An acoustic liner comprising: a first face sheet ; a second face sheet spaced from the first face sheet ; and a plurality of sidewalls extending between the first face sheet and the second face sheet , the plurality of sidewalls defining a plurality of cells , each cell of the plurality of cells defining a cavity between the first face sheet and the second face sheet , wherein the first face sheet defines a plurality of apertures extending through a thickness (T1) of the first face sheet , each aperture of the plurality of apertures aligned with a respective cell of the plurality of cells , wherein the acoustic liner further comprises: a bulk absorber disposed within at least one cell of the plurality of cells , the bulk absorber further defining the cavity of the at least one cell of the plurality of cells , a membrane disposed proximate the first face sheet and adjacent the first face sheet , characterised in that : the cavity is further defined between the bulk absorber and the second face sheet ; and the membrane is a membrane coating disposed on a surface of the bulk absorber closest to the first face sheet , and a length of the membrane is bounded by the at least one cell of the plurality of cells . a first face sheet ; a second face sheet spaced from the first face sheet ; and a plurality of sidewalls extending between the first face sheet and the second face sheet , the plurality of sidewalls defining a plurality of cells , each cell of the plurality of cells defining a cavity between the first face sheet and the second face sheet , wherein the first face sheet defines a plurality of apertures extending through a thickness (T1) of the first face sheet , each aperture of the plurality of apertures aligned with a respective cell of the plurality of cells , wherein the acoustic liner further comprises: a bulk absorber disposed within at least one cell of the plurality of cells , the bulk absorber further defining the cavity of the at least one cell of the plurality of cells , a membrane disposed proximate the first face sheet and adjacent the first face sheet , characterised in that : the cavity is further defined between the bulk absorber and the second face sheet ; and the membrane is a membrane coating disposed on a surface of the bulk absorber closest to the first face sheet , and a length of the membrane is bounded by the at least one cell of the plurality of cells . a bulk absorber disposed within at least one cell of the plurality of cells , the bulk absorber further defining the cavity of the at least one cell of the plurality of cells , a membrane disposed proximate the first face sheet and adjacent the first face sheet , characterised in that : the cavity is further defined between the bulk absorber and the second face sheet ; and the membrane is a membrane coating disposed on a surface of the bulk absorber closest to the first face sheet , and a length of the membrane is bounded by the at least one cell of the plurality of cells . the cavity is further defined between the bulk absorber and the second face sheet ; and the membrane is a membrane coating disposed on a surface of the bulk absorber closest to the first face sheet , and a length of the membrane is bounded by the at least one cell of the plurality of cells . <EOS>