Experimental characterization of periodic metamaterials in acoustic open water tank

The measurement of acoustic performance of underwater panels, such as their reflection and transmission coefficients, has always been a challenge. The finite size of the tank imposes a low frequency limit, while diffraction by the edges of the panel disturbs measurements. Despite these difficulties, it remains a valuable tool for comparison with numerical and analytical studies. In a previous paper [Roux, Pouille, Audoly, and Hladky, J. Acoust. Soc. Am 147(2), 1104-1112 (2020)], the three-point method has been proposed and tested on a homogeneous panel. The present paper proposes an extension of the method applicable to periodic metamaterials. First, the applicability and robustness of the three-point method are tested on a periodic panel. Then, since it behaves like a diffraction grating, above the grating cut-off frequency, the measurement method must separate the specular wave from the other propagative contributions. For that purpose, the five-point method is proposed as a solution to separate these contributions to retrieve the reflection and transmission coefficients. The three- and five-point methods are applied to a test panel made of steel bars. Limitations on these methods are discussed and a solution is proposed to reduce their impact. Finally, a correct measurement-simulation agreement is observed for the reflection and transmission coefficients for both the specular and the first-propagative diffraction order.