Arbitrary Shaped Acoustic Concentrators Enabled by Null Media

Based on the transformation acoustic methodology, an innovative approach for designing arbitrary shape concentrators is proposed. Unlike previous works, which utilized inhomogeneous and anisotropic materials to concentrate the incident acoustic waves in an arbitrary domain, the same functionality is attained by introducing only one homogeneous anisotropic medium, which is called the acoustic null medium (ANM). The other advantage of this method is that the obtained materials are not dependent on the shape of the concentrator. That is, regardless of the device geometry, a constant ANM will be used for each new shape and the output results do not alter. This will circumvent the conventional transformation acoustics' sophisticated and tedious calculations and could be easily implemented in real-life scenarios. To show these competencies, several numerical full-wave simulations are performed. In addition to the bulk material simulations, an arbitrary shape concentrator is realized via effective medium theory and it is shown that an ANM, as a kind of extreme medium, could be implemented with natural materials.