THE OBSERVED AEROACOUSTIC BEHAVIOUR OF SOME FLOW-EXCITED EXPANSION CHAMBERS

Expansion chambers are often installed in flow duct systems to reduce pressure pulsations and to act as silencers. However, in certain circumstances they can become flow-excited sound generators rather than attenuators. The physics of the flow acoustic interactions responsible for this are examined in detail. They are then illustrated by a systematic sequence of sound power flux measurements in the downpipe, expansion chamber and tailpipe of some representative examples. These measurements then identify both the position and spectral characteristics of the sources of sustained excitation by the flow in its relation to the local geometry and its associated reverberant acoustic behaviour.