An assembly including: - a three-dimensional structural element, having a curved shape and/or reliefs and/or recesses, interposed between an external environment and an internal volume to be thermally and/or acoustically protected, - and an inner lining element (39,391) of the structural element, the inner lining element (39,391) comprising at least one said assembly, which is thereby interposed between the external environment and the internal volume to be protected, the three-dimensionally shaped assembly comprising: -- a three-dimensional fibrous textile structure, woven or nonwoven, comprising polymeric, mineral or natural fibres, and having a curved shape and/or reliefs and/or recesses, and -- a closed sleeve: --- comprising a wall, and --- which encloses the fibrous textile structure, the shape of which is followed by the wall. the fibrous textile structure of said assembly defining a first fibrous textile structure comprising a porous material having a first density, the inner lining element (39,391) further comprising a second fibrous textile structure comprising the same porous material or different porous material, and the first and second fibrous textile structures (5.50) being superimposed, characterized in that: - the second fibrous textile structure has a second density lower than the first density, and - the second fibrous textile structure has a curved shape and/or reliefs and/or recesses, and - said assembly and the second fibrous textile structure: -- are enclosed together in a second closed sleeve comprising one said wall, and -- are interposed between two major surfaces (S10, S20) of said wall, which is thermoformed at said two major surfaces. - a three-dimensional structural element, having a curved shape and/or reliefs and/or recesses, interposed between an external environment and an internal volume to be thermally and/or acoustically protected, - and an inner lining element (39,391) of the structural element, the inner lining element (39,391) comprising at least one said assembly, which is thereby interposed between the external environment and the internal volume to be protected, the three-dimensionally shaped assembly comprising: -- a three-dimensional fibrous textile structure, woven or nonwoven, comprising polymeric, mineral or natural fibres, and having a curved shape and/or reliefs and/or recesses, and -- a closed sleeve: --- comprising a wall, and --- which encloses the fibrous textile structure, the shape of which is followed by the wall. -- a three-dimensional fibrous textile structure, woven or nonwoven, comprising polymeric, mineral or natural fibres, and having a curved shape and/or reliefs and/or recesses, and -- a closed sleeve: --- comprising a wall, and --- which encloses the fibrous textile structure, the shape of which is followed by the wall. --- comprising a wall, and --- which encloses the fibrous textile structure, the shape of which is followed by the wall. the fibrous textile structure of said assembly defining a first fibrous textile structure comprising a porous material having a first density, the inner lining element (39,391) further comprising a second fibrous textile structure comprising the same porous material or different porous material, and the first and second fibrous textile structures (5.50) being superimposed, characterized in that: - the second fibrous textile structure has a second density lower than the first density, and - the second fibrous textile structure has a curved shape and/or reliefs and/or recesses, and - said assembly and the second fibrous textile structure: -- are enclosed together in a second closed sleeve comprising one said wall, and -- are interposed between two major surfaces (S10, S20) of said wall, which is thermoformed at said two major surfaces. - the second fibrous textile structure has a second density lower than the first density, and - the second fibrous textile structure has a curved shape and/or reliefs and/or recesses, and - said assembly and the second fibrous textile structure: -- are enclosed together in a second closed sleeve comprising one said wall, and -- are interposed between two major surfaces (S10, S20) of said wall, which is thermoformed at said two major surfaces. -- are enclosed together in a second closed sleeve comprising one said wall, and -- are interposed between two major surfaces (S10, S20) of said wall, which is thermoformed at said two major surfaces.