An acoustic impedance control feature for airfoils of a gas turbine engine, the acoustic impedance control feature comprising: an acoustic resonator (500, 600, 900, 1000) comprising an acoustic resonator cell (502, 504, 602, 604, 714, 814, 906, 1006) having: a backing chamber (508, 512, 608, 612, 722, 822, 912, 1012) defining a respective volume; and a neck (506, 510, 606, 610, 720, 820, 910, 1010) arranged relative to the backing chamber and defining an opening (614, 718, 818, 916, 1016), wherein the neck has a length ( l) and a cross-sectional area ( a), wherein the acoustic resonator cell (502, 504, 602, 604, 714, 814, 906, 1006) satisfies the following relationships: l / L = 0.2-0.8, where l is the length of the neck (506, 510, 606, 610, 720, 820, 910, 1010) and L is a depth of the backing chamber (508, 512, 608, 612, 722, 822, 912, 1012); and a / A = 0.02-0.20, where a is the cross-sectional area of the neck and A is a cross-sectional area of the backing chamber characterized by at least one of: (i) the acoustic resonator (500, 600) includes a first acoustic resonator cell (502, 602) and a second acoustic resonator cell (504, 604), wherein: the first acoustic resonator cell (502, 602) comprises the backing chamber (508, 608) and the neck (506, 606); and the second acoustic resonator cell (504, 604) comprises a respective second backing chamber (512, 612) and a respective second neck (510, 610), wherein the first acoustic resonator cell (502, 602) is stacked on the second acoustic resonator cell (504, 604) and the first backing chamber (508, 608) and the second backing chamber (512, 612) are fluidly connected through the second neck (510, 610); (ii) the neck (506, 510, 820) is arranged outward from the backing chamber (508, 512, 822) and does not extend into the backing chamber (508, 512, 822); and (iii) the acoustic resonator (900, 1000) is integrally formed with the airfoil. a backing chamber (508, 512, 608, 612, 722, 822, 912, 1012) defining a respective volume; and a neck (506, 510, 606, 610, 720, 820, 910, 1010) arranged relative to the backing chamber and defining an opening (614, 718, 818, 916, 1016), wherein the neck has a length ( l) and a cross-sectional area ( a), wherein the acoustic resonator cell (502, 504, 602, 604, 714, 814, 906, 1006) satisfies the following relationships: l / L = 0.2-0.8, where l is the length of the neck (506, 510, 606, 610, 720, 820, 910, 1010) and L is a depth of the backing chamber (508, 512, 608, 612, 722, 822, 912, 1012); and a / A = 0.02-0.20, where a is the cross-sectional area of the neck and A is a cross-sectional area of the backing chamber characterized by at least one of: (i) the acoustic resonator (500, 600) includes a first acoustic resonator cell (502, 602) and a second acoustic resonator cell (504, 604), wherein: the first acoustic resonator cell (502, 602) comprises the backing chamber (508, 608) and the neck (506, 606); and the second acoustic resonator cell (504, 604) comprises a respective second backing chamber (512, 612) and a respective second neck (510, 610), wherein the first acoustic resonator cell (502, 602) is stacked on the second acoustic resonator cell (504, 604) and the first backing chamber (508, 608) and the second backing chamber (512, 612) are fluidly connected through the second neck (510, 610); (ii) the neck (506, 510, 820) is arranged outward from the backing chamber (508, 512, 822) and does not extend into the backing chamber (508, 512, 822); and (iii) the acoustic resonator (900, 1000) is integrally formed with the airfoil. l / L = 0.2-0.8, where l is the length of the neck (506, 510, 606, 610, 720, 820, 910, 1010) and L is a depth of the backing chamber (508, 512, 608, 612, 722, 822, 912, 1012); and a / A = 0.02-0.20, where a is the cross-sectional area of the neck and A is a cross-sectional area of the backing chamber (i) the acoustic resonator (500, 600) includes a first acoustic resonator cell (502, 602) and a second acoustic resonator cell (504, 604), wherein: the first acoustic resonator cell (502, 602) comprises the backing chamber (508, 608) and the neck (506, 606); and the second acoustic resonator cell (504, 604) comprises a respective second backing chamber (512, 612) and a respective second neck (510, 610), wherein the first acoustic resonator cell (502, 602) is stacked on the second acoustic resonator cell (504, 604) and the first backing chamber (508, 608) and the second backing chamber (512, 612) are fluidly connected through the second neck (510, 610); (ii) the neck (506, 510, 820) is arranged outward from the backing chamber (508, 512, 822) and does not extend into the backing chamber (508, 512, 822); and (iii) the acoustic resonator (900, 1000) is integrally formed with the airfoil.