Abstract: Sound attenuating structure including a combination of one or more core layers comprising cells directed perpendicular to the main surface of the structure. The core layers are separated from each other by perforated separating sheets, a closed sheet is adhered to one side of said combination and a perforated sheet to the other side of said combination, at which side the sound waves impinge upon the structure. The passages in said last mentioned sheet as well as in the separating sheets have an asymmetric cross section having its smallest diameter somewhat less than the sheet thickness of the structure and increasing from this diameter fluently towards both surfaces of the respective sheet.