a facing sheet that comprises holes, the facing sheet having a porosity and a facing sheet thickness,

a backing sheet,

a plurality of cells arranged between the facing sheet and the backing sheet, the cells each having a cell depth and each cell defining a cavity,

a plurality of internal necks beginning at the facing sheet and extending from an inner side of the facing sheet towards the backing sheet but ending prior to reaching the backing sheet so that lengths of the internal necks are less than the cell depth and the internal necks are spaced a distance from the backing sheet, each of the internal necks being located around a respective one of the holes of the facing sheet, thereby extending a longitudinal length of the hole,

wherein the acoustic liner is adapted to have a peak attenuation at a frequency in a range between 4,000 Hz and 8,000 Hz in a temperature condition in which a sound velocity is 600 m/s,

wherein the cell depth of the cells is in a range between 2 mm and 5 mm,

wherein each of the internal necks has a length in a range between 0.1 mm and 2 mm,

wherein the facing sheet including the internal necks, walls of the cells and the backing sheet are formed as one part, manufactured by 3D printing;

wherein each of the holes has a closed perimeter;

wherein each of the holes in the facing sheet includes a respective one of the internal necks;

wherein each cell of the acoustic liner is connected with a defined plurality of the holes in the facing sheet and the defined plurality of internal necks respectively surrounding the defined plurality of holes;

wherein the facing sheet is an outermost element of the acoustic liner facing a volume to be attenuated;

wherein the facing sheet defines an outermost boundary of the cells;

wherein the facing sheet forms an outer portion of each of the internal necks;

wherein the facing sheet is monolithic with each of the internal necks.