Abstract: An acoustic liner for a gas turbine engine comprises a folded cavity with an opening and a resistive facing sheet covering the opening, characterised in that the folded cavity further comprises a resistive septum that increases attenuation at high frequencies. The presence of the septum permits the attenuation of noise over a much wider range of frequencies than in known liners. By suitable choice of the resistance and reactance of the septum and of its position in the cavity, the liner can provide low frequency attenuation comparable with known liners while also providing improved attenuation at higher frequencies.