Inclined space-coiling metamaterials for highly efficient sound energy transmission at various incident angles

In many acoustic components, strong directional response exerts considerable influence on sound wave energy transmission, restricting their widespread applications. Here, we report new designs of an inclined metamaterial channel that enable improved transmission behaviors and broadband performance under variable-angle incident radiation. For the two-dimensional sound wave guide, the tilt angle is tuned to cater for oblique incidences. The inclined metamaterial is modified based on the normal space-coiling sound wave guide with impedance matching layers. At different incident angles, the inclined waveguides are capable of providing proper gradient parameter distributions to match the impedance value of the outside environment and the inner structure. Both experimental and numerical results show that the inclined metamaterial designs improve the broadband transmission efficiency for 2kHz with specific incident angles. The anomalous refraction behaviors are demonstrated based on the inclined metamaterial units, showing a huge potential for wavefront engineering and acoustic transparency.