Zero-reflection acoustic metamaterial with a negative refractive index

Waves are reflected to varying degrees at the boundary between two different media. A reduction of this reflection is important for many materials, including acoustic metamaterials. Here, we theoretically develop a balanced acoustic metamaterial that exhibits zero reflection between the metamaterial and air over its entire refractive index range, regardless of the sign of the refractive index. The metamaterial is realized using unit cell structures containing membranes and open tubes, and the material performance is verified experimentally, demonstrating sound reflectance of less than 10% in the frequency region that satisfies the homogeneous medium condition. The zero-reflection characteristic of the acoustic balanced metamaterial can improve the performance of new acoustic devices such as the invisibility cloak and super-lens and can also contribute to basic research of acoustic metamaterials.