A flutter damper, comprising: an acoustic liner for a bypass duct of a fan section of a gas turbine engine, the gas turbine engine having a central longitudinal axis, wherein a circumferential direction and a radial direction are relative to the central longitudinal axis, the acoustic liner in fluid communication with a fluid flow the acoustic liner being configured for peak acoustical energy absorption at a frequency range that is greater than a frequency range associated with fan flutter; and a plurality of modular chambers configured for peak acoustical energy absorption at a frequency range that is associated with one or more fan flutter modes; the plurality of modular chambers being disposed radially outside of and in acoustic communication with the acoustic liner, the plurality of modular chambers including a circumferential gap (212; 232; 250) between proximate circumferential ends (214; 216) of at least one adjacent pair of modular chambers, and the plurality of modular chambers each including a plurality of circumferentially aligned and connected chamber segments; and wherein at least one of the chambers in the plurality of modular chambers has a mutually unique length, width and/or height, or shape; and one or more of: the plurality of modular chambers have a same volume; at least two of the plurality of modular chambers are configured for peak acoustical energy absorption at a frequency range that is associated with mutually distinct fan flutter modes; and the acoustic chamber segments are elliptical. an acoustic liner for a bypass duct of a fan section of a gas turbine engine, the gas turbine engine having a central longitudinal axis, wherein a circumferential direction and a radial direction are relative to the central longitudinal axis, the acoustic liner in fluid communication with a fluid flow the acoustic liner being configured for peak acoustical energy absorption at a frequency range that is greater than a frequency range associated with fan flutter; and a plurality of modular chambers configured for peak acoustical energy absorption at a frequency range that is associated with one or more fan flutter modes; the plurality of modular chambers being disposed radially outside of and in acoustic communication with the acoustic liner, the plurality of modular chambers including a circumferential gap (212; 232; 250) between proximate circumferential ends (214; 216) of at least one adjacent pair of modular chambers, and the plurality of modular chambers each including a plurality of circumferentially aligned and connected chamber segments; and wherein at least one of the chambers in the plurality of modular chambers has a mutually unique length, width and/or height, or shape; and one or more of: the plurality of modular chambers have a same volume; at least two of the plurality of modular chambers are configured for peak acoustical energy absorption at a frequency range that is associated with mutually distinct fan flutter modes; and the acoustic chamber segments are elliptical. the plurality of modular chambers have a same volume; at least two of the plurality of modular chambers are configured for peak acoustical energy absorption at a frequency range that is associated with mutually distinct fan flutter modes; and the acoustic chamber segments are elliptical.