Compressibility-Near-Zero Directive Sound

A hallmark of a near-zero-index material is its constancy of field amplitude and phase as waves propagate with static-like properties in these materials. This feature has been exploited in electromagnetics for several applications, including for the realization of compact highly directive antennas illuminated by a single source. Here we show the promise of near-zero radiation in acoustics, where the equivalence between a waveguide operating near cutoff and the dispersion of compressibility leads to ideal aperture illumination. The near-zero behavior also makes the source robust to disorder, which is shown using a statistical description of the radiation patterns of randomly generated geometries.