Active control of multi-modal propagation of tonal noise in ducts

The active control of tonal noise propagating in ducts at frequencies where many modes are able to propagate is considered. The final objective of the work reported is to cancel the prominent 450-500 Hz blade passing frequency of the rotary suction fans found in chimney stacks of power stations. Three sets of experimental results are presented and supported by results from computer simulations. Thus, a number of observations and guidelines for the design of such a control system that so far have been drawn only from theory and simulation studies are discussed based on experimental results from the laboratory. The study of the full control system, which comprises 30 secondary sources and 32 error sensors, has revealed a number of new results, in particular with respect to the positioning of the sources and sensors along the length of the duct. Eventually, a reduction of the radiated power of between 20 and 50 dB was obtained under laboratory conditions at frequencies where up to 25 modes were able to propagate. The final experiments carried out in an out-of-use chimney stack showed slightly poorer results and a number of explanations for this discrepancy are discussed. (C) 1996 Academic Press Limited