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[References: 22 tit.]

The ability of focus waves with concave or convex surfaces is well known both in optics and in acoustics. Nowadays, the possibility of beamforming sound with flat lenses is a hot topic because of its application in different areas such as biomedical engineering or non-destructive techniques. In this paper, we propose a gas filled cuboid lens that has a different sound speed than that of the surrounding medium (air in our case) as a beamforming acoustic device. This constitutes an experimental visualization of the capability of sound focusing with flat surfaces lens and allows understanding the corresponding physic phenomenon.