Underwater sound absorption characteristic of the rubber core sandwich structure with funnel-shaped cavities reinforced by carbon fiber columns

This paper introduces a multi-functional rubber core sandwich structure with funnel-shaped cavities (RCSSFC) reinforced by carbon fiber columns (CFC), which has the potential to meet requirements of the pressure resistance and acoustic stealth for the underwater equipment in deep sea. A test sample of the RCSSFC with CFC is firstly manufactured and the acoustic propagation experiment is conducted by the pulse tube method to gain the sound absorption coefficient. Then the approximate theoretical model of the RCSSFC is built to predict the sound absorption performance via the layered gradient discretization of cavities, the equivalent theory of the cylindrical tube and the transfer matrix method. Both the obtained experimental and theoretical results are compared with the ones from finite element (FE) analyses conducted in the software COMSOL and the good agreements are achieved. Characteristic analyses are further done to investigate the effects of the diameter of CFC, the shape of cavities, the backing of the structure and the underwater pressure. The design of localized resonances is introduced to improve the low-frequency sound absorption performance. It is found that there are three sound absorption peaks (SAPs) on the curves at lower, middle and higher frequencies, which are respectively dominated by the longitudinal vibration of the structure, the resonances of cavities and the local vibration of the face sheet. Characteristic parameters mainly affect the sound absorption coefficients by changing the above three vibration behaviors. Besides, the CFC greatly improves the pressure resistance property, and the sound absorption performance of the RCSSFC with CFC is insensitive to the change of the pressure. Localized resonances show the excellent effect on increasing the low-frequency sound absorption of the RCSSFC with CFC.