Characterisation of metamaterials for underwater acoustics using the 5-point method

The characterization of metamaterials by measurement is essential to reveal the true efficiency of a material's reflection and transmission in water. The current 3-point method developed by L. Roux et al, J. Acoust. Soc. Am. 147, 2 (2020) 1104-1112 provides important advantages, such as consideration of the pressure field diffracted by the edges of the panel and late reflections, due respectively to the finite size of the panel and the water tank. However, this method only considers specularly reflected and transmitted waves. For metamaterial panels based on periodic arrays of macro-inclusions there exists a diffraction cut-off frequency, above which several propagating waves are generated in water, with frequency-dependent propagation directions, in both the reflected and transmitted fields. Due to this multiplication of waves, the 3-point method is no longer adequate. The 5-point method consider these components, in addition to those considered by the 3-point method allowing measurement beyond the material's first cut-off frequency by using 5 hydrophone position instead of three. Tests were carried out on a panel with an adaptable grating periodicity to test the robustness of the 5 points method. Results are compared with the 3-point method when both are valid and numerical results obtained by FEM.