Broadband unidirectional invisibility for airborne sound

We present a metafluid-based broadband cloak capable of guiding acoustic waves around obstacles along given directions while maintaining the wavefront undisturbed. The required parameter distribution of the proposed cloak is derived by coordinate transformation and practically implemented by employing the acoustic metafluid formed with periodically arranged slabs in acoustic chambers. The method for independently modulating the effective mass density and bulk modulus of the metafluid is developed by tuning the geometry parameters and the temperature of the acoustic chamber in a specific process. By virtue of this free-modulated method, the range of realizable effective parameters is substantially broadened, and the acoustic impedance of the anisotropic structures can be well matched to the background. The performance of the designed structure is quantitatively evaluated in the frequency range of 3–4 kHz by the averaged invisibility factor. The results show that the proposed cloak is effective in manipulating the acoustic field along the given direction and suppressing the wave scattering from the hidden object.