Figure 4. The apparent negative reflection behavior of the gradient acoustic metasurface when ξ = −0.508 k₀.

(a) Relation between sin θ_i and sin θ_re when k_s = −0.508. The blue solid line indicates the anomalous reflection state , the green solid line represents the allowed apparent negative reflection . The red dotted line represents the specular reflection , the blue and cyan dash-dot line correspond to the forbidden apparent reflection state and . (b) The relation between the incident angle and reflected angle when k_s = −0.508. (c) Calculated reflected acoustic field of the gradient acoustic metasurface when k_s = −0.508. When the acoustic plane wave is incident at 30°, 0°, −30° and −40°, the beam would reflect at −0.8°, −31°, 31° and 62° correspondingly.