Acoustic transmissive cloaking with adjustable capacity to the incident direction.

Zero-refractive-index (ZRI) phononic crystals (PhCs), in which acoustic waves can be transmitted without phase variations, have considerable potential for engineering wavefronts and thus are applicable to invisibility cloaking. However, the creation of the transmissive cloaking achieved by ZRI-PhCs is challenging under an oblique incidence, which substantially hinders their practical applications. Here, we experimentally demonstrate acoustic transmissive cloaking with the adjustable capacity to the incident direction. Acoustic transmissive cloaking of arbitrarily shaped obstacles can be obtained through a hybrid acoustic structure consisting of one outer layer of a programmable phase-engineered metasurface (PPEM) and one inner layer of a double zero-refractive-index (DZRI)-PhC. The DZRI-PhC is functionally the same as an equiphase area and can guide acoustic waves around the obstacle, a process known as acoustic tunneling. The PPEM perpendicularly transfers the incident acoustic waves to the DZRI-PhC and allows the emergent waves from the DZRI-PhC to transmit along the original incident direction. The DZRI-PhC is made of an array of iron squares in the air. The reciprocal of the effective bulk modulus and the effective mass density is approximately zero at a frequency of 3015 Hz (0.5187