Abstract:

Tonal noise emitted from large spray dryer exhaust stacks 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 the frequency. In many food processing industries the use of sound absorptive materials in silences is not acceptable and, particularly when the fan speed is variable, it is difficult to achieve an acceptable passive solution at a reasonable cost. Here the active control of tonal noise propagating above the cut-on frequency of the first higher order mode in a large industrial cylindrical exhaust stack is investigated. Real time control was performed on in-service equipment and control performance was evaluated in the community. The results demonstrated that tonal noise above the first cuton frequency of the exhaust stack can be actively controlled.