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Tonal noise emitted from large spray dryer exhaust stacks used in the dairy industry can give rise to complaints from nearby communities. In many cases, the tone at the fan blade passing frequency is characterised by a frequency above the first mode cut on frequency of the exhaust stack and varies in both amplitude and frequency. In food processing industries, the use of sound absorptive materials in silencers is often unacceptable and particularly when the fan speed is variable, it is difficult to achieve an acceptable passive solution at a reasonable cost.
On this on-going project, which is funded by Dairy Australia, the active control of tonal noise propagating above the cut-on frequency of the first higher order mode in cylindrical industrial exhaust stacks was investigated. Real time control has been performed on a large size in-service exhaust stack and control performance was evaluated in the community. The results demonstrated that where ever it could be measured in the community, the tonal noise emitted from the exhaust stack at the fan blade passing frequency (above the first mode cut on frequency of the exhaust stack) was reduced almost to the background noise level using the active control system, even though the blade passing frequency varied significantly in amplitude and frequency over short periods of time. The current system has now been operational for more than a year and current work is directed at adapting the ANC system to a spray dryer with a wet scrubber (and a corresponding very humid environment).
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