Experimental characterization of a low-frequency underwater metamaterial absorber

For many years, the absorption of underwater acoustic waves has aroused strong interest for military applications of underwater detection and stealth. More recently, environmental issues have been added due to increasing human activity at sea (maritime traffic, seismic prospecting, development of off-shore installations for the production of energy (tidal turbine, wind turbine), etc...). Therefore, noise reduction remains today an important issue. We present a novel metamaterial absorber [1] with structured impedance-matched composite comprises tungsten particles dispersed in a polyurethane (PU) polymer matrix, geometrically structured into rods. The broadband functionality is achieved by simultaneously engineering the distribution of Fabry-Perot resonances and the characteristic impedance of the metamaterial to match that of water. The absorbing performance of a large-size sample (0.92 m×0.92 m) was verified by measurements in a water pool with a time-domain approach. One of the main challenges with such low frequency experimental study is to deal with long wavelengths and therefore extracted the useful signals from the spurious signals arising from the reflections by pool walls. An experimental protocol was developed that enables us to identify and isolate relevant signals and compute the reflection and transmission on the metamaterial absorber. At deep subwavelength regime (~λ/42), we measure average absorption greater than 90% from 4 to 20 kHz in very good agreement with the simulation. [1] arXiv:2111.0207