Measuring anisotropy in underwater inertial metamaterials

Abstract

The acoustic behavior of acoustic metafluids, designed and fabricated for underwater applications has been studied experimentally. Unit cells consist of elastic elements coupled via compliant layers to produce negligible shear modulus and anisotropic dynamic effective density and phase speed in orthogonal directions. Finite element simulations were used to design a unit cell that exhibits subsonic phase speeds in one direction and supersonic speeds in the orthogonal direction. Numerous samples were constructed to experimentally validate these predictions. Various combinations of materials were employed to enhance anisotropy, simplify construction, and reduce unwanted effects. Uniformly-spaced samples were tested from 0.05-10 kHz in a one-dimensional, resonator tube filled with degassed water. An electrodynamic shaker excited the system with frequency-modulated chirps. The system response was recorded using a hydrophone positioned near the top of the tube, and the resonance frequencies were used to infer the phase speeds for each mode. To extract material properties from these measurements, an effective medium model was used to represent the material-water mixture and an exact analytical dispersion relation was used to correct for elastic waveguide dispersion. Extracted material properties indicated anisotropy was achieved, and were found to be in good agreement with the as-designed properties. [Work supported by ONR.]