Abstract: A sound insulating building element or structural member includes a plurality of parallel sheet elements of which a first, inner, thick element is stiff and has a plurality of internal cavities, a second, inner, stiff element is substantially pervious to air and connected to one main surface of the first, inner element, and an outer, impervious element is arranged at a small distance from the second, inner element in such a manner that substantially the entire outer element can oscillate freely in relation to the second, inner element, the other main surface of the first, inner element being connected to a sheet material having an E-modulus greater than 10.sup.9 and which can be substantially impervious to air, or similar to the second, inner element, can be pervious to air and be connected over an air gap to an outer substantially impervious sheet element.