RAFAL: RANDOM FACED ACOUSTIC LENS USED TO MODEL INTERNAL WAVES EFFECTS ON UNDERWATER ACOUSTIC PROPAGATION

Abstract

International audienceWe present here an experimental protocol to reproduce the effects of linear internal waves (LIW) on acoustic wave propagation in a very controlled and reproducible manner. In fact, the experiment consists in propagating an ultrasonic wave through an acoustic lens presenting a plane input face and a randomly rough output face. The so-called RAFAL (Random Faced Acoustic Lens) was designed so that the roughness of the output face induce resulting acoustic pressure field featuring typical characteristics of propagation though LIW.To ensure representativeness of our model, we conducted analytical calculations leading to dimensionless parameters equivalent to the ones developed by Flatté (strength parameter Φ and diffraction parameter Λ). In our case, the strength parameter was calculated after evaluation of the phase of the average acoustic field propagated through the RAFALS, whereas our diffraction parameter was evaluated using the phase sensitivity kernel. On the other hand, we calculated the ratio of correlation length of the acoustic field to wavelength. Measurements were conducted on several RAFALS, corresponding to various realistic configurations. The regimes of saturation (full and partial) and unsaturation were explored. The results are presented in terms of order 2 (coherence function) and order 4 (intensity) statistics and demonstrate the accuracy of our experimental scheme with respect to real scale simulations and simplified theory. Other representations, such as phasors, also show a very meaningful behavior