The underwater acoustical model structured by two steel cylinders and a void cylinder embedded in rubber medium with steel and air backing or water backing is proposed in this paper, based on the theory of sound resonance absorption. Through the commercial software Comsol 5.4, the underwater absorption coefficient and displacement deformation of phononic crystal are performed. Then, sound absorption coefficient of phononic crystal with steel and air backing or with water backing is plotted, which provides a perfect idea to re-construct anechoic tile. The numerical results show that the underwater sound absorption frequency band could be broadened by utilizing coupled multi-resonator modes between steel cylinder and void. In addition, results reveal that mass-spring resonant mode has significant impacts at low-frequency absorption and the conversation from longitudinal waves to the shear waves is inspected at mid-frequency band through observation of displacement deformation of phononic crystal. This paper can offer a practical guidance to design and construct phononic crystal which is comprised by multi-periodic steel and void cylinders.
In recent years, there has been growing interests in the acoustic strength of underwater vehicle for many researchers. Various kinds of phononic crystals are designed and applied to the anechoic coating of submarines to reduce the reflection and increase the absorption of sound wave emitted from the active sonar, and anechoic coating constructed by phononic crystals could also modify the acoustic strength performance of underwater vehicles. In addition, increased attention has been focused on the study of acoustic absorption by using dark acoustic metamaterials (Mei, 2012), coiling up space (Li, 2012), active sound control (Kundu, 2011), gradient acoustic metasurface (Yuan, 2015). Besides, sonic crystals made from localized resonant structures perform spectral gaps with two orders of magnitude smaller than the relevant wavelength (Liu, 2000). However, these investigations are mainly focused on air sound absorption, and research and experiment about underwater acoustic absorption receive less attention additionally. In fact, acoustic wavelength in air is much less than 5 times compared with underwater acoustic wavelength for the same sound frequency. On the other hand, acoustic impedance in water is much larger than acoustic impedance in air, so that hard inclusion (steel) in air has to be considered as elastic material in water. These two disadvantages result in great limitation on the study of underwater acoustic absorption.
