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 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 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.