating frequency with smaller size individual elements. The proposed design with the advantage of feasibility and effectiveness in broadband shows potential for a wide range of wave-steering applications.