A method for damping thermo acoustic vibrations, especially in the combustion chamber of a gas turbine engine uses at least one Helmholtz resonator (2) of a fixed volume and connected to the combustion chamber via a duct (5) with variable transmission. This enables the control system to vary the resonant frequency to match a range of operating conditions. The duct can comprise a number of sub ducts selected y a movable baffle, a rotating/sliding baffle to vary the effective length of the duct, or any suitable means.
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