Extraordinary Sound Isolation Using an Ultrasparse Array of Degenerate Anisotropic Scatterers

Subwavelength acoustic scatterers with extraordinary multipole scattering properties have attracted much research interest. We introduce an ultrasparse anisotropic acoustic scatterer array, which has degenerate monopole and dipole resonances, for total absorption of sound based on scattering cancellation of monopole and dipole moments. The dipolar scattering property of the scatterer is extremely anisotropic so that the dipole resonance can be switched off by rotating the scatterer about its own center by ${90}^{\ensuremath{\circ}}$. This feature leads to the scatterer-orientation dependence of mode degeneracy as well as the sound-absorption capability. Simulation and experimental results demonstrate the orientation-dependent sound-isolation property of the scatterer array. Moreover, the scatterer array is ultrasparse in that the spatial period exceeds the limit bounded by 4 times the absorption cross section of a subwavelength resonator ($dg4{\ensuremath{\sigma}}_{\mathrm{abs}}=2\ensuremath{\lambda}/\ensuremath{\pi}$). This ultrasparse and controllable sound-isolating material may have applications in many areas such as infrastructures, consumer electronics, and the automotive industry.