ACTIVE CONTROL OF THE NOISE FROM A RIJKE TUBE

Instabilities caused by flames or other heat sources in resonators can be suppressed with a feedback system. It consists of a microphone inside the resonator, a phase-shifter, an amplifier and a loudspeaker at one end of the resonator. The pressure field produced by the loudspeaker changes the boundary conditions at the end of the resonator. This annuls the unstable oscillations in a phase range of up to 150 degrees if the amplification is sufficiently high. A theory explaining the effect of the feedback system on the stability has been developed. Noise contamination in the feedback system is taken into account. The stability behavior of the resonator and the reflection coefficients at the resonator end are predicted with the theory. Experiments with a Rijke tube driven by a hot gauze have been performed which confirm the predictions of the theory. The feedback system reduces flame-driven oscillations in a tube by over 40 db.