Design of Acoustic Meta-surfaces for Underwater Stealth in Low Frequency Ranges

Abstract

In this study, it is aimed to protect underwater vehicles against active sonar systems by anechoic metamaterial coating. A mixture of matrix material with close acoustic impedance to water, resistivity to hydrostatic pressure, suitability for the marine environment, and high material loss factor are selected. At low frequencies, the inclusions in different shapes and sizes are added to the matrix material. Since solid inclusions will increase the density considerably, air cavities are preferred as inclusions. More attention is paid to low frequency absorption, especially below 1 kHz, because of advancing sonar technology. The acoustic performance of the designed models is compared in three frequency range: low (0–3 kHz), middle (3–6 kHz), and high (6–10 kHz). The designed models are constructed by considering hydrostatic pressure; hence, volume of air cavities is tried to decrease while absorption performance is aimed to increase. Therefore, a conical air cavity is optimized by chancing its dimensions and location. Also, novel approaches, gong shape air cavity, and sandglass air cavities are introduced. The results show that, not only cavity shape, but also its location and dimensions are highly influential on absorption performance. High volume air cavities increase the absorption performance at the low frequency range, but they are not effective at high frequencies. The gong shape, and sandglass air cavities shows broadband absorption, also, gong shape air cavity volume is less than literature models. Thus, its usability increases at deep waters. The results of this study provide novel underwater meta surfaces for various applications.