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Practical issues associated with the installation of an active noise control (ANC) system in an 80m high exhaust stack, containing a hot, wet and dirty air flow, are discussed. The noise problem to be controlled was a 165Hz tone generated by the fan at the bottom of the stack and radiated by the stack into the surrounding community from which complaints were received on a regular basis, sometimes from residents living more than 1km away. The stack was split into three parallel axial sections in the vicinity of the ANC system to ensure that only plane waves were present. The use of the ANC system resulted in reductions of the tonal noise of 10dB inside one section of the duct and 20dB inside the other two sections. It is expected that similar reductions in the tonal noise would be measured in the community. However this latter measurement was not done because it was found that the tonal noise was masked by the background noise, even with the ANC system switched off. This was a result of the installation of the axial splitters which divided the duct into 3 sections such that the acoustic path length difference was 1/3 of a wavelength between adjacent sections. However, in the event that further complaints of tonal noise are received from the community, the work reported here indicates that an ANC system will be a viable and cost effective solution.
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