Highly anisotropic hexagonal lattice material for low frequency water sound insulation

Insulation of water sound through impedance mismatch has the advantage of broadband effectiveness compared to using materials with bandgaps induced either by local resonance or Bragg scattering. In general, acoustic impedance of an isotropic solid under normal incidence condition is the product of mass density and longitudinal wave velocity. It is derived here, the acoustic impedance of an anisotropic solid depends additionally on a new parameter, and a carefully designed anisotropic solid can achieve a very small impedance along a specific direction. Honeycomb beam lattice is proposed as an example to achieve a much smaller effective impedance than water based on the above principle. Numerical simulation shows, a thin slab, with an overall thickness being two orders of magnitude smaller than the water wavelength, designed from the highly anisotropic lattice can reflect almost 97.7% of incident acoustic energy. A deep subwavelength sample with a thickness 21 mm is then fabricated and measured in a water tube. The experiment shows, the sample can reduce sound transmission by nearly 18.7 dB over the low frequency range 1.5 kHz ∼3.5 kHz. This study demonstrates the potential of anisotropic lattices in engineering effective impedance for insulating water sound at low frequency.