An Ultrasonic Testbench for Emulating the Degradation of Sonar Performance in Fluctuating Media

A scaled experimental protocol, in a water tank, is proposed to mimic the effects of medium heterogeneities on underwater acoustic propagation. The procedure consists in transmitting an ultrasonic wave (f = 2.25 MHz) through a RAndom Faced Acoustic Lens (RAFAL) in order to induce a spatially fluctuating sound pressure field. A scaling process based on a dimensional analysis ensures the representativeness of the experiment, when compared to the fluctuations of a few kHz continuous waves in the ocean. The regimes of saturation and unsaturation are explored by tuning the statistical parameters of the RAFAL's output face, the signal frequency and the propagation distance in the water tank. The control and the reproducibility of the measurements are therefore ensured. The influence of volume effects is especially studied, since the experiments were conducted in free field conditions. Results in terms of mutual coherence function (second-order moment) are presented; a good agreement is found between the values of radii of coherence calculated using experimental and simulation data, as well as theoretical results. The evaluation of the complex pressure distribution highlights the relevance of the experimental scheme, since behaviors typical of the regimes of fluctuations (saturation and unsaturation) are found.