Design and testing of sub-wavelength panels for underwater acoustic isolation

Underwater sound isolation is an important area of research for both environmental and military applications. This work explores present research in airborne thin panel acoustic metamaterials and underwater acoustic isolation using encapsulated bubbles. These ideas are combined in the design of sub-wavelength, free-standing underwater panels for acoustic isolation. This work investigates the resonance of cylindrical air cavities in water with at aspect ratios. The spherical resonance approximation proposed by Minnaert for gas bubbles in water is shown to provide a good approximation of the resonance frequency of cylindrical inclusions. Panels with cylindrical inclusions are developed and tested in the 500 Hz to 1.3 kHz range and the 2 kHz to 5 kHz range. Panels with a void fraction of 1.3% and non-dimensional kT value of 0.02 to 0.07 show a frequency averaged insertion loss of 3 dB to 8 dB. Additionally, it is shown that an increase in void fraction yields an increase in panel isolation capability. It is the hope that this and future work in the area of sub-wavelength panels will improve the underwater environment for marine life and underwater naval applications.