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 $\sim$3.5 kHz. This study demonstrates the potential of anisotropic lattices in engineering effective impedance for insulating water sound at low frequency.

与使用局部共振或布拉格散射引起的带隙材料相比，通过阻抗失配来隔离水声具有宽带有效性的优势。通常，在垂直入射条件下各向同性固体的声阻抗是质量密度和纵向波速的乘积。由此得出，各向异性固体的声阻抗还取决于新参数，精心设计的各向异性固体可以沿特定方向获得非常小的阻抗。基于上述原理，提出以蜂窝状梁格为例，以实现比水小的有效阻抗。数值模拟表明，一块薄板，总厚度比水波长小两个数量级，从高度各向异性的晶格进行设计可以反射近97.7％的入射声能。然后制作厚21毫米的深亚波长样品，并在水管中进行测量。实验表明，在1.5 kHz低频范围内，样品可以将声音传输降低近18.7 dB$〜$3.5 kHz。这项研究证明了各向异性晶格在降低水中水声的工程有效阻抗中的潜力。