A sandwich anechoic coating embedded with a micro-perforated panel in high-viscosity condition for underwater sound absorption

The interest of this article lies in the underwater application of micro-perforated panel (MPP). In this paper, a new type of sound absorption structure based on MPP and viscoelastic substrate for underwater usage is proposed. The MPP in the condition of high-viscosity castor oil generates absorption peak in low-frequency bandwidth, and the rubber substrate of appropriate thickness achieves good absorption performance in medium-high frequency. The sound absorption formula for the new structure is derived from the transfer matrix method based on wave propagation equation, the equation for elastomer vibration, Maa's equation for MPP and continuity conditions for sound waves. The proposed model is tested in a hydroacoustic impedance tube filled with water, which shows good agreement with theoretical results. The absorption coefficient (alpha) is clearly improved within the frequency above 1 kHz similar to 15 kHz due to MPP. It can be seen that such structures can enhance the absorption dominantly in the low-frequency range, broaden the absorption bandwidth, and have the potential to be utilized as an underwater anechoic coating.