Fink et al. for incoherent processing of ultrasonic fields have utilized random Phase Screens (RPS). These RPSs have shown interesting properties but they do not allow any modification of their randomness. It has seemed interesting to get a system where the shape of the phase screen could be controlled through a simple parameter without modifying the ultrasonic emission. We present the results we have obtained when propagating an impulsional ultrasonic wave through an instable liquid layer. The surface of the layer is vertically modulated at very low frequency (under 100 Hz). When increasing the acceleration that generates the well-known Faraday instability. The surface goes from a plane to a surface covered by waves (rolls), then to a surface where squared, hexagonal patterns or even Faraday crispation appear. This allows us to modulate the acoustical field by a phase screen of periodical, quasi-periodical or chaotic shape. When the surface is crispated or covered by hexagons, the study of the diffracted acoustical field in air shows results similar to those obtained previously for RPSs.
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