Highly anisotropic pentamode materials for low frequency water sound insulation

Insulating sound propagation is of great importance for many practical engineering applications, particularly for water-born sound. Conventional insulation technique is often employed based on impedance mismatch, where a large impedance difference can lead to a high reflection and insulation. However, due to the large impedance of water itself, this method hardly works for insulating water-born sound. Therefore, in this work, we propose an alternative mechanism for underwater sound insulation with highly anisotropic materials. It is demonstrated that an anisotropic pentamode metamaterial (PM) can achieve a much lower impedance at certain material off-axis angle and a good sound insulation is obtained for low and broadband frequency range. Finally the anisotropic PM sample is machined by advance electric discharging machining (EDM) technology, and tested in water tube. We show that nearly 99% of the underwater incident acoustic energy can be blocked by a thin PM layer of 21mm thickness for a frequency range over 1.5 kHz to 3.5 kHz. Our work paves the ways for innovating new devices based on underwater low-frequency sound insulation.