The concept of cancelling a sound by adding an out-ofphase sound is neither very new nor very difficult to imagine. Neither is the idea that adding a sound wave to waves already present in a duct will alter the standing wave pattern. This study arose from the hope that both concepts might be applied in some useful and practical way to control the sound transported in ducts containing fluids. At the time of initiating the study, the literature contained little in the way of practical application of sound concepts to problems of air or water carried in ducts used in the air-conditioning industry. Therefore, several experimental setups were devised and tried, some air-filled and some water-filled, to pinpoint possible difficulties and limitations. An interest was expressed to the author to make his exploratory experimentation known, although progress was not yet sufficient to indicate practical methods for noise control. The report must be viewed therefore as an outline of concepts, a review of technical theory, a presentation of exploratory experiments, and thoughts on possible uses or applications of the concepts.