A panel for attenuating noise, comprising: a first skin; a second skin; and a core forming a plurality of cavities vertically between the first skin and the second skin, the core including a plurality of walls, a plurality of baffles and a plurality of septums; each of the cavities is formed laterally between a first of the plurality of walls and a second of the plurality of walls; wherein a first of the plurality of septums divides the respective cavity into fluidly coupled sub-cavities (54A,54B) and is angularly offset from a first of the plurality of walls by an acute angle; and wherein each of the cavities is fluidly coupled with one or more respective perforations in the first skin; characterised in that each of the cavities is formed longitudinally between the respective first of the plurality of baffles and the respective second of the plurality of baffles; wherein the respective first of the plurality of septums is longitudinally between the respective first of the plurality of baffles and the respective second of the plurality of baffles, wherein the respective first of the plurality of baffles and the respective second of the plurality of baffles are angularly offset from the first of the plurality of walls by an acute angle; wherein the respective first of the plurality of septums is connected to the respective first of the plurality of baffles and the respective second of the plurality of baffles; wherein the respective first of the plurality of baffles, the respective second of the plurality of baffles and the respective first of the plurality of septums each extends between and is connected to the first of the plurality of walls and the second of the plurality of walls; and wherein the respective first of the plurality of baffles is angularly offset from the first skin by an acute angle. a first skin; a second skin; and a core forming a plurality of cavities vertically between the first skin and the second skin, the core including a plurality of walls, a plurality of baffles and a plurality of septums; each of the cavities is formed laterally between a first of the plurality of walls and a second of the plurality of walls; wherein a first of the plurality of septums divides the respective cavity into fluidly coupled sub-cavities (54A,54B) and is angularly offset from a first of the plurality of walls by an acute angle; and wherein each of the cavities is fluidly coupled with one or more respective perforations in the first skin; characterised in that each of the cavities is formed longitudinally between the respective first of the plurality of baffles and the respective second of the plurality of baffles; wherein the respective first of the plurality of septums is longitudinally between the respective first of the plurality of baffles and the respective second of the plurality of baffles, wherein the respective first of the plurality of baffles and the respective second of the plurality of baffles are angularly offset from the first of the plurality of walls by an acute angle; wherein the respective first of the plurality of septums is connected to the respective first of the plurality of baffles and the respective second of the plurality of baffles; wherein the respective first of the plurality of baffles, the respective second of the plurality of baffles and the respective first of the plurality of septums each extends between and is connected to the first of the plurality of walls and the second of the plurality of walls; and wherein the respective first of the plurality of baffles is angularly offset from the first skin by an acute angle.