A panel (20;700) for attenuating noise, comprising: a first skin; a second skin, wherein one of the first skin and the second skin is perforated; and a core comprising a corrugation and a stiffener and forming a plurality of cavities (52B,54) vertically between the first skin and the second skin, the corrugation including a first baffle and a first septum, and the stiffener including a first wall and a second wall; a first of the cavities formed laterally between the first baffle and the first wall, wherein the first septum is laterally between the first baffle and the first wall and divides the first of the cavities into fluidly coupled sub-cavities (104A,B); and a second of the cavities formed laterally between the first wall and the second wall; wherein the first baffle is connected to and offset from the first skin by an acute first angle, and the first wall is connected to and offset from the first skin by an acute second angle that is substantially equal to the first angle; wherein the first septum is connected to and offset from the first skin by a third angle, and the second wall is connected to and offset from the first skin by a fourth angle that is substantially equal to the third angle; characterised in that: the stiffener further includes a vertical stem portion providing the stiffener with a Y-shaped cross-sectional geometry; wherein: a stiffness of the stiffener is greater than a stiffness of the corrugation; and/or the first baffle has a first thickness and the first wall has a second thickness that is greater than the first thickness. a first skin; a second skin, wherein one of the first skin and the second skin is perforated; and a core comprising a corrugation and a stiffener and forming a plurality of cavities (52B,54) vertically between the first skin and the second skin, the corrugation including a first baffle and a first septum, and the stiffener including a first wall and a second wall; a first of the cavities formed laterally between the first baffle and the first wall, wherein the first septum is laterally between the first baffle and the first wall and divides the first of the cavities into fluidly coupled sub-cavities (104A,B); and a second of the cavities formed laterally between the first wall and the second wall; wherein the first baffle is connected to and offset from the first skin by an acute first angle, and the first wall is connected to and offset from the first skin by an acute second angle that is substantially equal to the first angle; wherein the first septum is connected to and offset from the first skin by a third angle, and the second wall is connected to and offset from the first skin by a fourth angle that is substantially equal to the third angle; characterised in that: the stiffener further includes a vertical stem portion providing the stiffener with a Y-shaped cross-sectional geometry; wherein: a stiffness of the stiffener is greater than a stiffness of the corrugation; and/or the first baffle has a first thickness and the first wall has a second thickness that is greater than the first thickness. the stiffener further includes a vertical stem portion providing the stiffener with a Y-shaped cross-sectional geometry; wherein: a stiffness of the stiffener is greater than a stiffness of the corrugation; and/or the first baffle has a first thickness and the first wall has a second thickness that is greater than the first thickness.