of the structural element has depressions of from 0 1 to 10 mm in diameter most preferably 0 2 to 4 mm in diameter the depressions may be circular oval cross shaped or slit shaped and combinations of various shapes may be employed the depth of the depressions is 0 1 to 7 cm preferably 1 to 4 cm and is less than the thickness of the panel the difference between the depth of the depressions and the thickness of the panel is preferably at least 5 mm the number of depressions in a surface of the structural element is 1 to 18 per cm 2 preferably 4 to 15 per cm 2 the structural element may have depressions on the front or rear side or on both sides it has also been found that the use of outer layers leads to an improvement in the sound absorbing properties of the structural element according to the invention in this context either only one side of the polyurethane rigid foam panel of the structural element or alternatively both sides may be provided with an outer layer the front and rear sides may be provided with identical or different outer layers mineral fleece with bulk densities of 40 to 80 kg m 3 is preferably used as the outer layer natron kraft paper bituminized paper bituminized felt crepe paper polyethylene coated glass fleece and aluminum foils may however also be used as the outer layer the thickness of the outer layer s used depends on the material being used a thickness of 0 03 mm is suitable for example for aluminum foil a thickness of up to 3 mm is suitable for bituminized felt other suitable outer layers.