A noise attenuation panel for a structure within a propulsion system, comprising: a first plurality of unit cells; and a first plurality of mass elements disposed within the first plurality of unit cells, wherein the first plurality of unit cells includes a first periodic structure having a first unit cell, a second unit cell, a third unit cell, and a fourth unit cell, and wherein each of the first unit cell, the second unit cell, the third unit cell, and the fourth unit cell includes a central body interconnected via a plurality of lateral tubes extending from the central body, the first periodic structure forming a first lateral layer of unit cells, wherein exterior surfaces of the first plurality of unit cells define a noise attenuation flow path for a flow of air therebetween, characterised in that the noise attenuation panel further comprises: a second plurality of unit cells (2702 2); a perforated plate (2760 1) disposed between the first plurality of unit cells (2702 1) and the second plurality of unit cells (2702 2); a first plurality of apertures disposed in the perforated plate (2760 1) and configured to accommodate a plurality of tubes extending from the first plurality of unit cells (2702 1) to the second plurality of unit cells (2702 2); and a second plurality of apertures disposed in the perforated plate (2760 1) and at least partially defining the noise attenuation flow path. a first plurality of unit cells; and a first plurality of mass elements disposed within the first plurality of unit cells, wherein the first plurality of unit cells includes a first periodic structure having a first unit cell, a second unit cell, a third unit cell, and a fourth unit cell, and wherein each of the first unit cell, the second unit cell, the third unit cell, and the fourth unit cell includes a central body interconnected via a plurality of lateral tubes extending from the central body, the first periodic structure forming a first lateral layer of unit cells, wherein exterior surfaces of the first plurality of unit cells define a noise attenuation flow path for a flow of air therebetween, characterised in that the noise attenuation panel further comprises: a second plurality of unit cells (2702 2); a perforated plate (2760 1) disposed between the first plurality of unit cells (2702 1) and the second plurality of unit cells (2702 2); a first plurality of apertures disposed in the perforated plate (2760 1) and configured to accommodate a plurality of tubes extending from the first plurality of unit cells (2702 1) to the second plurality of unit cells (2702 2); and a second plurality of apertures disposed in the perforated plate (2760 1) and at least partially defining the noise attenuation flow path. wherein the first plurality of unit cells includes a first periodic structure having a first unit cell, a second unit cell, a third unit cell, and a fourth unit cell, and wherein each of the first unit cell, the second unit cell, the third unit cell, and the fourth unit cell includes a central body interconnected via a plurality of lateral tubes extending from the central body, the first periodic structure forming a first lateral layer of unit cells, wherein exterior surfaces of the first plurality of unit cells define a noise attenuation flow path for a flow of air therebetween, characterised in that the noise attenuation panel further comprises: a second plurality of unit cells (2702 2); a perforated plate (2760 1) disposed between the first plurality of unit cells (2702 1) and the second plurality of unit cells (2702 2); a first plurality of apertures disposed in the perforated plate (2760 1) and configured to accommodate a plurality of tubes extending from the first plurality of unit cells (2702 1) to the second plurality of unit cells (2702 2); and a second plurality of apertures disposed in the perforated plate (2760 1) and at least partially defining the noise attenuation flow path. a second plurality of unit cells (2702 2); a perforated plate (2760 1) disposed between the first plurality of unit cells (2702 1) and the second plurality of unit cells (2702 2); a first plurality of apertures disposed in the perforated plate (2760 1) and configured to accommodate a plurality of tubes extending from the first plurality of unit cells (2702 1) to the second plurality of unit cells (2702 2); and a second plurality of apertures disposed in the perforated plate (2760 1) and at least partially defining the noise attenuation flow path.