Active Control of the Noise from a Rijke Tube

Abstract

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 chosen to be sufficiently high. A theory explaining the effect of the feedback system on the stability behaviour will be developed. Experiments using a Rijke tube driven by a hot gauze have been performed which confirm the predictions of the theory. In a practical situation with background noise, the feedback system achieves a noise reduction of over 40 dB.