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 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 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 . <EOS>