Active reduction of the dominant clear tone component of the noise pollution from power plant chimneys

An investigation of a system for active reduction of an irritating dominant clear tone emitted as a part of the noise pollution from a power plant chimney is described. Experiments were carried out on a scale model with diameters 3-4 times less than the actual chimney. Reduction of 25-30 decibels was achieved in relation to the clear tone in the frequency range in connection with which it could be expected that the noise-muffling system would work. The system was also seen to be able to follow the expected variation in the noise signals and to be stable during a longer period of time. The theories on which the experiment was based, the experimental setup and measurement methods and results are presented. (AB)