A sensor (11) detects the acoustic pressure at a first location in a combustion chamber (7) of a test rig I and produces an input signal which is a function of the acoustic pressure. A controller (13) which receives the input signal and produces an output signal which is a function of the input signal. An acoustic actuator (16-18) receives the output signal and introduces into the combustion chamber (7) at a second location an acoustic pressure which is a function of the output signal. The acoustic actuator may comprise a fuel injector (18) or a loudspeaker. By using an appropriate control algorithm, acoustic boundary conditions corresponding to a particular gas turbine combustion chamber may be produced, at least in a certain frequency range.