A sound-proofing covering comprising a cellular structure, each cell of which comprises a duct extending at least between the first face and the second face of the covering. The duct is formed between an outer wall and an inner wall of the cell and has a restriction of its cross section. The outer and inner walls have rounded forms, without sharp edges. The cell comprises a cavity in which the duct emerges. Thus, each cell forms a resonator comprising a neck formed by the duct and the cavity. Each cell is conformed such that the duct and the cavity are formed on either side of the inner wall. Such a covering is particularly suitable for reducing acoustic waves in aircraft engine plant nacelles, particularly in the low frequencies.