Experimental Acoustic Ground Cloak in Air

We present the design, fabrication, and performance analysis for a class of two-dimensional acoustic cloaking coatings in air. Our approach takes advantage of transformation acoustics and linear coordinate transformations that result in shells which are homogeneous, broadband, and compact. The required material parameters are highly anisotropic; however, we show that they are easily achievable in practice in metamaterials made of perforated plastic plates. The good performance of the fabricated design is assessed from measurements of the sound field produced around the cloak by a broadband source. The remarkably low complexity of the device made of perforated plastic plates shows that sound in air can be fully and effectively manipulated using realizable transformation acoustics devices.

Figure 1

Simulated acoustic signatures of triangular object (top) and object covered by cloak (bottom) placed on a reflecting horizontal surface. The signature is computed as the difference between the pressure fields with and without the target. The cloak is characterized by the realizable material parameters given by Eq. (3). The dashed rectangle surrounds the region in which we will experimentally measure the pressure fields.

Figure 2

Experimental setup: (a) Picture of cloak covering the object. The angle made by the perforated plastic plates with the vertical is given by Eq. (3). Inset: unit cell ($d=5\mathrm{mm}$; $s=1.6\mathrm{mm}$; $t=1\mathrm{mm}$). (b) Two-dimensional parallel plate waveguide used to map the acoustic field around the cloak. The mapped region is highlighted together with the pulse produced by the source.

Figure 3

Measured scattered fields. Left panels: specular reflection from the ground plane. Middle panels: pressure field scattered by the object placed on the ground plane. Right panels: scattering from the object covered by cloak. Bottom panels represent wave forms measured $880\mu \mathrm{s}$ after the measurements shown in the top panels.

Figure 4

Measured acoustic signature of object covered by cloak (bottom) is significantly reduced when compared with the acoustic signature of the object (top) in agreement with the theoretical prediction of Fig. 1. The signature of each target is computed as the difference between the fields with and without the target.